Depreciation and amortization decreased by approximately $70,000 to $715,000 for the three months ended June 30, 2021 from $785,000 for the three months ended June 30, 2020. For the three months ended June 30, 2021, depreciation and amortization expense consisted of depreciation for 237 11th of approximately $405,000 and the amortization of lease commissions, acquired in-place leases and warrants of approximately $310,000. For the three months ended June 30, 2020, depreciation and amortization expense consisted of depreciation for 237 11th property of approximately $389,000 and the amortization of trademarks and lease commissions and acquired in-place leases of approximately $396,000. The decrease in depreciation and amortization expense for the three months ended June 30, 2021 compared to June 30, 2020 was primarily due to amortization of warrants.
Gain on sale of condominium of $24.2 million for the three months ended June 30, 2020 consists of the sale of the school condominium to the SCA of $20.0 million and an additional gain of $4.2 million related to the recognition of our construction supervision fee. This gain was recorded upon the closing on the sale of the school condominium to the SCA in April 2020.
Equity in net loss from unconsolidated joint ventures increased by approximately $129,000 to $264,000 for the three months ended June 30, 2021 from $135,000 for the three months ended June 30, 2020. Equity in net loss from unconsolidated joint ventures represents our 50% share in The Berkley and our 10% share in 250 North 10th. For the three months ended June 30, 2021, our share of the loss is primarily comprised of operating income before depreciation of $456,000 offset by depreciation and amortization of $362,000, interest expense of $187,000 and the change in the fair market value of the interest rate swap of $171,000. For the three months ended June 30, 2020, our share of the loss is primarily comprised of operating income before depreciation of $498,000 offset by depreciation and amortization of $451,000 and interest expense of $182,000.
Unrealized gain on warrants decreased by approximately $121,000 to $67,000 for the three months ended June 30, 2021 from $188,000 for the three months ended June 30, 2020. This represents the change in the fair market valuation of the warrants.
Interest expense, net increased by approximately $627,000 to $881,000 for the three months ended June 30, 2021 from $254,000 net for the three months ended June 30, 2020. For the three months ended June 30, 2021, there was approximately $5.3 million of gross interest expense incurred, $4.4 million of which was capitalized, and $1,000 of interest income. For the three months ended June 30, 2020, there was approximately $4.2 million of gross interest expense incurred, $3.9 million of which was capitalized, and no interest income. The increase in gross interest expense is due to the larger and growing borrowings outstanding on the 77 Greenwich Construction Facility during the period, as well as new borrowings under the Corporate Credit Facility, Mezzanine Loan and secured line of credit as described in more detail in the Liquidity and Capital Resources section below.
Interest expense - amortization of deferred finance costs increased approximately $515,000 to $623,000 for the three months ended June 30, 2021 from $108,000 for the three months ended June 30, 2020. The increase was principally due to deferred finance costs of $567,000 that were expensed in June 2021 due to the refinancing of the 237 11th Loan, as well as the of amortization of finance costs for our loans and secured line of credit that were not capitalized as part of real estate under development.
We recorded $99,000 in tax expense for the three months ended June 30, 2021 compared to $102,000 in tax expense for the three months ended June 30, 2020.
Net loss attributable to common stockholders increased by approximately $25.2 million to $4.7 million for the three months ended June 30, 2021 from income of $20.5 million for the three months ended June 30, 2020 as a result of the changes discussed above, principally the sale of the school condominium to the SCA in April 2020.
Results of Operations for the Six Months Ended June 30, 2021 Compared to the Six Months Ended June 30, 2020
Rental revenues in total increased by approximately $146,000 to $724,000 for the six months ended June 30, 2021 from $578,000 for the six months ended June 30, 2020. This consisted of an increase in rent revenues by approximately $168,000 to $696,000 for the six months ended June 30, 2021 from $528,000 for the six months ended June 30, 2020, offset by a decrease in tenant reimbursements by approximately $22,000 to $28,000 for the six months ended June 30, 2021 from $50,000 for the six months ended June 30, 2020. The increase in total revenues and its related components was due to higher occupancy, higher face rents and less rent concessions at 237 11th during the six months ended June 30, 2021