Acquisitions (Details) $ in Thousands						3 Months Ended										12 Months Ended
	Aug. 19, 2016 USD ($)	Mar. 17, 2016 USD ($)	Dec. 03, 2015 USD ($) item	May 16, 2014 USD ($)	Dec. 12, 2013 USD ($)	Sep. 30, 2016 USD ($) item	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Sep. 30, 2016 USD ($) item	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Nov. 13, 2015
Acquisitions
Consideration paid net of cash acquired																$ 342,362	$ 5,650	$ 95,604
Number of product categories in niche roofing | item						3										3
Estimated fair values of assets acquired and liabilities assumed
Current liabilities						$ (12,400)				$ (400)						$ (12,400)	(400)
Goodwill						290,503				178,199				$ 175,586		290,503	178,199	175,586
Intangible assets acquired:
Gain on acquired assets held for sale																4,450
Income tax expense (benefit)										(96,800)						22,756	(94,458)	3,574
Revenue						291,591	$ 262,466	$ 202,332	$ 218,418	272,717	$ 243,294	$ 179,725	$ 199,597			974,807	895,333	791,447
Net income (loss)						16,873	17,785	2,384	12,721	126,774	$ 23,008	$ (25,198)	7,084			49,763	131,668	16,058
Non-controlling interest in consolidated subsidiary
Balance									12,431							12,431
Net income attributable to non-controlling interest																1,667	869	774
Adjustment of estimated redemption value																		176
Balance						13,363				12,431						13,363	12,431
Acquisitions 2014 and 2015 and 2016
Pro forma consolidated revenue and net income
Pro forma revenue																1,096,658	1,061,072	835,864
Pro forma net income																65,049	145,291	20,409
Acquisitions 2014 and 2015 and 2016 | Acquisition-related Costs
Pro forma consolidated revenue and net income
Fees for advisory, legal and other professional services						900								700		900		700
Acquisitions 2014 and 2015 and 2016 | Fair Value Adjustment to Inventory
Pro forma consolidated revenue and net income
Nonrecurring expense related to the fair value adjustment to acquisition-date inventory						1,100								1,200		1,100		1,200
Acquisitions 2014
Intangible assets acquired:
Revenue																		47,600
Net income (loss)																		5,200
Roof Tile Acquisition LLC
Acquisitions
Percentage of equity interests acquired					80.00%
Entegra
Acquisitions
Total cash consideration of acquisition					$ 57,500
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:
Period over which goodwill is expected to be deductible for tax purpose					15 years
Minimum period following the acquisition date after which remaining noncontrolling equity interest may be acquired					5 years
Pro forma consolidated revenue and net income
Fees for advisory, legal and other professional services														400				400
Noncontrolling equity interest (as a percent)					20.00%
Non-controlling interest in consolidated subsidiary
Balance									$ 12,431				13,252			12,431	13,252
Estimated fair value as of acquisition date					$ 13,252									13,252				13,252
Net income attributable to non-controlling interest																1,667	869	774
Dividends paid to non-controlling interest																(735)	(1,690)	(950)
Adjustment of estimated redemption value																		176
Balance						$ 13,363				12,431				13,252		13,363	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 of the identified intangible assets					15 years
Gerard
Acquisitions
Total cash consideration of acquisition				$ 27,600
Estimated fair values of assets acquired and liabilities assumed
Current assets				9,236
Current liabilities				(1,691)
Property, plant and equipment				8,314
Goodwill				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
Pro forma consolidated revenue and net income
Fees for advisory, legal and other professional services														300				$ 300
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 of the identified intangible assets				15 years
Acquisitions 2015
Intangible assets acquired:
Revenue																	3,200
Net income (loss)																	$ (1,200)
Acquisitions 2016
Intangible assets acquired:
Revenue																49,300
Net income (loss)																4,700
Roofing Businesses
Acquisitions
Number of acquisitions aggregated | item			2
Consideration paid net of cash acquired			$ 57,000
Estimated fair values of assets acquired and liabilities assumed
Current assets			7,402
Current liabilities			(1,455)
Property, plant and equipment			3,138
Goodwill			27,218
Long-term liabilities			(3,015)
Net assets acquired			$ 56,995
Intangible assets acquired:
Period over which goodwill is expected to be deductible for tax purpose			15 years
Roofing Businesses | Trade names
Intangible assets acquired:
Indefinite lived intangible assets			$ 5,565
Roofing Businesses | Customer relationships
Intangible assets acquired:
Finite lived intangible assets			$ 8,040
Estimated useful lives of the identified intangible assets			12 years
Roofing Businesses | Intellectual property
Intangible assets acquired:
Finite lived intangible assets			$ 10,102
Estimated useful lives of the identified intangible assets			15 years
Metro Roof Products
Acquisitions
Percentage of equity interests acquired																			100.00%
SynMat
Acquisitions
Percentage of equity interests acquired		100.00%
Consideration paid net of cash acquired		$ 33,200
Future estimated consideration payments accrued		12,000
Estimated fair values of assets acquired and liabilities assumed
Current assets		3,965
Current liabilities		(1,491)
Property, plant and equipment		3,710
Goodwill		4,414
Net assets acquired		$ 33,168
Intangible assets acquired:
Period over which goodwill is expected to be deductible for tax purpose		15 years
SynMat | Trade names
Intangible assets acquired:
Indefinite lived intangible assets		$ 4,790
SynMat | Contracts
Intangible assets acquired:
Finite lived intangible assets		$ 15,220
Estimated useful lives of the identified intangible assets		20 years
SynMat | Customer relationships
Intangible assets acquired:
Finite lived intangible assets		$ 2,560
Estimated useful lives of the identified intangible assets		15 years
Krestmark Industries
Acquisitions
Total cash consideration of acquisition	$ 240,000
Estimated fair values of assets acquired and liabilities assumed
Current assets	20,731
Current liabilities	(3,884)
Property, plant and equipment	5,597
Goodwill	66,539
Net assets acquired	$ 235,433
Intangible assets acquired:
Period over which goodwill is expected to be deductible for tax purpose	15 years
Prepaid compensation for employee retention obligations	$ 4,600
Amortization period for prepaid compensation	2 years
Krestmark Industries | Trade names
Intangible assets acquired:
Indefinite lived intangible assets	$ 36,600
Krestmark Industries | Customer relationships
Intangible assets acquired:
Finite lived intangible assets	$ 107,500
Estimated useful lives of the identified intangible assets	20 years
Krestmark Industries | Noncompete agreements
Intangible assets acquired:
Finite lived intangible assets	$ 2,350
Estimated useful lives of the identified intangible assets	7 years
Other Business Acquisitions
Acquisitions
Total cash consideration of acquisition							$ 10,400	$ 6,300		$ 4,500				$ 7,400	$ 3,100
Intangible assets acquired:
Income tax expense (benefit)																2,000
Other Business Acquisitions | Other Income
Intangible assets acquired:
Gain on acquired assets held for sale																$ 4,500
Other Business Acquisitions | Equity Method Investee
Acquisitions
Total cash consideration of acquisition													$ 1,200
Percentage of equity interests acquired													100.00%