<PAGE>

                                                                   EXHIBIT 10.40

                              SECOND AMENDMENT TO
                         MANAGEMENT SERVICES AGREEMENT



     This SECOND AMENDMENT TO MANAGEMENT SERVICES AGREEMENT (this "Amendment"),
dated December 31, 1997, but effective as of January 1, 1998 (the "Effective
Date"), is by and between Cambridge Industries, Inc. (the "Company") and Bain
Capital, Inc. ("Bain").

     WHEREAS, Bain is acting as an advisor to the Company pursuant to the terms
of that certain Management Services Agreement dated as of November 17, 1995
between the Company and Bain, as amended by that certain Amendment to Management
Services Agreement dated March 1, 1996 (collectively, the "Agreement").

     WHEREAS, the Company and Bain wish to amend the Agreement.

     NOW THEREFORE, in consideration of the agreements herein contained, the
parties hereto agree as follows:

     1.  Paragraph 3 of the Agreement is hereby amended to read in its entirety
as follows, effective as of the Effective Date:

               3.  Advisory Fees and Transaction Fees. (a) Payment to Bain for
     services rendered in connection with the performance of services pursuant
     to this Agreement shall be $950,000 per year ("Advisory Fees") plus
     reasonable out-of-pocket expenses of Bain, payable by the Company on a
     quarterly basis in arrears.

               (b) During the term of this Agreement, Bain shall be entitled to
     receive from the Company a transaction fee in connection with (i) the
     consummation of each acquisition by the Company of an additional business,
     and (ii) the completion of a recapitalization of the Company, in either
     case in an amount equal to three quarters of one percent ( 3/4%) of the
     aggregate transaction value (each such payment, a "Transaction Fee");
     provided that, Bain may waive the payment of a Transaction Fee in
     connection with any acquisition as to which Bain and the Board mutually
     agree that a Transaction Fee is not appropriate. For purposes of this
     Agreement, in the event of a recapitalization completed in connection with
     the acquisition of an additional business, the transaction value shall be
     the greater of the value of the acquisition or the amount of the
     recapitalization.

     2.  Applicable Law.  All issues and questions concerning the construction,
validity, interpretation and enforceability of this Amendment shall be governed
by, and construed in accordance with, the law provisions (whether of the State
of Michigan or any other jurisdiction) that would cause the application of the
laws of any jurisdiction other than the State of Michigan.

     3.  Counterparts; Effectiveness.  This Amendment may be executed in any
number of counterparts and by different parties hereto in separate counterparts,
each of which when so executed and delivered shall be deemed an original, but
all such counterparts together shall constitute but one and
<PAGE>

the same instrument.  This Amendment shall become effective upon the execution
of a counterpart hereof by each of the parties hereto, and written or telephonic
notification of such execution and authorization of delivery thereof has been
received by each party hereto.

     IN WITNESS WHEREOF, the parties hereto have caused this Second Amendment to
Management Services Agreement to be executed by their respective officers
hereunto duly authorized  on December ___, 1997, but effective as of January 1,
1998.

                                    CAMBRIDGE INDUSTRIES, INC.


                                    By:  _________________________
                                         Name:
                                         Title:


                                    BAIN CAPITAL, INC.


                                    By:  __________________________

                                    Its:  _________________________

                                       2