In 2020, we had $8.8 billion in revenues and set records on an adjusted basis for earnings per diluted share of $6.40, operating income of $490 million and operating income margin of 5.6%.
We also had record operating cash flow of $806 million.
Our Electrical Construction segment performed well with 8.4% operating income margins.
We also had 8.4% operating income margins with exceptional performance across the commercial sector, again driven by telecommunications and data centers.
Our United States Building Services segment team showed grit and resilience as they faced the COVID-19 disruption in late March, April and May, with many of our customer sites not acceptable, bookings off as much as 40% in some of our subsidiary companies and in some of our product lines and a very cautious resumption of decision-making by our customers to allow us to resume service and projects.
We also served as the boots on the ground for our customers to keep lightly occupied buildings, campuses and schools operational, functioning and safe over the past 10 months.
We exit 2020 with our remaining performance obligations or RPOs at an all-time high of $4.6 billion, 13.8% higher than the year-ago period.
Consolidated revenues of $2.3 billion in quarter four are down $122.4 million or 5.1% from 2019.
Our fourth quarter results include $55.4 million of revenues attributable to businesses acquired pertaining to the period of time that such businesses were not owned by EMCOR in last year's fourth quarter.
Excluding the impact of businesses acquired, fourth quarter 2020 consolidated revenues decreased $177.9 million or 7.4% organically.
Specific segment revenue performance for the quarter is as follows: United States Electrical Construction segment revenues of $493.5 million decreased $71 million or 12.6% from quarter four 2019.
United States Mechanical Construction segment revenues of $969.4 million increased $73.8 million or 8.2% from quarter four 2019.
Excluding acquisition revenues of $24.2 million, the segment's revenues increased $49.6 million or 5.5% organically.
EMCOR's total domestic construction business fourth quarter revenues of $1.46 billion increased $2.7 million or less than 0.25%.
United States Building Services revenues of $568.1 million increased $29.1 million or 5.4%.
However, when excluding acquisition revenues of $31.2 million, this segment's quarterly revenues decreased $2.1 million or 40 basis points.
United States Industrial Services revenues of $135.5 million decreased by $163.7 million or 54.7% as this segment continues to be impacted by the negative macroeconomic conditions and uncertainty within the markets in which it operates.
United Kingdom Building Services segment revenues of $115 million increased $9.4 million or 8.9% from last year's quarter.
Additionally, fourth quarter 2020 revenues were positively impacted by $2.9 million as a result of favorable foreign exchange rate movement in the period.
Selling, general and administrative expenses of $244.6 million reflects an increase of $3.7 million from quarter four 2019.
The current period includes approximately $4.4 million of incremental expenses from businesses acquired, inclusive of intangible asset amortization, resulting in an organic quarter-over-quarter decrease of approximately $700,000.
As a percentage of revenues, selling, general and administrative expenses totaled 10.7% in quarter four 2020 versus 10% in the year-ago period.
Reported operating income for the quarter of $137.6 million represents a $14.7 million or 12% increase when compared to operating income of $122.9 million in last year's fourth quarter.
Our fourth quarter operating margin was 6%, which compares favorably to the 5.1% of operating margin reported in 2019's fourth quarter.
Specific quarterly performance by reportable segment is as follows: our United States Electrical Construction segment had operating income of $43.4 million, which increased by $2.1 million from the comparable 2019 period.
Reported quarterly operating margin is 8.8% and represents a 150 basis point improvement over 2019's fourth quarter.
Fourth quarter operating income of the United States Mechanical Construction Services segment of $100.4 million represents a $31.5 million increase from last year's quarter, while operating margin in the quarter of 10.4% represents a 270 basis point improvement over 2019.
Our combined U.S. construction business is reporting a 9.8% operating margin and $143.7 million of operating income, which has increased from 2019's fourth quarter by $33.5 million or 30.4%.
Operating income for United States Building Services of $28 million represents a $3.8 million increase from last year's fourth quarter, and operating margin of 4.9% represents an improvement of 40 basis points when compared to the prior year.
Our United States Industrial Services segment operating loss of $8.2 million represents a decline of $21.3 million, which compares to operating income of $13.1 million in last year's fourth quarter.
United Kingdom Building Services operating income of $4.2 million represents an increase of approximately $300,000 over quarter four of 2019.
Operating margin was 3.7% for both quarter periods.
Additional financial items of significance for the quarter not previously addressed are as follows: quarter four gross profit of $383.9 million or 16.8% of revenues is improved over last year's quarter by $19.1 million and 160 basis points of gross margin.
Diluted earnings per common share of $1.45 compares to $1.54 per diluted share in last year's fourth quarter.
Adjusting our 2020 quarterly performance for the negative impact on our income tax rate resulting from the nondeductible portion of the noncash impairment charges recording -- recorded during the second quarter of 2020, non-GAAP diluted earnings per share for the quarter ended December 31, 2020, is $1.86, which favorably compares to last year's fourth quarter by $0.32 or nearly 21%.
Our tax rate for quarter four of 2020 is 41.8%, which is significantly higher than the tax rate for the corresponding 2019 period due to the nondeductibility of the majority of the impairment charges just referenced.
My last comment on slide nine is with respect to our $259.5 million of operating cash flow in the quarter, which favorably compares to $178.8 million of operating cash flow in the year-ago period and reflects the continued effective management of working capital by our subsidiary leadership teams.
Additionally, the deferral of the employer's portion of social security taxes in the United States benefited our cash flow by approximately $35.2 million during the fourth quarter of 2020.
On a full year basis, the social security tax deferrals, coupled with the deferral of value-added tax in the United Kingdom, has favorably impacted our 12-month operating cash flow by approximately $117.3 million.
Consolidated revenues of $8.8 billion represent a decrease of $377.6 million or 4.1% when compared to our record annual revenues in 2019 of $9.17 billion.
Our year-to-date results include $269.6 million of revenues attributable to businesses acquired pertaining to the period of time that such businesses were not owned by EMCOR in the 2019 period.
Excluding the impact of businesses acquired, year-to-date revenues decreased organically 7.1%, primarily as a result of the significant revenue contraction experienced during quarter two as the majority of our operations were most significantly impacted by the COVID-19 pandemic during such period.
Discrete segment revenue performance for full year 2020 is as follows: United States Electrical Construction segment revenues of $1.97 billion decreased $243.2 million or 11% from 2019's $2.22 billion of revenues; acquisitions contributed $25.4 million of incremental revenues, resulting in an organic decline of $268.5 million or 12.1%.
United States Mechanical Construction revenues of $3.49 billion increased $145.2 million or 4.3% compared to 2019.
Acquisitions contributed $188.8 million of incremental revenues to the segment, which, when excluded, results in an organic revenue decline of $43.7 million or 1.3% from 2020.
United States Building Services segment revenues of $2.11 billion increased $3.2 million or less than 0.5%.
Acquisitions contributed $55.4 million of revenues, resulting in an organic revenue decline of 2.5% when compared to full year 2019.
United States Industrial Services segment revenues of $797.5 million decreased $290.1 million or 26.7% from 2019's $1.09 billion of revenues.
Revenues of our United Kingdom Building Services segment for 2020 increased 1.7% to $430.6 million, primarily as a result of new maintenance contract awards within the commercial and institutional market sectors.
Revenues were also favorably impacted by $2.3 million as a result of exchange rate movement in the pound sterling year-over-year.
Selling, general and administrative expenses of $903.6 million represent 10.3% of revenues as compared to $893.5 million or 9.7% of revenues in 2019.
Full year 2020 SG&A includes $29.6 million of incremental expenses, inclusive of intangible asset amortization pertaining to businesses acquired.
Excluding such incremental amounts, our SG&A has decreased $19.4 million on an organic basis, primarily as a result of certain cost reductions resulting from our actions taken in response to the COVID-19 pandemic.
2020's year-to-date operating income is $256.8 million.
Adjusting this amount to exclude the noncash impairment loss on goodwill, identifiable intangible assets and other long-lived assets recorded in the second quarter, our non-GAAP operating income for the year was $489.6 million.
This compares to operating income of $460.9 million for full year 2019 and represents a $28.7 million or 6.2% improvement year-over-year.
Of the two segments which did not, United States Building Services is reporting a modest decline of just over 1%, while our United States Industrial Services segment suffered a significant year-over-year reduction, resulting in an operating loss for 2020.
Their 2020 operating income of $166.5 million represents an all-time segment record, and it is an increase of $4.8 million or 3% compared to the prior year.
Operating margin for 2020 is 8.4%, which is 110 basis points higher than 2019.
United States Mechanical Construction operating income of $292.5 million increased $67.5 million or 30% over 2019 levels, and operating margin reached 8.4% versus 6.7% in the prior year.
Acquired companies contributed incremental operating income of $9.3 million, inclusive of $12.7 million of amortization expense associated with identifiable intangible assets.
The 170 basis point improvement in operating margin was also a result of our solid project execution and improved gross profit margin, most notably within the manufacturing and commercial market sectors.
United States Building Services operating income for 2020 of $113.4 million declined by $1.3 million or 1.2% due to a reduction in year-over-year large project activity within the segment's energy services division as well as decreased project and building control opportunities within their mechanical services division due to both temporary closure and restricted access to certain customer facilities impacted by the COVID-19 pandemic.
These reductions were partially offset by incremental operating income contribution from companies acquired, which totaled $4.5 million, inclusive of $3.2 million of amortization expense associated with identifiable intangible assets.
Operating margin of 5.4% was consistent with the prior year as a reduction in gross profit margin was offset by a decrease in the ratio of selling, general and administrative expenses to revenues due to certain cost-reduction measures taken during 2020.
Our United States Industrial Services segment incurred an operating loss of $2.8 million for 2020 as compared to operating income of $44.3 million in 2019.
Operating income of our United Kingdom Building Services segment of $20.7 million or 4.8% of revenues compares to operating income of $18.3 million or 4.3% of revenues in the prior year.
The $2.3 million improvement is largely due to an increase in gross profit from new maintenance contract awards, while the 50 basis point expansion in operating margin is attributable to both the increase in gross profit margin as well as a reduction in the ratio of selling and general and administrative expenses to revenues.
Additional key financial data on slide 12 not addressed during my full year commentary is as follows: year-to-date gross profit of $1.4 billion is greater than 2019's gross profit by $39.5 million, while gross margin of 15.9% is higher than last year's 14.8% by 110 basis points.
Total restructuring costs of $2.2 million are increased from 2019 due to actions taken during 2020 to both realign certain management functions as well as rightsize our cost structure in light of the revenue headwinds we faced.
Diluted earnings per common share was $2.40 compared to $5.75 per diluted share a year ago.
When adjusting this amount for the impact of the noncash impairment charges recorded in 2020 second quarter, non-GAAP diluted earnings per share of $6.40 as compared to the same $5.75 in last year's annual period.
This represents a $0.65 or 11.3% improvement year-over-year.
Our cash balance has increased from $358.8 million at December 31, 2019, to $902.9 million at the end of 2020.
Operating cash flow of $806.4 million, aided by the FICA and VAT cash tax deferrals previously referenced, was the primary driver of this increase.
Operating cash flow was partially offset by cash used in investing activities of nearly $95 million, predominantly representing payments for acquisitions of businesses and capital expenditures as well as cash used in financing activities, which totaled $172 million and consisted of the repurchase of our common stock, net repayments under our credit facility and dividends paid to our stockholders.
Working capital has increased by over $236 million as a result of the increase in our cash balance, partially offset by a reduction in accounts receivable given the lower organic revenue during the period as well as an increase in contract liabilities due to advanced billing on certain long-term construction projects.
Our identifiable intangible asset balance has decreased since the end of last year largely due to $60 million of amortization expense recorded during 2020, which was partially offset by incremental intangible assets recognized as a result of the acquisition of three businesses during calendar 2020.
Total debt has decreased by $35.7 million since the end of 2019, reflecting our net financing activity during the year.
And course, debt-to-capitalization ratio has decreased to 11.9% from 13.2% in the year-ago period.
As I stated in my remarks, total RPOs at the end of 2020 were a shade under $4.6 billion, up $559 million or 13.8% when compared to the year-ago level of $4.03 billion.
So I'm going to go on to page 14, remaining performance obligations by segment and market sector.
Look, they're up $559 million or 13.8% for those that didn't hear it.
Our domestic construction segments experienced strong project growth in 2020 with RPOs increasing $495 million or 15.2% since the end of 2019 as we continue -- and continue to see demand for electrical mechanical systems, both in new construction and retrofit projects.
It was quite a recovery from the March, April and May time frame when the segment was hit especially hard as described earlier, with bookings down 40% in many cases.
Commercial projects, which make up 41% of that total RPOs, increased $297 million or close to 19% for the year.
For the year, healthcare project RPOs increased $207 million or 56%.
And water and wastewater project RPOs grew similar by 57% to $173 million.
The nonresidential market, as measured by the U.S. Census Bureau for put-in-place activity, remains a very large market, and it was roughly $800 billion at the end of December 31, 2020.
It's down 5% in 2020.
And on nonresidential market, they decreased 5%.
I'm now on page 15, capital allocation.
We have not only invested in over 20 acquisitions and we spent around $555 million on those acquisitions since 2017 until today, but we also have returned significant cash to our shareholders through share repurchases and dividends.
I'm now on page 16, titled Resilient Markets.
We started building the largest data centers in the country for financial institutions and the original hosting providers almost 20 years ago.
I'm now going to wrap this up on page 17 and 18.
The oil and gas markets are still depressed, and the nonresidential market is expected to decline by another 3% to 5%.
We expect revenues of $9.2 billion to $9.4 billion and expect to earn $6.20 to $6.70 in earnings per diluted share.
We will have to execute very well to maintain the 2020 record levels of operating income margins of 8.4% in our Electrical and Mechanical Construction segments.
We are operating near 100% capability on our jobs with no meaningful COVID-19-induced issues.

Summary:
Diluted earnings per common share of $1.45 compares to $1.54 per diluted share in last year's fourth quarter.
Adjusting our 2020 quarterly performance for the negative impact on our income tax rate resulting from the nondeductible portion of the noncash impairment charges recording -- recorded during the second quarter of 2020, non-GAAP diluted earnings per share for the quarter ended December 31, 2020, is $1.86, which favorably compares to last year's fourth quarter by $0.32 or nearly 21%.
Total restructuring costs of $2.2 million are increased from 2019 due to actions taken during 2020 to both realign certain management functions as well as rightsize our cost structure in light of the revenue headwinds we faced.
We expect revenues of $9.2 billion to $9.4 billion and expect to earn $6.20 to $6.70 in earnings per diluted share.