Ronald Ballschmiede
Analyst · risk factors that could affect these projections and assumptions. The company assumes no obligation to update forward-looking statements as a result of new information, future events or otherwise.
Please also note that management may refer to EBITDA, adjusted EBITDA, adjusted net income or adjusted earnings per share on this call, which are all financial measures not recognized under U.S. GAAP. As required by SEC regulations and rules, these non-GAAP financial measures are reconciled to their most comparable GAAP financial measures in our earnings release issued yesterday afternoon. I will now turn the call over to Mr. Joe Cutillo. Thank you, sir. Please go ahead
Thanks, Joe, and good morning, everyone. I am pleased to provide a summary of our 2020 third quarter results. With this being a full year of including the Plateau acquisition in our consolidated results, most of the acquisition integration and new financing noise related onetime costs are now behind us, providing a cleaner picture of our financial performance.
The actions we have made progressing our multiyear strategy, including the transformational acquisition of Plateau, continues to be apparent in substantially all of our financial measures. Today's conference call, together with our earnings release, Form 10-Q, and the investor deck posted to our website, should provide insight into our strategic progress in delivering strong earnings and cash flow and improving liquidity while reducing risk. Not to be overlooked, this strong performance came during the most challenging period across our country in many decades.
Now let me take you through our financial highlights. Starting with our backlog metrics on Slide #5. At September 30, 2020, our backlog totaled a record high $1.238 billion, a 16% increase over the beginning of the year.
Heavy civil backlog increased 13%, while specialty services backlog increased 25% from the beginning of 2020. The gross margin in our third quarter 2020 backlog was 12.4%, a 90 basis point increase from December 31, 2019. Unsigned low bid awards totaled $270 million at the end of September 2020.
We finished the third quarter with combined backlog of $1.508 billion, a 12% increase over the start of the year. The gross margin of our combined backlog increased to 11.6% at September 30, 2020, up from 11% at the beginning of the year. Our year-to-date 2020 book-to-burn factors for the combined heavy civil and specialty services segment were 118% and 117% for backlog and combined backlog, respectively.
Note that the book-to-burn computations include only the revenues from heavy civil and specialty services and excludes residential revenue as it is not a backlog driven business.
Please flip to Slide 6 for a summary of our consolidated results. Note that this slide includes quarterly results for 3 periods, the second and third quarters of 2020 and the comparable to prior year third quarter. Given the magnitude of the Plateau acquisition and the related acquisition financing and changes in NOL accounting, which occurred in late 2019, the second quarter of 2020 was included to highlight the "same-store" sequential quarter revenues. Our third quarter 2020 revenues of $383 million increased by $92 million or 31% over the comparable 2019 quarter.
Third quarter 2020 specialty services revenues increased by $107 million over the comparable 2019 period driven by the Plateau acquisition. The acquisition was also the major driver of the significant increase in gross profit and the 300 basis point gross margin improvement over the periods.
The Q3 2020 revenue decline from the sequential prior quarter reflects a $17 million reduction in heavy civil revenues. I will discuss the significant year-over-year segment results in a moment. The increased SG&A from Q3 2019 reflects the Plateau acquisition, including corporate-related costs. While SG&A -- probably SG&A decline in the sequential quarter was principally the result of lower stock-based compensation expense.
The third quarter 2020 decline in other operating expense net for both prior periods reflects a decline in gross profit from our 50% owned consolidated subsidiaries, reflecting a lower income sharing expense. This decline reflects the temporarily lower revenues during the -- and gross -- gross margin from a mix -- change in mix in Q3 2020. We continue to expect our full year other operating expense net to be in the $14 million to $16 million range.
Operating income for Q3 2020 was $29 million for an operating margin of 7.5%, which was consistent with our expectations. Operating income for Q2 2020 totaled $33 million and operating margin of 8.5 -- 8.25%.
Our record second quarter 2020 operating returns benefited from entering the second quarter with record combined backlog and poor weather in Q1 2020, which pushed incremental revenues and earnings into the 2020 second quarter.
Higher net interest expense in both 2020 periods over the 2019 period reflect the acquisition-related borrowings. Our effective income tax rate for the first 3 quarters of 2020 was 28.5% compared to 8.9% for the comparable 2019 period.
Beginning in 2020, our income tax expense includes a noncash tax provision of approximately 22% of our taxable income or approximately $11 million year-to-date. We expect our full year effective income tax rate to be approximately 28.5%, an increase from our previous expectation of 28.2%.
Our third quarter net income totaled $15.2 million or $0.54 per share compared to the third quarter of 2019 net income of $8 million or 30% per share.
Third quarter 2020 EBITDA increased more than 2x to $36.7 million from $15.4 million over the comparable 2019 period.
Slide 7 highlights the third quarter segment results. Heavy civil revenues decreased by $18 million or 8% in the third quarter, reflecting a $6 million increase in heavy highway revenues and a $24 million decrease in aviation and other heavy civil revenues. This temporary revenue mix change to a higher -- heavy highway component had a negative impact on our operating returns for the quarter.
Also during the quarter, heavy civil resulted results included a charge for increased estimated costs to complete a three bridge project in Texas. We continue to expect substantial completion of these 3 bridges in the first, second and third quarters of 2021.
The increase in specialty services revenues and operating income primarily reflects the inclusion of Plateau in the 2020 results. Additionally, Plateau entered the 2020 second quarter with record backlog.
And as I previously discussed, poor first quarter weather pushed work into the second quarter of 2020. Both of these factors, together with a favorable project mix contribution to the second quarter 2020 operating margins.
Third quarter 2020 residential revenues totaled $42.4 million or a 6.3% increase over the third quarter of 2019. Our Houston expansion continues to pick up speed. Houston accounted for 15% of residential's third quarter 2020 completed slabs compared to 11% in the third quarter of 2019.
Residential operating margins declined 50 basis points in the Q3 '20 period, resulting from temporarily -- temporary COVID-19 related pricing pressures from our customers and significant cost increase for lumber and concrete during the quarter.
Now let's move to Slide 18, which summarizes our cash flow generation and deleveraging strategy.