	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Lumber & lumber sheet goods	$	601,487	$	811,159	$	1,119,176	$	1,455,068
Manufactured products		374,299		370,891		691,651		665,069
Windows, doors & millwork		391,049		375,515		744,439		707,667
Gypsum, roofing & insulation		138,388		141,273		259,307		254,149
Siding, metal & concrete products		191,289		189,536		341,207		331,699
Other building products & services		208,011		201,514		380,043		376,672
Net sales	$	1,904,523	$	2,089,888	$	3,535,823	$	3,790,324

Information regarding disaggregation of sales by segment is discussed in Note 10 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 June 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 $49.6 million and $42.1 million as of June 30, 2019 and December 31, 2018, respectively.