Cover - USD ($)	12 Months Ended
	Oct. 31, 2020	Dec. 04, 2020	Apr. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Oct. 31, 2020
Current Fiscal Year End Date	--10-31
Document Transition Report	false
Entity File Number	1-33913
Entity Registrant Name	QUANEX BUILDING PRODUCTS CORPORATION
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-1561397
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
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	NX
Security Exchange Name	NYSE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 403,851,329
Entity Common Stock, Shares Outstanding		32,828,337
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Central Index Key	0001423221
Documents Incorporated by Reference	Portions of the Registrant’s definitive Proxy Statement for its 2021 Annual Meeting of Stockholders to be filed with the Commission within 120 days of October 31, 2020 are incorporated herein by reference in Part III of this Annual Report on Form 10-K.
ICFR Auditor Attestation Flag	true