Acquisitions (Details) $ in Thousands				1 Months Ended	3 Months Ended								12 Months Ended
	May. 16, 2014 USD ($)	Dec. 12, 2013 USD ($)	Dec. 31, 2012 USD ($)	Nov. 30, 2015 USD ($)	Sep. 30, 2015 USD ($) item	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Acquisitions
Consideration paid net of cash acquired													$ 5,650	$ 95,604	$ 43,250
Number of product categories in niche roofing | item					3								3
Estimated fair values of assets acquired and liabilities assumed
Goodwill					$ 178,199				$ 175,586				$ 178,199	175,586	137,198
Intangible assets acquired:
Revenue					272,717	$ 243,294	$ 179,725	$ 199,597	245,921	$ 223,399	$ 156,512	$ 165,615	895,333	791,447	702,576
Net income					126,774	$ 23,008	$ (25,198)	7,084	16,681	$ 10,893	(10,082)	$ (1,434)	131,668	16,058	7,137
Non-controlling interest in consolidated subsidiary
Balance								13,252					13,252
Net income attributable to non-controlling interest													869	774	0
Adjustment of estimated redemption value													0	176	0
Balance					12,431				13,252				12,431	13,252
Construction materials
Estimated fair values of assets acquired and liabilities assumed
Goodwill					118,435				118,899				118,435	118,899	115,999
Intangible assets acquired:
Revenue													352,263	309,337	293,000
Building products
Estimated fair values of assets acquired and liabilities assumed
Goodwill					59,764				56,687				59,764	56,687	21,199
Intangible assets acquired:
Revenue													523,643	472,434	394,324
Acquisitions 2013, 2014 and 2015
Pro forma consolidated revenue and net income (loss)
Revenue													902,537	835,864	784,395
Net income													132,084	20,409	12,387
Acquisitions 2013, 2014 and 2015 | Acquisition-related Costs
Acquisitions
Fees for advisory, legal and other professional services									700					700	900
Pro forma consolidated revenue and net income (loss)
Other costs															300
Acquisitions 2013, 2014 and 2015 | Fair Value Adjustment to Inventory
Pro forma consolidated revenue and net income (loss)
Nonrecurring expense related to the fair value adjustment to acquisition-date inventory									1,200					1,200	500
Acquisitions 2013
Intangible assets acquired:
Revenue															28,600
Net income															1,600
Kleer Lumber, Inc.
Acquisitions
Total cash consideration of acquisition			$ 43,300
Fees for advisory, legal and other professional services															$ 900
Estimated fair values of assets acquired and liabilities assumed
Current assets			5,818
Current liabilities			(3,093)
Property, plant and equipment			4,098
Goodwill			20,527
Net assets acquired			$ 43,250
Intangible assets acquired:
Period over which goodwill is expected to be deductible for tax purpose			15 years
Kleer Lumber, Inc. | Trade Names
Intangible assets acquired:
Indefinite lived intangible assets			$ 4,800
Kleer Lumber, Inc. | Customer Relationships
Intangible assets acquired:
Finite lived intangible assets			$ 11,100
Estimated useful lives			15 years
Acquisitions 2014
Intangible assets acquired:
Revenue														47,600
Net income														5,200
Roof Tile Acquisition LLC
Intangible assets acquired:
Percentage of equity interests acquired		80.00%
Entegra
Acquisitions
Total cash consideration of acquisition		$ 57,500
Fees for advisory, legal and other professional services									400					400
Estimated fair values of assets acquired and liabilities assumed
Current assets		8,261
Current liabilities		(3,422)
Property, plant and equipment		10,589
Goodwill		28,156
Net assets acquired		70,784
Less redeemable non-controlling interest		(13,252)							(13,252)					(13,252)
Net assets attributable to Headwaters		$ 57,532
Intangible assets acquired:
Noncontrolling equity interest (as a percent)		20.00%
Minimum period following the acquisition date after which remaining noncontrolling equity interest may be acquired		5 years
Period over which goodwill is expected to be deductible for tax purpose		15 years
Non-controlling interest in consolidated subsidiary
Balance								13,252					13,252
Estimated fair value as of acquisition date		$ 13,252							13,252					13,252
Net income attributable to non-controlling interest													869	774
Dividends paid to non-controlling interest													(1,690)	(950)
Adjustment of estimated redemption value														176
Balance					12,431				13,252				12,431	13,252
Entegra | Trade Names
Intangible assets acquired:
Indefinite lived intangible assets		6,600
Entegra | Customer Relationships
Intangible assets acquired:
Finite lived intangible assets		$ 20,600
Estimated useful lives		15 years
Gerard
Acquisitions
Total cash consideration of acquisition	$ 27,600
Fees for advisory, legal and other professional services									300					$ 300
Estimated fair values of assets acquired and liabilities assumed
Current assets	9,236
Current liabilities	(1,691)
Property, plant and equipment	8,314
Goodwill and intangible assets	7,719
Long-term liabilities	(3,906)
Net assets acquired	$ 27,572
Intangible assets acquired:
Period over which goodwill is expected to be deductible for tax purpose	15 years
Gerard | Trade Names
Intangible assets acquired:
Indefinite lived intangible assets	$ 3,900
Gerard | Customer Relationships
Intangible assets acquired:
Finite lived intangible assets	$ 4,000
Estimated useful lives	15 years
Acquisitions 2015
Intangible assets acquired:
Revenue													3,200
Net income													$ (1,200)
Other Business Acquisitions | Equity Method Investee
Acquisitions
Total cash consideration of acquisition								$ 1,200
Intangible assets acquired:
Percentage of equity interests acquired								100.00%
Other Business Acquisitions | Construction materials
Acquisitions
Total cash consideration of acquisition									$ 7,400		$ 3,100
Other Business Acquisitions | Building products
Acquisitions
Total cash consideration of acquisition					$ 4,500
Metro Roof Products | Building products
Acquisitions
Consideration paid net of cash acquired				$ 34,600
Intangible assets acquired:
Percentage of equity interests acquired				100.00%