| Cover | Oct. 07, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Oct. 7, 2020 |
| Entity File Number | 1-33913 |
| Entity Registrant Name | QUANEX BUILDING PRODUCTS CORPORATION |
| Entity Central Index Key | 0001423221 |
| Entity Tax Identification Number | 26-1561397 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1800 West Loop South |
| Entity Address, Address Line Two | Suite 1500 |
| Entity Address, City or Town | Houston |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 77027 |
| City Area Code | 713 |
| Local Phone Number | 961-4600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | NX |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
