Year Ended December 31, 2020 Compared to Year Ended December 31, 2019
Net Sales. Net sales were $480.2 million for the year ended December 31, 2020 compared to $571.7 million in 2019, a decrease of $91.5 million, or 16.0%. Net sales decreased for the year ended December 31, 2020 primarily due to lower volumes driven by below average snowfall, class 4-6 chassis availability constraints which we believe was impacted by the COVID-19 pandemic, and the effect of reduced shipments from our facilities being shut down as a result of the COVID-19 pandemic for several weeks throughout the first and second quarters. See below for a discussion of net sales for each of our segments.
Net sales at our Work Truck Attachment segment were $252.8 million for the year ended December 31, 2020 compared to $293.6 million in the year ended December 31, 2019, a decrease of $40.8 million primarily due to lower volumes due to below average snowfall for the snow season ended March 31, 2020. Snowfall in this most recent snow season was approximately 25% below the ten year average, and was the second below average snowfall season in a row.
Net sales at our Work Truck Solutions segment were $227.3 million for the year ended December 31, 2020 compared to $278.1 million in the year ended December 31, 2019, a decrease of $50.8 million due primarily to lower volumes as a result of class 4-6 chassis supply constraints, as well as result of the facilities shutdown associated with the COVID-19 pandemic leading to significantly reduced shipments in the first and second quarters of 2020.
Cost of Sales. Cost of sales was $351.9 million for the year ended December 31, 2020 compared to $402.9 million in 2019, a decrease of $51.0 million, or 12.7%. Cost of sales as a percentage of net sales increased from 70.5% for the year ended December 31, 2019 to 73.3% for the year ended December 31, 2020. The increase in cost of sales as a percentage of sales in the year ended December 31, 2020 when compared to the year ended December 31, 2019 was primarily due to lower sales volumes due to below average snowfall and reduced shipments related to facility shutdowns, as well as shutdown expenses related to COVID-19. Such shutdown expenses include the continuation of wages for employees who were not working during the shutdown, as well as an increase in fixed expenses and overhead, as these costs were not capitalized into inventory for the shutdown period, and increased inefficiencies due to absenteeism.
Gross Profit. Gross profit was $128.3 million for the year ended December 31, 2020 compared to $168.8 million in 2019, a decrease of $40.5 million, or 24.0%, due to the decrease in net sales described above under “—Net Sales.” As a percentage of net sales, gross profit decreased from 29.5% for the year ended December 31, 2019 to 26.7% for the corresponding period in 2020, as a result of the factors discussed above under “—Cost of Sales.”
Selling, General and Administrative Expense. Selling, general and administrative expenses, including intangible asset amortization, were $75.5 million for the year ended December 31, 2020 compared to $82.2 million for the year ended December 31, 2019, a decrease of $6.7 million, or 8.2%. The decrease compared to the year ended December 31, 2019 was in part due to $2.0 million of earnout valuation adjustments in the year ended December 31, 2020 compared to $0.4 million in the year ended December 31, 2019. The remainder of the decrease in the year ended December 31, 2020 is due to lower discretionary spending, including travel and advertising and promotions, as a result of facility shutdowns during the first and second quarters and decreased volumes as a result of the COVID-19 pandemic. As a percentage of net sales, selling, general and administrative expenses, including intangibles amortization, increased from 14.4% for the year ended December 31, 2019 to 15.8% for the corresponding period in 2020.
Impairment Charges. Impairment charges were $127.9 million for the year ended December 31, 2020. There were no impairment charges in the prior year. The impairment charges in 2020 relate to goodwill impairment taken on our Municipal and Dejana reporting units of $47.8 and $80.1 million, respectively. The increase in impairment charges was due to reduced performance in the current year and projected future years as a result of the COVID-19 pandemic and chassis and other supply chain constraints. See Note 2 for additional information.
Interest Expense. Interest expense was $20.2 million for the year ended December 31, 2020 compared to $16.8 million in the corresponding period in 2019. The increase in interest expense for the year ended December 31,