<PAGE>
 
                                                                   EXHIBIT 10.43

                     SECOND AMENDMENT TO CREDIT AGREEMENT
                     ------------------------------------

                SECOND AMENDMENT (this "Amendment"), dated as of July 1, 1998, 
among Cambridge Industries Holdings, Inc. ("Holdings"), Cambridge Industries, 
Inc. (the "Borrower"), the lenders party to the Credit Agreement referred to 
below (the "Banks"), and Bankers Trust Company, as Agent (in such capacity, 
the "Agent").  All capitalized terms used herein and not otherwise defined 
herein shall have the respective meanings provided such terms in the Credit 
Agreement referred to below.

                                  WITNESSETH:
                                  ----------

                WHEREAS, Holdings, the Borrower, the Banks and the Agent are
parties to a Credit Agreement, dated as of July 10, 1997 (as amended, modified
or supplemented through the date hereof, the "Credit Agreement"); and

                WHEREAS, the parties to the Credit Agreement wish to amend the 
Credit Agreement as herein provided;

                NOW, THEREFORE, it is agreed:

                1.      Section 8.10 of the Credit Agreement is hereby amended 
by deleting the ratio "1.7:1.0" opposite each of the dates "September 30, 1998" 
and "December 31, 1998" and inserting the ratio "1.6:1.0" in lieu thereof.

                2.      Section 8.11 of the Credit Agreement is hereby amended 
by deleting the ratio "6.0:1.0" opposite each of the dates "September 30, 1998" 
and "December 31, 1998" and inserting the ratio "6.5:1.0" in lieu thereof.

                3.      This Amendment is limited as specified and shall not 
constitute a modification, acceptance or waiver of any other provision of the 
Credit Agreement or any other Credit Document.

                4.      In order to induce the Banks to enter into this 
Amendment, each of Holdings and the Borrower hereby represents and warrants that
(x) no Default or Event of Default exists on the Second Amendment Effective Date
after giving effect to this Amendment and (y) all of the representations and 
warranties contained in the Credit Documents shall be true and correct in all 
material respects on the Second Amendment Effective Date both before and after 
giving effect to this Amendment with the same effect as though such 
representations and warranties had been made on and as of the Second Amendment 
Effective Date (it being understood that any representation or warranty made as 
of a specific date shall be true and correct in all material respects as of such
specific date).

                5.      This Amendment may be executed in any number of 
counterparts and by the different parties hereto on separate counterparts, each 
of which counterparts when executed and delivered shall be an original, but all 
of which shall together constitute one and the same instrument.  A complete set 
of counterparts shall be lodged with the Borrower and the Agent.



<PAGE>
 


                6.      This Amendment and the rights and obligations of the 
parties hereunder shall be construed in accordance with and governed by the law 
of the State of New York.

                7.      This Amendment shall become effective as of July 1, 1998
on the date (the "Second Amendment Effective Date") when each of Holdings, the 
Borrower and the Required Banks shall have signed a counterpart hereof (whether 
the same or different counterparts) and shall have delivered (including by way 
of telecopier) the same to the Agent at its Notice Office.  The Agent shall 
promptly notify the Borrower and the Banks in writing of the Second Amendment 
Effective Date.

                8.      From and after the Second Amendment Effective Date, all
references in the Credit Agreement and each of the other Credit Documents to the
Credit Agreement shall be deemed to be references to the Credit Agreement after
giving effect to this Amendment.



                                     -2- 







<PAGE>
 


                IN WITNESS WHEREOF, each of the parties hereto has caused a 
counterpart of this Amendment to be duly executed and delivered as of the date 
first above written.

                                       CAMBRIDGE INDUSTRIES HOLDINGS, INC.


                                       By /s/ John M. Colaianne
                                       ------------------------
                                       Title:   CFO

                                       CAMBRIDGE INDUSTRIES, INC.

                                       By /s/ John M. Colaianne
                                       ------------------------
                                       Title:   CFO                      
<PAGE>
 

                                            BANKERS TRUST COMPANY,
                                             Individually and as Agent

                                            By    /s/   Mary Kay Coyle
                                              ------------------------------
                                              Title:    MANAGING DIRECTOR
<PAGE>
 

                                            BANKBOSTON, N.A.

                                            By    /s/   Kimberly F. Harris
                                              ------------------------------
                                              Title:    VP

<PAGE>
 

                                            CITY NATIONAL BANK

                                            By    [signature illegible]
                                              ------------------------------
                                              Title:    VP


<PAGE>
 

                                            COMERICA BANK

                                            By    [signature illegible]
                                              ------------------------------
                                              Title:    VP



<PAGE>
 

                                            DRESDNER BANK AG, NEW YORK
                                            AND GRAND CAYMAN BRANCHES

                                            By    /s/ Beverly G. Cason
                                              ------------------------------
                                              Title:    BEVERLY G. CASON
                                                        VICE PRESIDENT

                                            By    /s/ John W. Sweeney
                                              ------------------------------
                                              Title:   JOHN W. SWEENEY 
                                                    ASSISTANT VICE PRESIDENT

                                             




<PAGE>
 

                                            DEEPROCK & COMPANY
                                            By  Eaton Vance Management, as  
                                                Investment Advisor

                                            By    /s/  Scott H. Page
                                              ------------------------------
                                              Title:   SCOTT H. PAGE
                                                      VICE PRESIDENT

                                             





<PAGE>
 

                                            SENIOR DEBT PORTFOLIO
                                            By: Boston Management and Research  
                                                as Investment Advisor

                                            By    /s/  Scott H. Page
                                              ------------------------------
                                              Title:   SCOTT H. PAGE
                                                      VICE PRESIDENT

                                             






<PAGE>
 

                                            FLEET NATIONAL BANK


                                            By  [signature illegible]
                                              ------------------------------
                                              Title:  Senior Vice-President 
                                                      

                                             







<PAGE>
 

                                INDOSUEZ CAPITAL FUNDING III, LIMITED
                                By:  Indosuez Capital Luxembourg, as Collateral
                                     Manager
                                        
                                
                                By  /s/  Francoise Berthelot
                                ------------------------------
                                Title:  FRANCOISE BERTHELOT
                                       AUTHORIZED SIGNATORY               

                                             








<PAGE>
 

                                INDOSUEZ CAPITAL FUNDING IIA, LIMITED
                                By:  Indosuez Capital Luxembourg, as Collateral
                                     Manager
                                        
                                
                                By  /s/  Francoise Berthelot
                                ------------------------------
                                Title:  
                                       

                                             









<PAGE>
 

                                      PILGRIM AMERICA PRIME RATE TRUST


                                      By  /s/  Robert L. Wilson
                                        ------------------------------
                                        Title:  Robert L. Wilson
                                                 Vice-President 
                                                      

                                             








<PAGE>
 

                                      SANWA BUSINESS CREDIT CORPORATION


                                      By  /s/  Stanley Kaminski
                                        ------------------------------
                                        Title:  Stanley Kaminski
                                                 Vice-President 
                                                      

                                             









<PAGE>
 

                                      TRANSAMERICA BUSINESS CREDIT
                                      CORPORATION


                                      By  /s/  Perry Vavoules
                                        ------------------------------
                                        Title:  Perry Vavoules
                                                Senior Vice-President 
                                                      

                                             










<PAGE>
 

                                      VAN KAMPEN AMERICAN CAPITAL PRIME
                                      RATE INCOME TRUST


                                      By  /s/  Jeffrey W. Maillet
                                        --------------------------------
                                        Title:  JEFFREY W. MAILLET
                                               SR. VICE PRES. & DIRECTOR
                                                      

                                             











<PAGE>
 

                                            COMPAGNIE FINANCIERE DE CIC ET DE
                                            L'UNION EUROPEENNE

                                            By  /s/ Sean Mounier
                                              ------------------------------
                                              Title:  First Vice President  
                                                       

                                            By:    /s/ Brian O'Leary
                                               ----------------------------
                                              Title:  Vice President   
                                                    

                                             




 

<PAGE>
 

                                            GENERAL ELECTRIC CAPITAL
                                            CORPORATION

                                            By  /s/ Murray Stegelmann
                                              ---------------------------------
                                              Title:  Murray Stegelmann
                                                      Duly Authorized Signatory
                                                       

                                            
                                                    

                                             




 


<PAGE>
 

                                            OCTAGON LOAN TRUST

                                            By  Octagon Credit Incestors,
                                                   its manager      
                                               /s/ Richard W. Stewart
                                              ---------------------------
                                              Title:  RICHARD W. STEWART
                                                       MANAGING DIRECTOR
                                                       

                                            
                                                    

                                             




 



<PAGE>
 

                                      BANK POLSKA KASA OPIEKI S.A., PEKAO
                                      S.A. GROUP, NEW YORK BRANCH


                                      By  [signature illegible]
                                        --------------------------------
                                        Title:  Vice President
                                                      

                                             












<PAGE>
 

                                      DLJ CAPITAL FUNDING, INC.


                                      By  [signature illegible]
                                        --------------------------------
                                        Title:  
                                                      

                                             












 

<PAGE>
 

                                      ML CLO XII PILGRIM AMERICA
                                         (CAYMAN) LTD.
                                        By: Pilgrim America Investments, Inc.
                                            as its Investment Manager

                                      By  /s/  Robert L. Wilson
                                        --------------------------------
                                        Name:    Robert L. Wilson
                                        Title:    Vice President
                                                      

                                             












 


<PAGE>
 

                                      ML CLO XV PILGRIM AMERICA
                                         (CAYMAN) LTD.
                                        By: Pilgrim America Investments, Inc.
                                            as its Investment Manager

                                      By  /s/  Robert L. Wilson
                                        --------------------------------
                                        Name:    Robert L. Wilson
                                        Title:    Vice President
                                                      

                                             












 



<PAGE>
 

                                      PAMCO CAYMAN LTD (PROTECTIVE)
                                      By: Highland Capital Management, L.P.
                                          as Collateral Manager

                                      By  /s/  James Dondero
                                        ---------------------------------------
                                        Name:  James Dondero, CFA, CPA
                                        Title: President
                                               Highland Capital Management L.P.

                                             












 




<PAGE>
 

                                      CERES FINANCE LTD.


                                      By  /s/  John H. Cullinane
                                        --------------------------------
                                        Name:    John H. Cullinane
                                        Title:     Director
                                                      

                                             












 




<PAGE>
 
                          THIRD WAIVER AND AMENDMENT

                THIRD WAIVER AND AMENDMENT (this "Waiver"), dated as of December
31, 1998, among CAMBRIDGE INDUSTRIES HOLDINGS, INC. ("Holdings"), CAMBRIDGE 
INDUSTRIES, INC., (the "Borrower"), the lenders from time to time party to the 
Credit Agreement described below (each, a "Bank" and collectively, the "Banks"),
and BANKERS TRUST COMPANY, as Agent (the "Agent"). All capitalized terms used 
herein and not otherwise defined herein shall have the respective meanings 
provided such terms in the Credit Agreement.

                                  WITNESSETH:

                WHEREAS, Holdings, the Borrower, the Banks and the Agent are 
party to a Credit Agreement, dated as of July 10, 1997 (as amended, modified and
supplemented prior to the date hereof, the "Credit Agreement"); and

                WHEREAS, Holdings, the Borrower, the Banks and the Agent intend 
to enter into an amendment to the Credit Agreement amending, on or prior to 
March 15, 1999, certain provisions of the Credit Agreement, including but not 
limited to, Sections 8.09, Section 8.10 and Section 8.11 of the Credit Agreement
(the "Permanent Amendment", and the date of effectiveness of the Permanent 
Amendment, the "Permanent Amendment Effective Date");

                WHEREAS, the Borrower has requested that the Banks provide the 
Waiver provided for herein and the Banks have agreed to provide such Waiver on 
the terms and conditions set forth herein;

                NOW, THEREFORE, it is agreed:

                1.      On the Permanent Amendment Effective Date the definition
of "Applicable Base Rate Margin" in Section 10 of the Credit Agreement shall 
retroactive to January 1, 1999, be amended by (i) deleting the percentage 
"1.50%" appearing therein and inserting "2.00%" in lieu thereof and (ii) 
deleting the percentage "2.00%" appearing therein and inserting "2.50%" in lieu 
thereof provided however, that interest on Base Rate Loans that is retroactively
amended shall be payable on the later of (x) the Permanent Amendment Effective 
Date and (y) the date such interest is otherwise due.

                2.      On the Permanent Amendment Effective Date the definition
of "Applicable Eurodollar Margin" in Section 10 of the Credit Agreement shall 
retroactive to January 1, 1999, be amended by (i) deleting the percentage 
"2.50%" appearing therein and inserting "3.00%" in lieu thereof and (ii) 
deleting the percentage "3.00%" appearing therein and inserting "3.50%" in lieu 
thereof.


                                       1
<PAGE>
 
                3.      On the Permanent Amendment Effective Date the definition
of "Interest Reduction Discount" shall retroactive to January 1, 1999, be 
amended by deleting such definition in its entirety and inserting the following 
new definition in lieu thereof.

                "Interest Reduction Discount" shall mean initially zero, 
provided that from and after any Start Date to and including the corresponding 
End Date, the Interest Reduction Discount shall be (I) in the case of A Term 
Loans and Revolving Loans, the respective percentage per annum set forth in 
clause (A), (B), (C), (D), (E), or (F) below if, but only if, as of the Test 
Date for such Start Date the applicable condition set forth in clause (A), (B), 
(C), (D), (E), or (F) below, as the case may be, is met:

                (A)     .250% if, but only if, the Leverage Ratio on such Test
        Date is less than 5.50:1.00 but greater than or equal to 5.00:1.00; or

                (B)     .500% if, but only if, the Leverage Ratio on such Test
        Date is less than 5.00:1.00 but greater than or equal to 4.50:1.00; or

                (C)     .750% if, but only if, the Leverage Ratio on such Test
        Date is less than 4.50:1.00 but greater than or equal to 4.00:1.00; or

                (D)     1.00% if, but only if, the Leverage Ratio on such Test
        Date is less than 4.00:1.00 but greater than or equal to 3.50:1.00; or

                (E)     1.25% if, but only if, the Leverage Ratio on such Test
        Date is less than 3.50:1.00 but greater than or equal to 3.00:1.00; or

                (F)     1.50% if, but only if, the Leverage Ratio on such Test
        Date is less than 3.00:1.00;

and (II) in the case of B Term Loans, the respective percentage per annum set 
forth in clause (A) or (B) below if, but only if, as of the Test Date for such 
Start Date the applicable condition set forth in clause (A) or (B) below, as the
case may be, is met:

                (A)     .250% if, but only if, the Leverage Ratio on such Test
        Date is less than 5.50:1.00 but greater than or equal to 5.00:1.00; or

                (B)     .500% if, but only if, the Leverage Ratio on such Test
        Date is less than 5.00:1.00.

                Notwithstanding anything to the contrary contained above in this
definition, the Interest Reduction Discount shall be zero at any time when an 
Event of Default shall exist.

                4.      Effective for any Test Period or any fiscal quarter 
ending on December 31, 1998 through and including March 15, 1999 (the "Waiver 
Termination Date"), the Banks hereby waive compliance with the provisions of 
Section 8.09, Section 8.10 and Section 8.11 of the Credit Agreement. This Waiver
shall be effective only for the period from December 31, 1998 to and


                                       2






<PAGE>
 
including the Waiver Termination Date (the "Waiver Period") and shall be of no 
force or effect at any other time.

                5.      In order to induce the Banks to enter into this Waiver, 
the Borrower agrees that at all times from the Waiver Effective Date (as defined
below) to and including the Waiver Termination Date, the sum of (i) the 
aggregate outstanding principal amount of Revolving Loans and Swingline Loans 
and (ii) the Letter of Credit Outstandings under the Credit Agreement shall not 
exceed $45,000,000.

                6.      In order to induce the Banks to enter into this Waiver, 
each of Holdings and the borrower hereby represents and warrants that (i) no 
Default or Event of Default exists as of the Waiver Effective Date (as defined 
below) after giving effect to this Waiver and (ii) on the Waiver Effective Date,
both before and after giving effect to this Waiver, all representations and 
warranties contained in the Credit Agreement and in the other Credit Documents 
are true and correct in all material respects.

                7.      This Waiver shall become effective on the date (the 
"Waiver Effective Date") when (i) the Required Banks, Holdings and the Borrower 
shall have signed a counterpart hereof (whether the same or different 
counterparts) and shall have delivered (including by way of facsimile 
transmission) the same to the Agent at its Notice Office and (ii) each Bank 
which shall have signed and delivered a copy of this Waiver prior to the close 
of business on January 13, 1999, shall have received a waiver fee equal to 1/8 
of 1% on the Revolving Loan Commitment and/or outstanding Term Loans of such 
Bank as in effect on such date.

                8.      This Waiver is limited as specified and shall not 
constitute a modification, acceptance or waiver of any other provision of the 
Credit Agreement or any other Credit Document.

                9.      This Waiver may be executed in any number of 
counterparts and by the different parties hereto on separate counterparts, each 
of which counterparts when executed and delivered shall be an original, but all 
of which shall together constitute one and the same instrument. A complete set 
of counterparts shall be lodged with the Borrower and the Agent.

                10.     At all times during the Waiver Period, all references in
the Credit Agreement and each of the Credit Documents to the Credit Agreement 
shall be deemend to be references to such Credit Agreement after giving effect 
to this Waiver.

                11.     THIS WAIVER AND THE RIGHTS AND OBLIGATIONS OF THE 
PARTIES HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED IN THE ACCORDANCE WITH THE 
LAWS OF THE STATE OF NEW YORK.


                                      ***


                                       3
<PAGE>
 
                IN WITNESS WHEREOF, each of the parties hereto has caused a 
counterpart of this Waiver to be duly executed and delivered as of the date 
hereof.


 

                                            CAMBRIDGE INDUSTRIES HOLDINGS,
                                                INC.

                                            By    /s/   John Colaianne   
                                              ------------------------------
                                              Name: John Colaianne
                                              Title:    CFO


                                            CAMBRIDGE INDUSTRIES, INC.   
                                                INC.

                                            By    /s/   John Colaianne   
                                              ------------------------------
                                              Name: John Colaianne
                                              Title:    CFO




                                       4

<PAGE>
 
 

                                            BANKERS TRUST COMPANY,
                                                Individually and as Agent

                                            By    /s/   Mary Kay Coyle    
                                              ------------------------------
                                              Name: MARY KAY COYLE
                                              Title: Managing Director



                                       5


<PAGE>
 

                                            BANKBOSTON, N.A. 

                                            By    /s/   Kimberly F. Harris
                                              ------------------------------
                                              Title:    VP



                                       6


<PAGE>
 

                                            CITY NATIONAL BANK

                                            By    /s/   Lalward Vassell   
                                              ------------------------------
                                              Title:    V.P.




                                       7


<PAGE>
 

                                            COMERICA BANK        

                                            By    Mark Reifel             
                                              ------------------------------
                                              Title:    V.P.



                                       8


<PAGE>
 
 

                                            CREDIT LYONNAIS CHICAGO BRANCH

                                            By    /s/ Lee E. Greve        
                                              ------------------------------
                                              Title:   LEE E. GREVE
                                                     FIRST VICE PRESIDENT




                                       9
<PAGE>
 
 
 
                                            CYPRESS TREE INVESTMENT       
                                            MANAGEMENT COMPANY, INC.
                                              As: Attorney-in-Fact and on 
                                            Behalf of First Allmerica Financial
                                            Life Insurance Company

                                            By    /s/ Catherine C. McDermott
                                              ------------------------------
                                              Title:   CATHERINE C. McDERMOTT
                                                           PRINCIPAL      



                                      10
<PAGE>
 
 
 
                                            DRESDNER BANK AG, NEW YORK    
                                            AND GRAND CAYMAN BRANCHES

                                            By    /s/ Christopher E. Sarisky
                                              ------------------------------
                                              Title: CHRISTOPHER E. SARISKY  
                                                     Assistant Vice President

                                            By    /s/ John W. Sweeney        
                                              ------------------------------
                                              Title: JOHN W. SWEENEY         
                                                     ASSISTANT VICE PRESIDENT



                                      11

<PAGE>
 
 
 
                                            DEEPROCK & COMPANY            

                                            By: Eaton Vance Management, as  
                                                Investment Advisor         


                                            By    /s/ Scott H. Page          
                                              ------------------------------
                                              Title: SCOTT H. PAGE           
                                                     VICE PRESIDENT



                                      12


<PAGE>
 
 
 
                                            SENIOR DEBT PORTFOLIO         

                                          By  Boston Management and Research, as
                                              Investment Advisor


                                          By    /s/ Scott H. Page          
                                              ------------------------------
                                              Title: SCOTT H. PAGE           
                                                     VICE PRESIDENT



                                      13

<PAGE>
 

                                            FLEET NATIONAL BANK

                                            By    George O. Januga              
                                              ------------------------------
                                              Title: Vice President



                                      14

<PAGE>
 

                                            HELLER FINANCIAL, INC.

                                            By    Craig Gallehugh               
                                              ------------------------------
                                              Title: Vice President



                                      15


<PAGE>
 
 
 
                                          INDOSUEZ CAPITAL FUNDING III, LIMITED
                                          By  Indosuez Capital Luxembourg, as  
                                              Collateral Manaager


                                          By    /s/ Francoise Berthelot    
                                              ------------------------------
                                              Title: FRANCOISE BERTHELOT     
                                                     AUTHORIZED SIGNATORY



                                      16


<PAGE>
 
 
                                          INDOSUEZ CAPITAL FUNDING IIA, LIMITED
                                          By  Indosuez Capital Luxembourg, as  
                                              Collateral Manaager


                                          By    /s/ Francoise Berthelot    
                                              ------------------------------
                                              Title: FRANCOISE BERTHELOT     
                                                     AUTHORIZED SIGNATORY



                                      17



<PAGE>
 
 
                                          PILGRIM AMERICA PRIME RATE TRUST
                                          By Pilgrim Investments, Inc.         
                                              as its Investment Manager


                                          By    /s/ Robert L. Wilson       
                                              ------------------------------
                                              Title: Robert L. Wilson        
                                                     Vice President      



                                      18




<PAGE>
 


                                       SANWA BUSINESS CREDIT CORPORATION

                                       By:  No Signature Received
                                           --------------------------
                                           Title:


                                      19

<PAGE>
 
 
                                            TRANSAMERICA BUSINESS CREDIT  
                                            CORPORATION               

                                            By    /s/ Perry Vavoules        
                                              ------------------------------
                                              Title: Perry Vavoules          
                                                     Senior Vice President



                                      20

<PAGE>
 
 
                                            VAN KAMPEN PRIME RATE INCOME TRUST

                                            By    /s/ Jeffrey W. Marlet     
                                              ------------------------------
                                              Title: JEFFREY W. MARLET       
                                               Senior Vice President & Director



                                      21


<PAGE>
 
 
                                            KZH PAMCO LLC                     

                                            By    /s/ Michael M. Wong
                                              ------------------------------
                                              Title: Michael M. Wong         
                                                     Authorized Agent



                                      22



<PAGE>
 
 
                                            COMPAGNIE FINANCIERE DE CIC ET DE 
                                            L'UNION EUROPEENNE

                                            By    /s/ Sean Mounier          
                                              ------------------------------
                                              Title: First Vice President    



                                            By    /s/ Brian O'Leary         
                                              ------------------------------
                                              Title: Vice President    


                                      23



<PAGE>
 
 
                                            GENERAL ELECTRIC CAPITAL          
                                            CORPORATION         

                                            By    /s/ Michael McGonigle     
                                              ------------------------------
                                              Title: MICHAEL McGONIGLE       
                                              DULY AUTHORIZED SIGNATORY




                                      24




<PAGE>
 
 
                                            BANK POLSAK KASA OPIEKI S.A., PEKAO
                                            S.A. GROUP, NEW YORK BRANCH

                                            By    Harvey Winter                
                                              ------------------------------
                                              Title: Vice President    





                                      25




<PAGE>
 
 


                                       FIRST UNION NATIONAL BANK        

                                       By:  No Signature Received
                                           --------------------------
                                           Title:


                                      26


<PAGE>
 
 
                                            ML CLO XII PILGRIM AMERICA (CAYMAN)
                                            LTD.  By: Pilgrim Investments, Inc.
                                                      as its Investment Manager

                                            By  /s/ Robert L. Wilson        
                                              ------------------------------
                                              Name:  Robert L. Wilson
                                              Title: Vice President





                                      27





<PAGE>
 
 
                                            ML CLO XV PILGRIM AMERICA
                                            (CAYMAN) LTD.  
                                            By: Pilgrim Investments, Inc.
                                                as its Investment Manager

                                            By  /s/ Robert L. Wilson        
                                              ------------------------------
                                              Name:  Robert L. Wilson
                                              Title: Vice President





                                      28






<PAGE>
 
 
                                         PAMCO CAYMAN LTD         
                                         By: Highland Capital Management, L.P.
                                             as Collateral Manager

                                         By  [signature illegible]       
                                           ------------------------------
                                           Name:  
                                           Title: 





                                      29







<PAGE>
 
 
                                         TORONTO DOMINION (TEXAS), INC.

                                         By  Sonja R. Jordan       
                                           ------------------------------
                                           Name:  SONJA R. JORDAN
                                           Title: VICE PRESIDENT





                                      30








<PAGE>
 
 
 
                                         CERES FINANCE LTD.             

                                         By  [signature illegible]
                                           ------------------------------
                                           Name:  
                                           Title: 





                                      31









<PAGE>
 
 
 
                                            EATON VANCE SENIOR INCOME TRUST    
                                            BY: EATON VANCE MANAGEMENT         
                                                AS INVESTMENT ADVISOR          

                                            By  /s/ SCOTT H. PAGE           
                                              ------------------------------
                                              Name:  SCOTT H. PAGE   
                                              Title: VICE PRESIDENT





                                      32






<PAGE>
 
 
                                        CYPRESS TREE INVESTMENT            
                                        PARTNERS I, LTD.                   
                                         By: Cypress Tree Investment       
                                        Company, Inc., as Portfolio Manager

                                            By  /s/ Philip C. Robbins       
                                              ------------------------------
                                              Title: PHILIP C. ROBBINS
                                                     PRINCIPAL      





                                      33







<PAGE>
 
                         FOURTH WAIVER AND AMENDMENT 
                         ---------------------------

                FOURTH WAIVER AND AMENDMENT (this "Amendment"), dated as of
February 23, 1999, among CAMBRIDGE INDUSTRIES HOLDINGS, INC. ("Holdings"),
CAMBRIDGE INDUSTRIES, INC., (the "Borrower"), the lenders from time to time
party to the Credit Agreement described below (each, a "Bank" and collectively,
the "Banks"), and BANKERS TRUST COMPANY, as Agent (the "Agent"). All capitalized
terms used herein and not otherwise defined herein shall have the respective
meanings provided such terms in the Credit Agreement.

                                  WITNESSETH:
                                  ----------

                WHEREAS, Holdings, the Borrower, the Banks and the Agent are 
party to a Credit Agreement, dated as of July 10, 1997 (as amended, modified and
supplemented prior to the date hereof, the "Credit Agreement");

                WHEREAS, Holdings, the Borrower, the Banks and the Agent entered
into the Third Waiver and Amendment to the Credit Agreement ("Third Waiver") 
dated as of December 31, 1998, which Third Waiver contemplated, among other 
things, the entering into of a Permanent Amendment (as defined in the Third 
Waiver);

                WHEREAS, this Amendment is the Permanent Amendment, although 
certain terms and provisions of this Amendment differ from those contemplated by
the Third Waiver, and the parties hereto intend that as to any such differences 
the terms and provisions of this amendment shall govern;

                WHEREAS, the parties hereto wish to amend the Credit Agreement 
as provided herein;

                NOW, THEREFORE, it is agreed;

                1.      Sections 1, 2, and 3 of the Third Waiver are hereby 
deleted in their entirety and replaced by Sections 2, 3 and 7 of this Amendment.

                2.      The definition of "Applicable Base Rate Margin" in 
Section 10 of the Credit Agreement is hereby amended (retroactively to January 
1, 1999) by (i) deleting the percentage "1.50%" appearing therein and inserting 
"2.50% in lieu thereof and (ii) deleting the percentage "2.00%" appearing 
therein and inserting "3.00%" in lieu thereof (it being understood and agreed 
that to the extent any interest payments on Base Rate Loans have been made 
between January 1, 1999 and the Amendment Effective Date, the Borrower will make
a supplemental interest payment to the Banks on the Amendment Effective Date to 
give retroactive effect to the increase in interest rates effected by this 
Agreement).


                                       1
<PAGE>
 

                3.      The definition of "Applicable Eurodollar Margin" in 
Section 10 of the Credit Agreement is hereby amended (retroactively to January 
1, 1999) by (i) deleting the percentage "2.50%" appearing therein and inserting 
"3.50%" in lieu thereof and (ii) deleting the percentage "3.00%" appearing 
therein and inserting "4.00%" in lieu thereof (it being understood and agreed 
that to the extent any interest payments on Eurodollar Loans have been made 
between January 1, 1999 and the Amendment Effective Date, the Borrower will make
a supplemental interest payment to the Banks on the Amendment Effective Date to 
give retroactive effect to the increase in interest rates effected by this 
Amendment).

                4.      Section 8.09(a) of the Credit Agreement is hereby 
amended by deleting the portion of the table set forth therein from and 
including March 31, 1999 to and including March 31, 2000 (together with the 
amounts set forth opposite such dates), and by inserting in lieu thereof the 
following:

                "March 31, 1999         $ 40,000,000
                June 30, 1999           $ 39,500,000
                September 30, 1999      $ 42,000,000
                December 31, 1999       $ 53,000,000
                March 31, 2000          $ 60,000,000"

                5.      Section 8.10 of the Credit Agreement is hereby amended
by deleting the portion of the table set forth therein from and including March
31, 1999 to and including March 31, 2000 (together with the ratios set forth
opposite such dates), and by inserting in lieu thereof the following:

                "March 31, 1999         1.3:1.0
                June 30, 1999           1.3:1.0
                September 30, 1999      1.4:1.0
                December 31, 1999       1.8:1.0
                March 31, 2000          2.0:1.0"

                6.      Section 8.11 of the Credit Agreement is hereby amended
by deleting the portion of the table set forth therein from and including March
31, 1999 to and including March 31, 2000 (together with the ratios set forth
opposite such dates), and by inserting in lieu thereof the following:

                "March 31, 1999         8.6:1.0
                June 30, 1999           8.6:1.0
                September 30, 1999      8.1:1.0
                December 31, 2000       6.5:1.0
                March 31, 2000          5.7:1.0"


                                       2



<PAGE>
 
                7.      The definition of "Interest Reduction Discount" 
appearing in Section 10 of the Credit Agreement is hereby amended (retroactive 
to January 1, 1999) by deleting such definition in its entirety and inserting 
the following new definition in lieu thereof:

                "Interest Reduction Discount" shall mean initially zero, 
provided that from and after any Start Date to and including the corresponding 
End Date, the Interest Reduction Discount shall be (I) in the case of A Term 
Loans and Revolving Loans, the respective percentage per annum set forth in 
clause (A), (B), (C), (D), (E), (F), or (G) below if, but only if, as of the 
Test Date for such Start Date the applicable condition set forth in clause
(A), (B), (C), (D), (E), (F), or (G) below, as the case may be, is met:

        (A)     .500% if, but only if, the Leverage Ratio on such Test Date is 
   less than 6:00:1.00 but greater than or equal to 5:50:1.00; or

        (B)     .750% if, but only if, the Leverage Ratio on such Test Date is 
   less than 5.50:1.00 but greater than or equal to 5.00:1.00; or

        (C)     .1.00% if, but only if, the Leverage Ratio on such Test Date is 
   less than 5.00:1.00 but greater than or equal to 4.50:1.00; or

        (D)     1.25% if, but only if, the Leverage Ratio on such Test Date is 
   less than 4.50:1.00 but greater than or equal to 4.00:1.00; or

        (E)     1.50% if, but only if, the Leverage Ratio on such Test Date is 
   less than 4.00:1.00 but greater than or equal to 3.50:1.00; or

        (F)     1.75% if, but only if, the Leverage Ratio on such Test Date is 
   less than 3.50:1.00 but greater than or equal to 3.00:1.00; or

        (G)     2.00% if, but only if, the Leverage Ratio on such Test Date is 
   less than 3.00:1.00;

and (II) in the case of B Term Loans, the respective percentage per annum set 
forth in clause (A), (B) or (C) below if, but only if, as of the Test Date for 
such Start Date the applicable condition set forth in clause (A), (B) or (C) 
below, as the case may be, is met:

        (A)     .500% if, but only if, the Leverage Ratio on such Test Date is 
   less than 6:00:1.00 but greater than or equal to 5:50:1:00; or

        (B)     .750% if, but only if, the Leverage Ratio on such Test Date is 
   less than 5.50:1.00 but greater than or equal to 5.00:1.00; or

        (C)     1,00% if, but only if, the Leverage Ratio on such Test Date is 
   less than 5.00:1.00.

        Notwithstanding anything to the contrary contained above in this 
definition, the Interest Reduction Discount shall be zero at any time when an 
Event of Default shall exist.


                                       3

 

<PAGE>
 
        8.      Effective for any Test Period or any fiscal quarter ending on 
(i) December 31, 1998, the Banks hereby permanently waive compliance with the 
provisions of Section 8.09(a), Section 8.10 and section 8.11 of the Credit 
Agreement and (ii) December 31, 1998 through and including March 31, 2000, the 
Banks hereby waive compliance with the provisions of Section 8.09(b).

        9.      In order to induce the Banks to enter into this Amendment, the 
Borrower agrees that from the Amendment Effective Date (as defined below) to and
including March 31, 2000, the aggregate outstanding principal amount of 
Revolving Loans and Swingline Loans shall not (x) at any time exceed $65,000,000
and (y) on the last day of any month exceed $50,000,000.  The Borrower further 
agrees that at all times from the Amendment Effective Date to and including 
March 31, 2000, the Letter of Credit Outstandings under the Credit Agreement 
shall not exceed $5,300,000.

        10.     Notwithstanding anything to the contrary contained in Sections 
4.01, 4.02 (A)(b)(i) and 4.02(A)(b)(ii) of the Credit Agreement, on the 
Amendment Effective Date the Borrower shall prepay $12,500,000 of the principal 
amount of A Term Loans (i.e. the principal amount of A Term Loans required as 
Scheduled A Repayments for the periods from and including March, 1999 to and 
including December, 1999) and (y) $1,350,000 of the principal amount of B Term 
Loans (i.e. the principal amount of B Term Loans required as Scheduled B 
Repayments for the periods from and including March, 1999 to and including 
December, 1999), it being understood and agreed that such prepayments shall 
reduce the remaining Scheduled Repayments in direct order of maturity.

        11.     In order to induce the Banks to enter into this Amendment, each 
of Holdings and the Borrower hereby represents and warrants that (i) no Default 
or Event of Default exists as of the Amendment Effective date (as defined below)
after giving effect to this Amendment and (ii) on the Amendment Effective Date, 
both before and after giving effect to this Amendment, all representations and 
warranties contained in the Credit Agreement and in the other Credit Documents 
are true and correct in all material respects.

        12.     This Amendment shall become effective on the date (the
"Amendment Effective Date") when (i) the Required Banks, Holdings and the
Borrower shall have signed a counterpart hereof (whether the same or different
counterparts) and shall have delivered (including by way of facsimile
transmission) the same to the Agent at its Notice Office, (ii) the Borrower
shall have made the prepayments described in Section 10 of this Amendment and
(iii) each Bank which shall have signed and delivered a copy of this amendment
prior to the close of business on March 10, 1999, shall have received an
amendment fee equal to (x) 1/2 of 1% on the Revolving Loan Commitment and
outstanding A Term Loans of such Bank as in effect on such date and (y) 3/8 of
1% on the outstanding B Term Loans of such Bank as in effect on such date.

        13.     This Amendment is limited as specified and shall not constitute 
a modification, acceptance or waiver of any other provision of the Credit 
Agreement or any other Credit Document.


                                       4
<PAGE>
 
        14.     This Amendment may be executed in any number of counterparts and
by the different parties hereto on separate counterparts, each of which 
counterparts when executed and delivered shall be an original, but all of which 
shall together constitute one and the same instrument.  A complete set of 
counterparts shall be lodged with the Borrower and the Agent.

        15.     From and after the Amendment Effective Date, all references in 
the Credit Agreement and each of the Credit Documents to the Credit Agreement 
shall be deemed to be references to such Credit Agreement after giving effect to
this Amendment.

        16.     THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES 
HEREUNDER SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE 
STATE OF NEW YORK


                                     * * *



                                       5
<PAGE>
 

                IN WITNESS WHEREOF, each of the parties hereto has caused a 
counterpart of this Waiver to be duly executed and delivered as of the date 
hereof.


                                       CAMBRIDGE INDUSTRIES HOLDINGS, INC.


                                       By /s/ John M. Colaianne
                                       ------------------------
                                       Name: John M. Colaianne
                                       Title:   CFO

                                       CAMBRIDGE INDUSTRIES, INC.

                                       By /s/ John M. Colaianne
                                       ------------------------
                                       Name: John M. Colaianne
                                       Title:   CFO                      




                                       6
<PAGE>
 
 

                                            BANKERS TRUST COMPANY
                                             Individually and as Agent

                                            By    /s/   Mary Kay Coyle
                                              ------------------------------
                                              Name:    MARY KAY COYLE
                                              Title:    MANAGING DIRECTOR



                                       7
<PAGE>
 
 

                                            BANKBOSTON N.A.

                                            By    /s/   Kimberly F. Harris
                                              ------------------------------
                                              Title:    VP



                                       8

<PAGE>
 
 

                                            CITY NATIONAL BANK

                                            By    Edward Vassallo      
                                              ------------------------------
                                              Title:    VP


                                       9
<PAGE>
 
 

                                            DRESDNER BANK AG, NEW YORK
                                            AND GRAND CAYMAN BRANCHES


                                            By    /s/ John W. Sweeney
                                              ------------------------------
                                              Name:    JOHN W. SWEENEY 
                                              Title: ASSISTANT VICE PRESIDENT

                                             
                                            By    /s/ Beverly G. Cason
                                              ------------------------------
                                              Name:     BEVERLY G. CASON
                                              Title:    VICE PRESIDENT




                                      13

 
<PAGE>
 
 

                                            DEEPROCK & COMPANY
                                            By  Eaton Vance Management, as  
                                                Investment Advisor

                                            By    /s/  Scott H. Page
                                              ------------------------------
                                              Title:   SCOTT H. PAGE
                                                      VICE PRESIDENT

                                             



                                      14


<PAGE>
 
 

                                            SENIOR DEBT PORTFOLIO
                                            By: Boston Management and Research  
                                                as Investment Advisor

                                            By    /s/  Scott H. Page
                                              ------------------------------
                                              Title:   SCOTT H. PAGE
                                                      VICE PRESIDENT

                                             







<PAGE>
 
 

                                            FLEET NATIONAL BANK


                                            By  [signature illegible]    
                                              ------------------------------
                                              Title:  Vice-President 
                                                      

                                             




                                      16



<PAGE>
 
 

                                            HELLER FINANCIAL, INC.

                                            By    [sig illegible]         
                                              ------------------------------
                                              Title: Vice President



                                      17



<PAGE>
 

                                INDOSUEZ CAPITAL FUNDING III, LIMITED
                                By:  Indosuez Capital as Portfolio Advisor
                                        
                                
                                By  [signature illegible]
                                ------------------------------
                                Title:  Vice President

                                             




                                      18




<PAGE>
 

                                INDOSUEZ CAPITAL FUNDING IIA, LIMITED
                                By:  Indosuez Capital as Portfolio Advisor
                                        
                                
                                By  [signature illegible]
                                ------------------------------
                                Title:  Vice President
                                       

                                             




                                      19

<PAGE>
 
 
 
 
                                            EATON VANCE SENIOR INCOME TRUST    
                                            BY: Eaton Vance Management         
                                                as Investment Advisor          

                                            By  /s/ SCOTT H. PAGE           
                                              ------------------------------
                                              Name:  SCOTT H. PAGE   
                                              Title: VICE PRESIDENT




<PAGE>
 
 

                                            FLEET BUSINESS CREDIT CORPORATION
                                            F/K/A
                                            SANWA BUSINESS CREDIT CORPORATION

                                            By    Peter Levy                    
                                              ------------------------------
                                              Title: Sr. V.P.



                                      21


<PAGE>
 
 

                                      TRANSAMERICA BUSINESS CREDIT
                                      CORPORATION


                                      By  /s/  Perry Vavoules
                                        ------------------------------
                                        Title:  Perry Vavoules
                                                Senior Vice-President 
                                                      

                                             




                                      22






<PAGE>
 

                                      VAN KAMPEN PRIME RATE INCOME TRUST


                                      By  /s/  Jeffrey W. Maillet
                                        --------------------------------
                                        Title:  JEFFREY W. MAILLET
                                               Senior Vice President & Director
                                                      

                                             




                                      23


<PAGE>
 

                                      KZH-PAMCO LLC                     


                                      By  /s/  Shari Finkelstein
                                        --------------------------------
                                        Title:  SHARI FINKELSTEIN
                                                Authorized Agent
                                                      

                                             




                                      24



<PAGE>
 

                                            COMPAGNIE FINANCIERE DE CIC ET DE
                                            L'UNION EUROPEENNE

                                            By  /s/ Sean Mounier
                                              ------------------------------
                                              Title:  First Vice President  
                                                       

                                            By:    /s/ Anthony Rock 
                                               ----------------------------
                                              Title:  Vice President   
                                                    

                                             




                                      25


<PAGE>
 
 

                                            GENERAL ELECTRIC CAPITAL
                                            CORPORATION

                                            By    William Richardson   
                                               ----------------------------
                                              Title:  Duly Authorized Signatory
                                                     
                                                       

                                            
                                                    

                                             
                                      26



<PAGE>
 

                                      BANK POLSKA KASA OPIEKI S.A. PEKAO
                                      S.A. GROUP, NEW YORK BRANCH


                                      By  [signature illegible]
                                        --------------------------------
                                        Title:  Vice President
                                                      

                                             




                                      27
<PAGE>
 

                                      ML CLO XII PILGRIM AMERICA
                                         (CAYMAN) LTD.
                                        By: Pilgrim Investments, Inc.
                                            as its Investment Manager

                                      By  /s/  Robert L. Wilson
                                        --------------------------------
                                        Name:    Robert L. Wilson
                                        Title:    Vice President
                                                      

                                             






                                      29

<PAGE>
 
 

                                      ML CLO XV PILGRIM AMERICA
                                       (CAYMAN) LTD.    
                                      By: Pilgrim Investments, Inc.        
                                          as its investment manager

                                      By  /s/  Robert L. Wilson
                                        ---------------------------------------
                                        Name:  Robert L. Wilson
                                        Title: Vice President

                                             






                                      30

<PAGE>
 

                                      PAMCO CAYMAN LTD.
                                      By: Highland Capital Management, L.P.
                                          as Collateral Manager


                                      By  /s/  James Dondero
                                        --------------------------------
                                        Name:    James Dondero, CFA, CPA
                                        Title:   President
                                                 Highland Capital Management LP.
                                                      

                                             
                                      31

<PAGE>
 

                                      CERES FINANCE LTD.


                                      By  /s/  John H. Cullinane
                                        --------------------------------
                                        Name:    John H. Cullinane
                                        Title:     Director
                                                      

                                             




                                      33



<PAGE>
 

                                       BLACK DIAMOND CLO 1998-1 LTD.       


                                       By /s/ John H. Cullinane
                                       ------------------------
                                       Name: John H. Cullinane
                                       Title:   Director







                                      36