Hello, welcome to the Ashtead Group bondholder call for Quarter Four and the year-end results. Throughout the call, all participants will be in listen-only mode. Afterwards there will be a question and answer session. Just to remind you, this conference call is being recorded. Today, I am pleased to present Suzanne Wood, Finance Director. Please go ahead.
Thank you. Good afternoon to everyone on the call. With me today are Geoff Drabble, our Chief Executive, and Will Shaw, our Director of Investor Relations. As always, we appreciate your interest and are happy to have this opportunity to answer any questions you might have. The webcast of our earlier analyst meeting and the related slide presentation are available on our website. We'll refer to those slides during the call today. I will begin by covering some financial highlights from the release in the event you haven't had an opportunity to look at the webcast yet. Then we'll open up the call for your questions. As an overview, we once again had a very strong year with market-leading growth in both revenue and profitability.
We continued to make progress on our growth and our capital allocation priorities, while importantly, still maintaining leverage within our target range. I'll move on now to a bit more detail around our performance, starting with our fourth quarter results, which are shown on slide five in the presentation. Here, you'll note that our underlying pre-tax profit for the quarter was GBP 189 million, ahead of last year's GBP 163 million. The group's rental revenue grew by 11% at constant rates of exchange. However, the fourth quarter this year had two fewer billing days than last year. On a billings per day basis, rental revenue growth in the fourth quarter was 14%, which is right in line with our earlier quarters. This also had some impact on drop-through and margins in the quarter. EBITDA margin was unchanged at 46%, while operating profit margin was slightly lower, at 26%.
This resulted from higher depreciation expense associated with our larger fleet and a higher cost base reflecting certain central overhead investments we made. These were mainly in the areas of IT development and staff retention. I'm sure we'll come on to discuss that further in Q&A. On the next slide, we show the group's financial results for the full year. At constant rates of exchange, rental revenue increased by 13%, with our growth outpacing the markets in both the U.S. and the U.K. Similar to earlier quarters this year, the percentage change in our total revenue of 10% was slightly less than the change in our rental revenue. That had some impact on our performance, which we'll discuss more in a moment.
Our EBITDA margin improved to 47% for the year, and our operating profit margin reduced slightly to 28%. We think this margin performance was a solid one, particularly considering that we opened 61 new greenfield locations, completed 15 acquisitions with the associated cost of acquisition and integration, and invested in our infrastructure. As a result, our underlying pre-tax profit for the year increased by 7% on a constant currency basis, to GBP 793 million. On slide seven, it's worth pointing out that our growth rates after adjusting for the effect of gains on sale really reflect a more accurate picture of our underlying business performance. Excluding the sale of used equipment from both years, revenue increased by 13% and underlying profit by 10%. On the next couple of slides, we've shown the divisional performance for Sunbelt and A-Plant.
Beginning with Sunbelt, you'll note that rental revenue grew by 12% in the year as it continued to benefit from generally strong end markets. EBITDA margin increased to 49% for the full year, while operating profit margin was broadly flat at 30%, owing to the factors I mentioned earlier. Sunbelt's operating profit increased by 7% as compared to last year, as the continued operational efficiency of mature locations more than offset the drag effect of new stores. If we normalize for the effect of reduced fleet disposals by excluding gains on sale, then the underlying profit growth rate at Sunbelt was 10% in the year. With respect to A-Plant on slide nine, rental revenue grew by 16%. However, the operating cost base grew at a similar rate as four acquisitions were integrated, and as a result, margins remained broadly flat for the year.
Like Sunbelt, A-Plant's operating profit growth percentage change of 7% was adversely affected by fewer fleet disposals in the current year. Excluding the impact of lower gains on equipment sales, our underlying operating profit increased 11% over the prior year. On slide 10, we provided the details of our cash flow. The group's strong margins resulted in cash flow from operations of GBP 1.4 billion, significantly higher than last year. Ours is an inherently profitable cash-generating business, and it's that cash flow which gives us substantial flexibility. We used part of the GBP 1.4 billion to cover what I'll call non-discretionary items like interest, tax, and replacement CapEx. That left GBP 927 million available for discretionary items such as growth CapEx, M&A, and shareholder returns.
As in previous years, we chose to invest most heavily this year in growth CapEx to support the activity levels we see on the ground. Even after the investment in growth CapEx, we still generated free cash flow of GBP 319 million. We invested a further GBP 421 million on bolt-on acquisitions, increased our dividend, and repurchased shares. Slide 11 is the usual one on our debt and leverage profile, and we continue to show it because it reflects our ongoing commitment to investing responsibly, maintaining leverage within our target range, and ensuring that our debt structure remains flexible. At April 30th, our leverage ratio was 1.7 times, which was well within our target range. We also continued to maintain a wide margin between our net debt and the secondhand value of our fleet. The gap between those two is now GBP 1.4 billion.
Our strong balance sheet and leverage remains a competitive advantage and positions us well in the medium term. The other topic I will touch on quickly before moving on to Q&A is our 2018 CapEx plan, which is shown on slide 24. For 2018, we anticipate that our growth CapEx investment will be broadly similar to the year just ended, approximately GBP 900 million-GBP 1.2 billion on a growth basis. As you would expect, our CapEx is weighted more toward the U.S. and more toward growth than replacement, reflecting our young fleet age. This level of CapEx will support the organic growth plans discussed in our last Capital Markets Day today when we outlined our Project 2021. At this point, I am sure you have some questions you would like to ask, we will move on to the Q&A session. Operator, if you could give instructions for that, we would appreciate it.
Thank you very much. Ladies and gentlemen. We have a question now. Bear with me one moment. We have a question now from the line of Sean Wondrack of Deutsche Bank. Please go ahead. Your line is now open.
Hi, good afternoon, Sean. First question- We seem to have a technical issue. Apologies, Sean. I know you are speaking, but we can't actually hear you.
If I could ask you to wait for one moment while we resolve this technical issue. Thank you very much. Again, please accept my apologies for the delay while we just try to resolve this muting issue as we can hear Sean Wondrack's question. Thank you again for your patience. Again, apologies for the delay. Please bear with us while we try to resolve this.
Yeah, might I suggest, Sean, hopefully, you can hear me. This is Suzanne Wood. If we're having some difficulties with your line, I'm more than happy to speak to you immediately after the call. If you'd like to just ring me-
Sure
in the group office here in London. I suspect you have the number or can get access to that off our website. Will and I will be happy to chat with you about any questions you might have.
Okay, great.
Operator, hopefully, that works. I might suggest we go ahead and see if there are any other questions from participants to see if this technical issue only relates to Sean's line or is potentially more broad than that.
Okay. Unfortunately, the issue which is preventing us from hearing Sean's question is affecting all participants. While they are able to hear us, we will be unable to hear them if they have a question to ask.
All right. Well, I apologize for this. I will have to say this is the first conference call issue I've had in a really long time, if ever, that I can remember. For those of you on the call, I think there are 10 or 11 participants. I am here in the group office. Many of you have my mobile phone number. Please do give a call after this, and I will be happy to chat with each of you about any questions you might have. Thank you. I think, operator, at this point, we should just conclude the call, and hopefully, we will have the opportunity to speak with the participants individually.
Thank you very much. In that case, this now concludes our call. Thank you for attending. Participants, you may disconnect your lines.