| Document And Entity Information | May 03, 2021 |
| Document Information [Line Items] | |
| Entity, Registrant Name | NORTHWEST PIPE COMPANY |
| Document, Type | 8-K |
| Document, Period End Date | May 3, 2021 |
| Entity, Incorporation, State or Country Code | OR |
| Entity, File Number | 0-27140 |
| Entity, Tax Identification Number | 93-0557988 |
| Entity, Address, Address Line One | 201 NE Park Plaza Drive, Suite 100 |
| Entity, Address, City or Town | Vancouver |
| Entity, Address, State or Province | WA |
| Entity, Address, Postal Zip Code | 98684 |
| City Area Code | 360 |
| Local Phone Number | 397-6250 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | NWPX |
| Security Exchange Name | NASDAQ |
| Entity, Emerging Growth Company | false |
| Amendment Flag | false |
| Entity, Central Index Key | 0001001385 |
