	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2019		2018		2019		2018
Lumber & lumber sheet goods	$	605,494	$	818,693	$	1,724,670	$	2,273,761
Manufactured products		401,245		385,894		1,092,896		1,050,963
Windows, doors & millwork		407,397		372,453		1,151,836		1,080,120
Gypsum, roofing & insulation		149,676		146,648		408,983		400,797
Siding, metal & concrete products		201,068		196,591		542,275		528,290
Other building products & services		216,155		198,188		596,198		574,860
Net sales	$	1,981,035	$	2,118,467	$	5,516,858	$	5,908,791

Information regarding disaggregation of sales by segment is discussed in Note 13 to the condensed consolidated financial statements. Sales related to contracts with service elements represents less than 10% of the Company’s net sales for each period presented.

The timing of revenue recognition, billings and cash collections results in accounts receivable, unbilled receivables, contract assets and contract liabilities. Contract asset balances were not significant as of September 30, 2019 or December 31, 2018. Contract liabilities consist of deferred revenue and customer advances and deposits. Contract liability balances are included in accrued liabilities on our consolidated balance sheet and were $48.3 million and $42.1 million as of September 30, 2019 and December 31, 2018, respectively.