Northwest Pipe (NWPX) 10-Q 2008 Q3 Quarterly report

Exhibit 32.2

CERTIFICATION PURSUANT TO 18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

In connection with the Quarterly Report of Northwest Pipe Company (the “Company”) on Form 10-Q for the period ended September 30, 2008 as filed with the Securities and Exchange Commission on the date hereof (the “Report”), I, Stephanie J. Welty, Senior Vice President, Chief Financial Officer of the Company, certify, pursuant to 18 U.S.C. § 1350, as adopted pursuant to § 906 of the Sarbanes-Oxley Act of 2002, that:

(1) The Report fully complies with the requirements of section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

(2) The information contained in the Report fairly presents, in all material respects, the financial condition and result of operations of the Company.

/s/ Stephanie J. Welty

Stephanie J. Welty

Senior Vice President, Chief Financial Officer

November 7, 2008