<PAGE>   1
                                  EXHIBIT  4.9




THIRD AMENDMENT DATED OCTOBER 31, 1996 TO LOAN AGREEMENT DATED SEPTEMBER 19,
1993 AMONG ELCOR CORPORATION, NATIONSBANK OF TEXAS, N.A., AS ISSUER,
ADMINISTRATIVE LENDER, AND LENDER; AND BANK OF AMERICA-TEXAS, N.A. AND COMERICA
BANK - TEXAS AS LENDERS.

<PAGE>   2

                                  EXHIBIT 4.9

                       THIRD AMENDMENT TO LOAN AGREEMENT

         THIS THIRD AMENDMENT TO LOAN AGREEMENT (this "Third Amendment"), dated
as of October 31, 1996, is entered into among ELCOR CORPORATION, a Delaware
corporation ("Company"), the lenders listed on the signature pages hereof
("Lenders"), NATIONSBANK OF TEXAS, N.A., as Issuer (in said capacity,
"Issuer"), and NATIONSBANK OF TEXAS, N.A., as Administrative Lender (in said 
capacity, "Administrative Lender").

                                   BACKGROUND

         A.      Company, the Lenders, Issuer and Administrative Lender are
parties to that certain Loan Agreement, dated as of September 29, 1993, as
amended by that certain First Amendment to Loan Agreement, dated as of October
31, 1994, and that certain Second Amendment to Loan Agreement, dated as of
December 15, 1995 (said Loan Agreement, as amended, the "Loan Agreement"; the
terms defined in the Loan Agreement and not otherwise defined herein shall be
used herein as defined in the Loan Agreement).

         B.      Company, Lenders, Issuer and Administrative Lender desire to
amend the Loan Agreement to (i) increase the Commitment to $80,000,000, (ii)
extend the Termination Date, and (iii) make certain other amendments thereto.

         NOW, THEREFORE, in consideration of the covenants, conditions and
agreements hereafter set forth, and for other good and valuable consideration,
the receipt and adequacy of which are all hereby acknowledged, Company,
Lenders, Issuer and Administrative Lender covenant and agree as follows:

         1.      AMENDMENTS TO LOAN AGREEMENT.

                 (a)      The dollar amount of "$70,000,000" set forth in the
         Background paragraph of the Credit Agreement is hereby amended to be
         "$80,000,000".

                 (b)      The definition of "Applicable Margin" set forth in
         Article I of the Loan Agreement is hereby amended to read as follows:

                 "Applicable Margin" means the following per annum percentages,
         applicable in the following situations:
<PAGE>   3
<TABLE>
<CAPTION>
                                                               Prime Rate     LIBOR              
                             Applicability                       Basis        Basis              
                             -------------                     ----------     -----
     <S>     <C>                                                 <C>          <C>                
     (i)     If the Fixed Charge Coverage Ratio is less          0.000        1.000              
             than 1.25 to 1                                                                     
                                                                                                 
     (ii)    If the Fixed Charge Coverage Ratio is greater       0.000        0.625              
             than or equal to 1.25 to 1 but less than 1.50 to                                    
             1                                                                                   
                                                                                                 
     (iii)   If the Fixed Charge Coverage Ratio is greater       0.000        0.500              
             than or equal to 1.50 to 1                                      

</TABLE>
                 In addition, the per annum percentages set forth above (A) in
         all cases for the LIBOR Basis shall be increased by 0.25% if the
         Capitalization Ratio is not less than 40% and (B) in the case of
         clause (i) above only for the Prime Rate Basis shall be increased by
         0.25% if the Capitalization Ratio is not less than 40%. The Applicable
         Margin payable by Company on the Advances outstanding hereunder shall
         be subject to reduction or increase, as applicable and as set forth in
         the table above, on a quarterly basis according to the performance of
         Company as tested by the Fixed Charge Coverage Ratio and, where
         appropriate, the Capitalization Ratio. Any such increase or reduction
         in the Applicable Margin provided for herein shall be effective on the
         first calendar day of the month next succeeding the date of receipt by
         Administrative Lender of the applicable financial statements. If
         financial statements of Company setting forth the Fixed Charge
         Coverage Ratio and the Capitalization Ratio are not received by
         Administrative Lender by the date required pursuant to Section 5.5
         hereof, the Applicable Margin shall be determined as if the Fixed
         Charge Coverage Ratio is less than 1.25 to 1 and the Capitalization
         Ratio is not less than 40% until such time as such financial
         statements are received. For the final quarter of any fiscal year of
         Company, Company may provide its unaudited financial statements,
         subject only to year-end adjustments, for the purpose of adjusting the
         Applicable Margin."

                 (c)      The definition of "Commitment" set forth in Article 1
         of the Loan Agreement is hereby amended to read as follows:

                 "Commitment" means as to any Lender, the amount set forth
         opposite such Lender's name under the column titled "Commitment" on
         Schedule 7 hereto, as the same may be reduced or terminated pursuant
         to Article 2, which at no time shall exceed such Lender's Specified
         Percentage of $80,000,000."

                 (d)      The definition of "Termination Date" set forth in
         Article 1 of the Loan Agreement is hereby amended to read as follows:





                                      -2-
<PAGE>   4
                 "Termination Date" means October 31, 1999, or such earlier
         date that the Commitment is terminated or such later date that the
         Commitment is extended pursuant to Section 2.19 hereof."

                 (e)      Section 5.13 of the Loan Agreement is hereby amended
         to read as follows:

                 "5.13    Capitalization Ratio. Company and its Subsidiaries
         will maintain a Capitalization Ratio at the end of each fiscal quarter
         of not greater than 45%."

                 (f)      Schedule 1 to the Loan Agreement is hereby amended
         and supplemented as set forth on Schedule 1 to this Third Amendment.

                 (g)      Schedule 4 to the Loan Agreement is hereby amended
         and supplemented as set forth on Schedule 4 to this Third Amendment.

                 (h)      Schedule 5 to the Loan Agreement is hereby amended
         and supplemented as set forth on Schedule 5 to this Third Amendment.

                 (i)      Schedule 7 to the Loan Agreement is hereby amended to
         be in the form of Schedule 7 to this Third Amendment.

         2.      REPRESENTATIONS AND WARRANTIES TRUE: NO EVENT OF DEFAULT. By
its execution and delivery hereof, Company represents and warrants that, as of
the date hereof and after giving effect to the amendments contemplated by the
foregoing Section 1:

                 (a)      the representations and warranties contained in the
         Loan Agreement are true and correct on and as of the date hereof as if
         made on and as of such date;

                 (b)      no event has occurred and is continuing which
         constitutes a Default or an Event of Default;

                 (c)      Company has full power and authority to execute and
         deliver this Third Amendment, the $40,000,000 Note payable to the
         order of NationsBank in the form of Exhibit A hereto (the "NationsBank
         Note"), the $25,000,000 Note payable to the order of Bank of America
         in the form of Exhibit B hereto (the "Bank of America Note"), and the
         $15,000,000 Note payable to the order of Comerica in the form of
         Exhibit C hereto (the "Comerica Note") (the NationsBank Note, the Bank
         of America Note and the Comerica Note are collectively referred to
         herein as the "Notes"), and this Third Amendment, the Loan Agreement,
         as amended hereby, and the Notes constitute the legal, valid and
         binding obligations of Company, enforceable in accordance with their
         respective terms, except as enforceability may be limited by
         applicable debtor relief laws and by general principles of equity
         (regardless of whether enforcement is sought in a





                                      -3-
<PAGE>   5
         proceeding in equity or at law) and except as rights to indemnity may
         be limited by federal or state securities law; and

                 (d)      no authorization, approval, consent, or other action
         by, notice to, or filing with, any governmental authority or other
         Person (other than the Board of Directors of Company), is required for
         the execution, delivery or performance by Company of this Third
         Amendment or the Notes or the acknowledgement of this Third Amendment
         by each Subsidiary which executed the Guaranty Agreement (a
         "Guarantor").

         3.      CONDITIONS OF EFFECTIVENESS. This Third Amendment shall be
effective as of October 31, 1996, subject to the following:

                 (a)      Administrative Lender shall have received
         counterparts of this Third Amendment executed by each Lender and
         Issuer;

                 (b)      Administrative Lender shall have received
         counterparts of this Third Amendment executed by Company and
         acknowledged by each Guarantor;

                 (c)      Each Lender shall have received its respective Note
         executed by Company;

                 (d)      Administrative Lender shall have received certified
         copies of resolutions of Company authorizing execution, delivery and
         performance of this Third Amendment and the Notes; and

                 (e)      Administrative Lender shall have received, in form
         and substance satisfactory to Administrative Lender and its counsel,
         such other documents, certificates and instruments as Administrative
         Lender shall require.

         4.      GUARANTORS ACKNOWLEDGEMENT. By signing below, each of the
Guarantors (i) acknowledges and consents to the execution, delivery and
performance by Company of this Third Amendment, (ii) agrees that its
obligations in respect of the Guaranty Agreement (A) are not released,
modified, impaired or affected in any manner by this Third Amendment or any of
the provisions contemplated herein, and (B) cover, among other things, the
Commitment as increased by this Third Amendment, and (ii) acknowledges that it
has no claims or offsets against, or defenses or counterclaims to, the Guaranty
Agreement.

         5.      REFERENCE TO THE LOAN AGREEMENT.

                 (a)      Upon the effectiveness of this Third Amendment, each
         reference in the Loan Agreement to "this Agreement", "hereunder", or
         words of like import shall mean and be a reference to the Loan
         Agreement, as affected and amended by this Third Amendment.





                                      -4-
<PAGE>   6
                 (b)      The Loan Agreement, as amended by this Third
         Amendment, and all other Loan Papers shall remain in full force and
         effect and are hereby ratified and confined.

         6.      COSTS, EXPENSES AND TAXES. Company agrees to pay on demand
all costs and expenses of Administrative Lender in connection with the
preparation, reproduction, execution and delivery of this Third Amendment, the
Notes, and the other instruments and documents to be delivered hereunder
(including the reasonable fees and out-of-pocket expenses of counsel for
Administrative Lender with respect thereto and with respect to advising
Administrative Lender as to its rights and responsibilities under the Loan
Agreement, as amended by this Third Amendment).

         7.      EXECUTION IN COUNTERPARTS. This Third Amendment may be
executed in any number of counterparts and by different parties hereto in
separate counterparts, each which when so executed and delivered shall be
deemed to be an original and all of which taken together shall constitute but
one and the same instrument.

         8.      GOVERNING LAW: BINDING EFFECT. This Third Amendment shall be
governed by and construed in accordance with the laws of the State of Texas and
shall be binding upon Company, each Lender, Issuer and Administrative Lender
and their respective successors and assigns.

         9.      HEADINGS. Section headings in this Third Amendment are
included herein for convenience of reference only and shall not constitute a
part of this Third Amendment for any other purpose.

         10.     ENTIRE AGREEMENT. THE LOAN AGREEMENT, AS AMENDED BY THIS THIRD
AMENDMENT, AND THE OTHER LOAN PAPERS REPRESENT THE FINAL AGREEMENT BETWEEN THE
PARTIES AS TO THE SUBJECT MATTER THEREIN AND HEREIN AND MAY NOT BE CONTRADICTED
BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS BETWEEN
THE PARTIES.

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                                      -5-
<PAGE>   7

         IN WITNESS WHEREOF, the parties hereto have executed this Third
Amendment as of the date first above written.

                                ELCOR CORPORATION

                                By:    /s/ RICHARD J. ROSEBERY 
                                       ----------------------------------------
                                       Richard J. Rosebery, Executive Vice
                                       President, Treasurer, Chief
                                       Administrative and Financial Officer


                                NATIONSBANK OF TEXAS, N.A. 
                                as Administrative Lender, Lender and Issuer

                                By:    /s/ BIANCA HEMMEN
                                       ----------------------------------------
                                       Name: Bianca Hemmen
                                            -----------------------------------
                                       Title: Senior Vice President
                                             ----------------------------------


                                BANK OF AMERICA - TEXAS, N.A.

                                By:    /s/ DONALD P. HELLMAN
                                       ----------------------------------------
                                       Name:  Donald P. Hellman
                                            -----------------------------------
                                       Title: Vice President
                                             ----------------------------------


                                COMERICA BANK - TEXAS

                                By:    /s/ GARY L. EMERY
                                       ----------------------------------------
                                       Name: Gary L. Emery
                                            -----------------------------------
                                       Title: Vice President
                                             ----------------------------------




                                      -6-
<PAGE>   8
ELK CORPORATION OF DALLAS
ELK CORPORATION OF TEXAS
ELK CORPORATION OF AMERICA
ELK CORPORATION OF ARKANSAS
ELK CORPORATION OF ALABAMA
CHROMIUM CORPORATION
OEL, LTD.


By: /s/ RICHARD J. ROSEBERY           
    -------------------------------------
    Richard J. Rosebery
    Vice President for all


GA INDUSTRIES CORPORATION
M MACHINERY COMPANY,
      INCORPORATED
      (formerly known as Mosley
      Machinery Company, Incorporated)
M SERVICE CORPORATION
      (formerly known as Mosley
      Service Corporation)



By: /s/ RICHARD J. ROSEBERY               
    -------------------------------------
    Richard J. Rosebery
    President for all


ELCOR SERVICE CORPORATION


By: /s/ RICHARD J. ROSEBERY                 
    -------------------------------------
    Richard J. Rosebery
    Executive Vice President





                                      -7-
<PAGE>   9

                                   EXHIBIT A

                                PROMISSORY NOTE

$40,000,000.00                                              Dated:        ,1996
                                                                  --------


         FOR VALUE RECEIVED, the undersigned, ELCOR CORPORATION, a Delaware
corporation ("Company"), hereby promises to pay to the order of NATIONSBANK OF
TEXAS, N.A. ("Lender") the principal amount of each Advance made by Lender to 
Company pursuant to the Loan Agreement (as hereinafter defined). All Advances 
remaining unpaid shall be repaid in full on the Termination Date. Company 
promises to pay interest on the unpaid principal balance of each Advance from 
the date of such Advance until said principal amount is paid in full, at the 
times and at the rate or rates as specified in the Loan Agreement.

         Both principal and interest are payable in lawful money of the United
States of America to NationsBank of Texas, N.A., as Administrative Lender, at
901 Main Street, Dallas, Texas 75202, in immediately available funds. Each
Advance made by Lender to Company pursuant to the Loan Agreement and all
payments made on account of principal hereof shall be recorded by Lender and,
prior to any transfer hereof, endorsed on the grid attached hereto which is
part of this Note; provided, however, failure to make any such recordation or
endorsement shall not affect the obligations of Company hereunder or under the
Loan Agreement.

         This Note is one of the Notes referred to in, and is entitled to the
benefits of and obligations pertaining to, the Loan Agreement dated as of
September 29, 1993 (said Loan Agreement, as amended, modified or supplemented
from time to time, the "Loan Agreement") among Company, Lender, certain other
Lenders, and NationsBank of Texas, N.A., as Administrative Lender, and this
Note is a substitution for (but is not an extinguishment or novation of any
indebtedness in respect of) that certain Note of Company payable to the order
of Lender dated December 15, 1995, in the principal amount of $40,000,000.00.

         The Loan Agreement, among other things, (i) provides for the making of
Advances by Lender to Company from time to time in an aggregate amount not to
exceed at any time outstanding the U.S. dollar amount first above mentioned,
the indebtedness of Company resulting from each such Advance being evidenced by
this Note, and (ii) contains provisions for acceleration of the maturity hereof
upon the happening of certain stated events and also for prepayments on account
of principal hereof prior to the maturity hereof upon the terms and conditions
therein specified. All terms not expressly defined herein shall have the same
definitions as set forth in the Loan Agreement.

                                ELCOR CORPORATION

                                By:
                                      ---------------------------------------
                                      Richard J. Rosebery
                                      Executive Vice President, Treasurer,
                                      Chief Administrative and Financial Officer
<PAGE>   10

                                   EXHIBIT B

                                PROMISSORY NOTE

$25,000,000.00                                                Dated:      ,1996
                                                                    ------

         FOR VALUE RECEIVED, the undersigned, ELCOR CORPORATION, a Delaware
corporation ("Company"), hereby promises to pay to the order of BANK OF AMERICA
TEXAS, N.A. ("Lender") the principal amount of each Advance made by Lender to
Company pursuant to the Loan Agreement (as hereinafter defined). All Advances
remaining unpaid shall be repaid in full on the Termination Date. Company
promises to pay interest on the unpaid principal balance of each Advance from
the date of such Advance until said principal amount is paid in full, at the
times and at the rate or rates as specified in the Loan Agreement.

         Both principal and interest are payable in lawful money of the United
States of America to NationsBank of Texas, N.A., as Administrative Lender, at
901 Main Street, Dallas, Texas 75202, in immediately available funds. Each
Advance made by Lender to Company pursuant to the Loan Agreement and all
payments made on account of principal hereof shall be recorded by Lender and,
prior to any transfer hereof, endorsed on the grid attached hereto which is
part of this Note; provided, however, failure to make any such recordation or
endorsement shall not affect the obligations of Company hereunder or under the
Loan Agreement.

         This Note is one of the Notes referred to in, and is entitled to the
benefits of and obligations pertaining to, the Loan Agreement dated as of
September 29, 1993 (said Loan Agreement, as amended, modified or supplemented
from time to time, the "Loan Agreement") among Company, Lender, certain other
Lenders, and NationsBank of Texas, N.A., as Administrative Lender, and this
Note is a substitution for (but is not an extinguishment or novation of any
indebtedness in respect of) that certain Note of Company payable to the order
of Lender dated December 15, 1995 in the principal amount of $20,000,000.00.

         The Loan Agreement, among other things, (i) provides for the making of
Advances by Lender to Company from time to time in an aggregate amount not to
exceed at any time outstanding the U.S. dollar amount first above mentioned,
the indebtedness of Company resulting from each such Advance being evidenced by
this Note, and (ii) contains provisions for acceleration of the maturity hereof
upon the happening of certain stated events and also for prepayments on account
of principal hereof prior to the maturity hereof upon the terms and conditions
therein specified. All terms not expressly defined herein shall have the same
definitions as set forth in the Loan Agreement.

                                ELCOR CORPORATION

                                By:
                                     ------------------------------------------
                                     Richard J. Rosebery
                                     Executive Vice President, Treasurer,
                                     Chief Administrative and Financial Officer
<PAGE>   11

                                   EXHIBIT C

                                PROMISSORY NOTE

$15,000,000.00                                                Dated:      ,1996 
                                                                    ------

         FOR VALUE RECEIVED, the undersigned, ELCOR CORPORATION, a Delaware
corporation ("Company"), hereby promises to pay to the order of COMERICA BANK -
TEXAS ("Lender") the principal amount of each Advance made by Lender to Company
pursuant to the Loan Agreement (as hereinafter defined). All Advances remaining
unpaid shall be repaid in full on the Termination Date. Company promises to pay
interest on the unpaid principal balance of each Advance from the date of such
Advance until said principal amount is paid in full, at the times and at the
rate or rates as specified in the Loan Agreement.

         Both principal and interest are payable in lawful money of the United
States of America to NationsBank of Texas, N.A., as Administrative Lender, at
901 Main Street, Dallas, Texas 75202, in immediately available funds. Each
Advance made by Lender to Company pursuant to the Loan Agreement and all
payments made on account of principal hereof shall be recorded by Lender and,
prior to any transfer hereof, endorsed on the grid attached hereto which is
part of this Note; provided, however, failure to make any such recordation or
endorsement shall not affect the obligations of Company hereunder or under the
Loan Agreement.

         This Note is one of the Notes referred to in, and is entitled to the
benefits of and obligations pertaining to, the Loan Agreement dated as of
September 29, 1993 (said Loan Agreement, as amended, modified or supplemented
from time to time, the "Loan Agreement") among Company, Lender, certain other
Lenders, and NationsBank of Texas, N.A., as Administrative Lender, and this
Note is a substitution for (but is not an extinguishment or novation of any
indebtedness in respect of) that certain Note of Company payable to the order
of Lender dated December 15, 1995 in the principal amount of $10,000,000.00.

         The Loan Agreement, among other things, (i) provides for the making of
Advances by Lender to Company from time to time in an aggregate amount not to
exceed at any time outstanding the U.S. dollar amount first above mentioned,
the indebtedness of Company resulting from each such Advance being evidenced by
this Note, and (ii) contains provisions for acceleration of the maturity hereof
upon the happening of certain stated events and also for prepayments on account
of principal hereof prior to the maturity hereof upon the terms and conditions
therein specified. All terms not expressly defined herein shall have the same
definitions as set forth in the Loan Agreement.

                                ELCOR CORPORATION

                                By:
                                     ------------------------------------------
                                     Richard J. Rosebery
                                     Executive Vice President, Treasurer,
                                     Chief Administrative and Financial Officer
<PAGE>   12

                                   SCHEDULE 1

                              EXISTING LITIGATION

              This schedule is confidential and has been omitted.
<PAGE>   13
                                   SCHEDULE 4

                           FIXED ASSETS HELD FOR SALE

              This schedule is confidential and has been omitted.
<PAGE>   14
                                   SCHEDULE 5

                             ENVIRONMENTAL MATTERS

              This schedule is confidential and has been omitted.
<PAGE>   15
                                   SCHEDULE 7

<TABLE>
<CAPTION>
       LENDER                      COMMITMENT               SPECIFIED PERCENTAGE
       ------                      ----------               --------------------
<S>                               <C>                               <C>
NationsBank of Texas, N.A.        $40,000,000                       50.00%

Bank of America - Texas, N.A.     $25,000,000                       31.25%

Comerica Bank - Texas             $15,000,000                       18.75%
</TABLE>