Our process discipline enabled Wabash National to observe a notable reduction in volume, while minimizing the impact to operating income as shown through 14% decremental margins for the full year of 2020.
We generated $104 million of free cash flow during 2020, which enabled us to maintain our dividend through the cycle, a feat never remotely accomplished during a significantly challenging environment in the history of Wabash National.
While traditionally constructed refrigerated cargo vans are insulated using spray foam, which can be subject to off-gassing and mold intrusion, Gruau inserts are engineered to fit specific manned models and provide a superior finish with 30% to 50% thermal efficiency than standard refrigerated body construction, thus improving total cost of ownership, reducing spoilage and improving food safety.
Overall, backlog ended the fourth quarter at approximately $1.5 billion, up sequentially by approximately $500 million from the end of Q3.
While I believe this to remain true based on our own experience and feedback from suppliers, customers and peers, I do want to call out that we were able to successfully hire approximately 600 new employees across our business during the fourth quarter.
This hiring activity equated to adding to our workforce by about 15%.
We fully expect to add another 900 employees during the first half of 2021 based on our progress to date.
We are initiating our full year revenue outlook at just under $2 billion.
In this environment, we are seeing earnings per share of approximately $0.75 at the midpoint.
On a consolidated basis, fourth quarter revenue was $404 million.
Consolidated new trailer shipments were approximately 10,600 units during the quarter.
In terms of operating results, consolidated gross profit for the quarter was $45.5 million or 11.3% of sales.
The company generated operating income of $10 million and operating margin of 2.5% during the fourth quarter.
Consolidated decremental margins were 12% during the fourth quarter, which is a performance we're very proud to have achieved.
Compared to Q4 of last year, SG&A expense was lower by about $5.6 million or 16%.
Operating EBITDA for the fourth quarter was $25.2 million or 6.2% of sales.
Finally, for the quarter, GAAP net income was $5.5 million or $0.10 per diluted share.
From a segment perspective, Commercial Trailer Products performed very well with revenue of $283 million and non-GAAP adjusted operating income of $23.3 million.
Average selling price for new trailers within CTP was about $27,000 in the fourth quarter, which is roughly flat with the same quarter of last year.
Diversified Products Group generated $75 million of revenue in the quarter with non-GAAP adjusted operating income of $3.3 million.
This business is responsible for approximately $20 million during 2020, which is revenue that will not be part of DPG's results going forward.
FMP generated $52 million of revenue during the quarter with an operating loss of $4.5 million.
Due to the burden of depreciation and increasing amortization in the business, it's important to point out that FMPs fourth quarter EBITDA was a loss of only $600,000.
With operating cash flow of approximately $124 million, roughly $20 million was reinvested to be a capital expenditure, leaving $104 million of free cash flow.
We are extremely pleased with the work the team did to register $104 million of free cash flow during a pandemic.
With regard to capital allocation during the fourth quarter, we utilized $11.2 million to pay down debt, $8.7 million to repurchase shares and invested $6.4 million in capital projects and paid our quarterly dividend of $4.2 million.
We expect revenue of approximately $1.9 billion to $2 billion.
SG&A as a percent of revenue is expected to be approximately 6.5% for the full year and we remain positioned to sustain the reduction in our cost structure by $20 million from 2019 with around $15 million of that cost-out residing within SG&A.
Operating margins are expected to be 4% at the midpoint.
While we've talked about both incremental and decremental margins for the company being in the 20% range on a normalized basis, the base on which we're calculating incremental margins for 2021 have considerable furlough savings included, which does temporarily serve to depress incremental margins.
We had approximately $25 million to $30 million of one-time reductions in areas such as furloughs and incentive compensation in 2020 that will return in 2021.
But we would expect 20% incrementals from 2021 to calendar year 2022.
Lastly, I'd like to make on the full year calendar -- consolidated P&L is that amortization of the tangibles does step up again in 2021 by about $2 million.
On a segment basis, the step-up in amortization will be seen entirely within FMP, bringing this segment's full-year amortization to $12.4 million.
In total, we estimate 2021 capital spending of between $35 million and $40 million.
Combining those seasonal trends with the massive capacity ramp we are undertaking to keep up with the demand, that has us adding roughly 1,500 hourly employees from September 30 to March 31, and we would expect Q1 to be pressured.
Our expectation is for first quarter revenue to come in between $390 million and $420 million with new trailer shipments of 9,500 to 10,500 and to be approximately breakeven from an earnings per share perspective.
We continue to look for opportunities to drive structural improvements to working capital, though, in the short term and we do expect in 2021 to consume upwards of $50 million of cash, most of which will occur in the first half of the year.
We had outlined targets that centered around achieving a consolidated operating margin of 8%.
While the world has obviously changed immensely since we initially released these targets, I do want to reiterate that the team still see the 8% operating margin as a reasonable goal in the medium term.
Given our longer-term planning, I believe the 8% operating margin is achievable over the next two to three years.

Summary:
Overall, backlog ended the fourth quarter at approximately $1.5 billion, up sequentially by approximately $500 million from the end of Q3.
In this environment, we are seeing earnings per share of approximately $0.75 at the midpoint.
Finally, for the quarter, GAAP net income was $5.5 million or $0.10 per diluted share.
We expect revenue of approximately $1.9 billion to $2 billion.