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    EXHIBIT 10.22

FIFTH AMENDMENT TO LOAN AGREEMENT

    This amendment to Loan Agreement ("Amendment") is made as of May 11, 2000 by
and among the following parties:

    Bank of America, N.A., formerly known as Bank of America National Trust and
Savings Association ("Bank of America" and a "Lender")

    U.S. Bank National Association ("U.S. Bank" and a "Lender")

    Bank of America, N.A., formerly known as Bank of America National Trust and
Savings Association, in its capacity as Agent ("Agent")

    Each of the several financial institutions which subsequently becomes party
to the Loan Agreement pursuant to Section 11.7 (each individually a "Lender")

    Northwest Pipe Company, an Oregon corporation ("Borrower")

R E C I T A L S

    A.  The Borrower, the Lenders and the Agent are parties to that certain
Amended and Restated Loan Agreement dated as of June 30, 1998, as amended as of
December 23, 1998, June 16, 1999, November 30, 1999 and December 30, 1999, and
as the same may be further amended, modified or extended from time to time (the
"Loan Agreement") and the related Loan Documents described therein.

    B.  The parties desire to amend the Loan Agreement as set forth below:

    NOW, THEREFORE, the parties agree as follows:

A G R E E M E N T

    1.  Definitions.  Capitalized terms used herein and not otherwise defined
shall have the meaning given in the Loan Agreement.

    2.  Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended
by revising the following definition of "Applicable Margin":

"Applicable Margin" is amended by adding to the end of such definition the
following sentence: Notwithstanding the foregoing, the Applicable Margin for
Temporary Supplemental Revolving Loans shall always be 2.25%.

    Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended by
revising the following definition of "Loans":

"Loans" means the Revolving Loans, the Term Loans, and the Temporary
Supplemental Revolving Loans.

    3.  Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended
by revising the following definition of "Reference Related Rate":

"Reference Related Rate" is amended by adding to the end of such definition the
following sentence: Notwithstanding the foregoing, the Reference Related Rate
for all Temporary Supplemental Revolving Loans shall always be equal to the
Reference Rate.

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    4.  Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended
by adding the definition of "Temporary Supplemental Revolving Loan Commitment"
as follows:

"Temporary Supplemental Revolving Loan Commitment" means Ten Million Dollars
($10,000,000.00) until August 31, 2000, after which there shall be no Temporary
Supplemental Revolving Loan Commitment.

    5.  Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended
by adding the following definition of "Temporary Supplemental Revolving Loan
Maturity Date":

"Temporary Supplemental Revolving Loan Maturity Date" means August 31, 2000.

    6.  Amendment to Section 1.1.  Section 1.1 of the Loan Agreement is amended
by revising the following definition of "Total Commitment":

"Total Commitment" is amended by adding to the end of such definition the
following sentence: "Total Commitment" shall not include the Temporary
Supplemental Revolving Loan Commitment.

    Amendment to Section 6.6 "Restriction on Acquisitions."  Section 6.6 of the
Loan Agreement is amended to provide in full:

    "Section 6.6    Restriction on Acquisitions. Borrower shall not, and shall
not permit any Subsidiary to acquire any portion of any business directly or
indirectly, whether by stock acquisition, asset acquisition, the acquisition of
equity interests in any entity or by way of merger or reorganization."

    7.  Addition of Article 12.  Article 12, as set for the below, is hereby
added to the Loan Agreement:

ARTICLE 12
TEMPORARY SUPPLEMENTAL REVOLVING LOANS

    Section 12.1    Temporary Supplemental Revolving Loans. Subject to the terms
and conditions of this Agreement, each Lender hereby severally agrees, during
the period beginning on the date hereof and ending August 31, 2000, to make
temporary supplemental revolving loans duly requested hereunder (the "Temporary
Supplemental Revolving Loans") to Borrower in an amount equal to such Lender's
Revolving Loan Pro Rata share of each requested loan, provided that after giving
effect to any requested loan, absent such Lender's consent, the aggregate of all
Temporary Supplemental Revolving Loans outstanding from such Lender will not
exceed at any one time, its Revolving Loan Pro Rata share of the total Temporary
Supplemental Revolving Loan Commitment. The Temporary Supplemental Revolving
Loans described in this section constitute a revolving credit and, within the
amount and time specified, Borrower may pay, prepay and reborrow.

    Section 12.2    Applicability of Provisions. Except for Sections 2.1, 2.4,
2.5, 2.9 and 5.1, the provisions of the Loan Agreement shall apply to the
Temporary Supplemental Revolving Loans.

    Section 12.3    Repayment of Principal.

    (a) On each day that the principal balance of all outstanding Temporary
Supplemental Revolving Loans exceeds the Temporary Supplemental Revolving Loan
Commitment, Borrower shall repay the Temporary Supplemental Revolving Loans in
such amount as is necessary to reduce the principal balance of them to an amount
equal to or less than the Temporary Supplemental Revolving Loan Commitment. If
Borrower shall pay any Offshore Related Rate Loan pursuant to this section,
prior to the end of the Applicable Interest Period, Borrower shall include with
such payment any amount payable pursuant to Section 2.7 and applicable to the
payment of such Offshore Related Rate Loan prior to the termination of the
Applicable Interest Period.

    (b) Borrower shall repay the principal amount of the Temporary Supplemental
Revolving Loans on or before the Temporary Supplemental Revolving Loan Maturity
Date.

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    Section 12.4    Use of Temporary Supplemental Revolving Loan Proceeds. The
proceeds from the Temporary Supplemental Revolving Loans will be used to finance
accounts receivable and inventory of Borrower.

    8.  Fees.  Upon execution of this Amendment, Borrower agrees to pay Agent
for the benefit of Lenders, a fee of Thirty Thousand Dollars ($30,000.00) to be
divided in proportion to their Revolving Loan Pro Rata shares.

    9.  No Further Amendment; Fees.  Except as expressly modified by this
Amendment, the Loan Agreement and the other Loan Documents shall remain
unmodified and in full force and effect and the parties hereby ratify their
respective obligations thereunder. Without limiting the foregoing, the Borrower
expressly reaffirms and ratifies its obligation to pay or reimburse the Agent
and the Lender on request for all reasonable expenses, including legal fees,
actually incurred by the Agent or such Lender in connection with the preparation
of this Amendment, any other amendment documents, and the closing of the
transactions contemplated hereby and thereby.

    10.  Miscellaneous.  

    (a)  Entire Agreement.  This Amendment comprises the entire agreement of the
parties with respect to the subject matter hereof and supersedes all prior oral
or written agreements, representations or commitments.

    (b)  Counterparts.  This Amendment may be executed in any number of
counterparts and by different parties hereto in separate counterparts, each of
which when so executed shall be deemed to be an original, and all of which taken
together shall constitute one and the same Amendment.

    (c)  Governing Law.  This Amendment and the other agreements provided for
herein and the rights and obligations of the parties hereto and thereto shall be
construed and interpreted in accordance with the laws of the State of Oregon.

    (d)  Certain Agreements Not Enforceable.  

UNDER OREGON LAW, MOST AGREEMENTS, PROMISES AND COMMITMENTS MADE BY THE LENDERS
AFTER OCTOBER 3, 1989, CONCERNING LOANS AND OTHER CREDIT EXTENSIONS WHICH ARE
NOT FOR PERSONAL, FAMILY OR HOUSEHOLD PURPOSES OR SECURED SOLELY BY THE
BORROWER'S RESIDENCE MUST BE IN WRITING, EXPRESS CONSIDERATION, AND BE SIGNED BY
THE LENDERS TO BE ENFORCEABLE.

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    EXECUTED AND DELIVERED by the duly authorized officers of the parties as of
the date first above written.

BORROWER:   NORTHWEST PIPE COMPANY  
   
   
By: BRIAN DUNHAM     Its: PRESIDENT  
   
   
      Address:      12005 N. Burgard
                   Portland OR 97203
                   Fax No. (503) 240-6615  
LENDER:  
   
BANK OF AMERICA, N.A.  
   
   
By: ED KLUSS     Its: VICE PRESIDENT  
   
   
      Address:      Commercial Banking
                   121 SW Morrison Street, Suite 1700
                   Portland OR 97204
                   Fax No. (503) 275-1391
                   Attn: Larry C. Ellis  
   
   
U.S. BANK NATIONAL ASSOCIATION  
   
   
By: J. STEPHEN MITCHELL     Its: VICE PRESIDENT  
   
   
      Address:      Oregon Corporate Banking, T-4
                   111 SW Fifth Avenue, Suite 400
                   Portland OR 97208
                   Fax No. (503) 275-7290
                   Attn: Stephen Mitchell  
AGENT:  
   
BANK OF AMERICA, N.A.  
   
   
By: DORA A. BROWN     Its: VICE PRESIDENT  
   
   
      Address:      Agency Services
                   701 Fifth Avenue, Floor 16
                   Seattle WA 98104
                   Fax No. (206) 358-0971
                   Attn: Dora A. Brown

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QUICKLINKS

FIFTH AMENDMENT TO LOAN AGREEMENT
R E C I T A L S
A G R E E M E N T
ARTICLE 12 TEMPORARY SUPPLEMENTAL REVOLVING LOANS