7. Long-Lived Assets

Property, Plant and Equipment—Property, plant and equipment consisted of the following at September 30:

(in thousands of dollars)	Estimated useful lives	2014			2015
Land and improvements	15 - 40 years	$	15,298		$	15,904
Buildings and improvements	5 - 40 years		69,018			69,645
Equipment and vehicles	3 - 20 years		221,987			239,759
Dies and molds	3 - 20 years		83,299			85,880
Construction in progress	—		16,402			18,271

			406,004			429,459
Less accumulated depreciation			(224,706	)		(243,741	)

Net property, plant and equipment		$	181,298		$	185,718

Depreciation expense was approximately $32.1 million, $33.8 million and $35.8 million in 2013, 2014 and 2015, respectively.

Intangible Assets—Headwaters' identified intangible assets are being amortized over the estimated useful lives shown in the table below. The table also summarizes the gross carrying amounts and related accumulated amortization of intangible assets as of September 30:

		2014				2015
(in thousands of dollars)	Estimated useful lives	Gross Carrying Amount		Accumulated Amortization		Gross Carrying Amount		Accumulated Amortization
Trade names	Indefinite	$	15,300	$	—	$	15,300	$	—
CCP contracts	15 - 20 years		112,300		64,192		112,300		69,875
Customer relationships	5 - 17 years		107,953		50,355		109,078		57,844
Trade names	5 - 20 years		67,220		33,394		66,960		36,554
Patents and patented technologies	5 - 19 years		14,526		11,686		14,526		12,574
Other	5 - 17 years		3,935		1,744		4,585		2,184

		$	321,234	$	161,371	$	322,749	$	179,031

The above table includes provisional amounts for the intangible assets acquired in 2015 because the process of identifying and valuing those assets has not been completed. Total amortization expense related to intangible assets was approximately $20.2 million, $21.3 million and $18.2 million in 2013, 2014 and 2015, respectively. The primary reason for the decrease in amortization expense from 2014 to 2015 is that certain assets have been fully amortized. Total estimated annual amortization expense for 2016 through 2020 is shown in the following table:

Year ending September 30:	(in thousands)
2016	$	17,974
2017		17,096
2018		17,046
2019		16,018
2020		11,771

Goodwill—Changes in the carrying amount of goodwill, by segment, are as follows for the two-year period ended September 30, 2015:

(in thousands)	Building products		Construction materials			Total
Balances as of September 30, 2013	$	21,199	$	115,999		$	137,198
Goodwill related to 2014 acquisitions		35,488		2,900			38,388

Balances as of September 30, 2014		56,687		118,899			175,586
Goodwill related to 2015 acquisitions		2,687		0			2,687
Adjustments to previously recorded amounts for 2014 acquisitions		390		(464	)		(74	)

Balances as of September 30, 2015	$	59,764	$	118,435		$	178,199

Impairment Testing—In accordance with the requirements of ASC Topic 350 Intangibles—Goodwill and Other, Headwaters does not amortize goodwill or indefinite-lived intangible assets, all of which relate to acquisitions. However, Headwaters is required to periodically test these assets for impairment at least annually, or sooner if indicators of possible impairment arise. Headwaters performs its annual impairment testing during the fourth quarter of its fiscal year using a June 30 test date and a one- to three-step process. Headwaters' reporting units for purposes of impairment testing are the same as its operating segments.

Headwaters evaluates qualitative factors, including macroeconomic conditions, industry and market considerations, overall financial performance and cost factors, to determine whether it is necessary to perform step 1 of the two-step impairment test. This qualitative evaluation is commonly referred to as "step 0." After assessing the appropriate qualitative factors, only if it is determined that it is more likely than not that the fair value of a reporting unit is less than its carrying amount, is it necessary to perform step 1.

Step 1 of impairment testing consists of determining and comparing the fair value of a reporting unit, calculated primarily using discounted expected future cash flows, to the carrying value of the reporting unit. If step 1 is failed for a reporting unit, indicating a potential impairment, Headwaters is required to complete step 2, which is a more detailed test to calculate the implied fair value of goodwill and indefinite-lived intangible assets, and compare that value to the carrying value. If the carrying value of goodwill and indefinite-lived assets exceeds the implied fair value, an impairment loss is required to be recorded.

For all years presented, Headwaters performed a step 0 qualitative evaluation for both the construction materials and building products reporting units and concluded that it was more likely than not that the fair values exceeded the carrying amounts of goodwill and indefinite-lived assets. Accordingly, further step 1 and step 2 testing for impairment was not required to be performed.