September 27, 2018. As a percentage of net sales, our selling and store operating expenses increased approximately 130 basis points to 26.3% from 25.0% in the corresponding prior year period primarily driven by new stores open less than one year. Comparable store selling and store operating expenses as a percentage of comparable store sales increased by approximately 20 basis points, driven by higher advertising and other operating expenses.
Selling and store operating expenses during the thirty-nine weeks ended September 26, 2019 increased $78.6 million, or 24.5%, compared to the corresponding prior year period, due primarily to opening 18 new stores since September 27, 2018. As a percentage of net sales, our selling and store operating expenses increased approximately 110 basis points to 26.3% from 25.2% in the corresponding prior year period driven by new stores open less than one year. Comparable store selling and store operating expenses as a percentage of comparable store sales decreased by approximately 20 basis points due primarily to leveraging personnel expenses on higher net sales.
General and Administrative Expenses
General and administrative expenses, which are typically expenses incurred outside of our stores, increased $10.8 million, or 40.7%, during the thirteen weeks ended September 26, 2019 compared to the corresponding prior year period due to our continued investments in personnel for our store support functions, increased depreciation due to technology investments to support store growth, and a $4.1 million impairment charge related to the operating lease right-of-use asset for our former Store Support Center in Smyrna, Georgia. Our general and administrative expenses as a percentage of net sales increased approximately 100 basis points to 7.1% from 6.1% in the corresponding prior year period, of which 80 basis points of the increase was due to the impairment charge.
General and administrative expenses during the thirty-nine weeks ended September 26, 2019 increased $23.4 million, or 31.2%, compared to the corresponding prior year period due to our continued investments in personnel for our store support functions, increased depreciation due to technology investments to support store growth, and a $4.1 million impairment charge related to the operating lease right-of-use asset for our former Store Support Center in Smyrna, Georgia. Our general and administrative expenses as a percentage of net sales increased approximately 60 basis points to 6.5% from 5.9% in the corresponding prior year period, of which 30 basis points of the increase was due to the impairment charge.
Pre-Opening Expenses
Pre-opening expenses during the thirteen weeks ended September 26, 2019 decreased $0.1 million, or 1.8%, compared to the corresponding prior year period. The decrease is primarily due to an enhanced store opening process which shortens the period it takes to open new stores, thereby lessening pre-occupancy costs, as well as opening fewer stores in higher cost metropolitan markets. We opened seven stores during each of the thirteen weeks ended September 26, 2019 and September 27, 2018. Of the seven store openings during the thirteen weeks ended September 26, 2019, three were in existing markets as compared to one store opening in existing markets during the corresponding prior year period.
Pre-opening expenses during the thirty-nine weeks ended September 26, 2019 increased $0.7 million, or 3.8%, compared to the corresponding prior year period. For the first nine months of fiscal 2019, this expense grew at a slower rate than sales due to an enhanced store opening process which allows us to shorten the period it takes to open new stores, thereby lessening pre-opening occupancy costs. We opened thirteen stores during the thirty-nine weeks ended September 26, 2019 compared to opening twelve stores during the corresponding prior year period.
Interest Expense
Interest expense during the thirteen weeks ended September 26, 2019 decreased $0.2 million, or 8.9%, compared to the corresponding prior year period. The decrease in interest expense was primarily due to lowering our average debt balance to $146.6 million during the thirteen weeks ended September 26, 2019 compared to an average debt balance of $160.6 million in the corresponding prior year period.
Interest expense during the thirty-nine weeks ended September 26, 2019 increased $1.0 million, or 16.8%, compared to the corresponding prior year period. The increase in interest expense was primarily due to the increase in