Flat Roof Cost

Thank you for visiting us here at Quotation Check – Your guide to costs and charges within the building industry. This page is all about flat roof replacement costs and the written quotes you can expect to receive. Our complete price list is located here.We last updated this page on the .

By using the term “flat roof” we are of course referring to a felted flat roof that can be seen on many garages and some extensions. To keep things simple, on this page we are going to give you some example prices/costs for a replacement felt system to a flat roof; we are going to quote for bitumen based felt that is “torched” onto the roof with a gas-powered blow torch.

In case you were wondering how this works; the bitumen is already on the rear side of the felt, and the gas torch melts this bitumen onto the roof. We are not referring to rubber, fibreglass or any other type of roofing material, see our other pages for those price estimates.

Our Cost Example 1 – Double Garage Flat Roof

The price/costed example below is for a flat roof on a double garage, there are four sides to the garage, with a gutter on just one side. The quote is to replace all the felt and supply a 10-year guarantee.

Here is a list of all the work involved:

  • Sheet over contents of garage for protection
  • Remove any stone chippings from existing roof and rip up existing felt
  • Inspect existing timber decking and replace as required (extra cost)
  • Supply and install one underlay to decking timber
  • Supply and install one cap sheet with a green mineral finish on surface
  • Supply and install all edgings including upstands and drip edges which overhang into the gutter
  • Quote includes timber fillets and timber batten as required
  • Dispose of all waste material
LocationSmall Business /TraderLarge Business
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London Area£1050.00£1300.00
South, SW and Midlands£1050.00£1250.00
Outer Region and North£900.00£1000.00

Price Example 2 – Extension Roof

Cost to replace felted flat roof covering

Typical Flat Roof

In our second cost example, we have an extension roof at the rear of the property that requires some scaffold to access safely, the roofing contractor may erect an access tower instead of using an external scaffold company. This roof is approximately 1.5 times the size of a double garage roof.

There are also some differences compared to a simple garage roof; there are wall upstands and some lead overlaps that must be installed.

The added height of the roof means disposing of the waste material is going to take longer as it needs to be transported down to ground level and then to the front of the property.

Because the extension is over a habitable part of the home (i.e., not a garage), the roof will be finished with three layers of felt and will also have additional insulation placed under the new timber decking which needs replacing.

Here is the works schedule:

  • Erect scaffold tower up to work height
  • Sheet over ground floor for protection
  • Remove any stone chippings from existing roof and rip up existing felt
  • Replace existing timber decking
  • Supply and install one underlay, tacked (nailed) to decking timber
  • Supply and install one mid-layer sheet torched to underlay
  • Supply and install one cap sheet with a green mineral finish on surface torched to mid-sheet
  • Supply and install all edgings including upstands and drip edges which overhang into the gutter
  • Quote includes timber fillets and timber batten as required
  • Quote includes lead dressed onto felt to prevent water ingress (see photo 2)
  • Supply and install insulation placed between roof rafters underneath the new decking
  • Dispose of all waste material
LocationSmall Business /TraderLarge Business
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London Area£1450.00£1950.00
South, SW and Midlands£1100.00£1500.00
Outer Region and North£975.00£1200.00

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Where Did We Get These Figures From?

Unlike some other websites, we don;t just grab figures from thin air. All of the cost examples we publish here on Quotation Check are based on real quotes and estimates sent to us from trade people and businesses in the UK.

We have used the same method for over two years; first we source written quotes from traders in different locations around the UK and calculate an average cost.

The team here then displays the cost on our pages, along with any relevant information gathered from the tradespeople we selected.

You can read more about how we gather quotations on this page.

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Added Extras – (A Must Read!)

Lead Overhanging Onto Flat Roof

Lead Overhanging Onto Flat Roof

Here at QuotationCheck we prefer fixed quotations rather than estimates, this way the customer knows exactly how much the work is going to cost, without any nasty surprises when the bill arrives.

Unfortunately, flat roofs can be difficult to quote for as it’s sometimes impossible to see the condition of the timber beneath the felt. Your roofing contractor should give you a separate quote to replace the timber decking, just in case it’s in an unacceptable condition. If you have good quality wood under the felt, then lucky you! That has just saved you some money. If however your timber is made from thin, low-quality ply board or chipboard then chances are it will need to be replaced. You do not want to put the new felt onto a timber base that is rotten, sagging or failing.

Replacement decking boards (OSB grade) will cost you between £40-£55 each, supplied and installed as part of the replacement flat roof.

Changing the slope of the roof – You would think that a flat roof is “flat” – right?  Well, actually they are not, a “flat roof “should have a slight slope so rainwater runs off the felt and into the gutter. If the roof was perfectly flat then water would “pond” on top of the felt, this would shorten the life expectancy of the felt. Your contractor should check your existing roof and advise you of your options, and then the roofer can add some additional timber to raise one end of the roof by a couple of inches. You will need to weigh up the “pros” and “cons” of paying this additional sum, how many extra years will this add to the life expectancy of the felt? Is it worth the extra cost?

Stones and chippings – Do you need to have stones/chippings placed on the top of the felt? Stones/chippings were used decades ago as a way to protect the felt from UV rays which would shorten the life expectancy of the felt by drying it out and causing cracks.  In the price examples above, we have specified “mineral finish” felt, this type of cap sheet is somewhat UV resistant so no, you do not need to put stones/chippings on the roof felt, that would be an optional extra. Some homeowners just don’t like the appearance of felt so hiding it under a sea of stones is certainly possible, if that’s your preference.

Insulation – If the flat roof is located above a habitable part of your home then you will want the roofing contractor to check the quality of the roof insulation, make sure you get a quote for additional insulation before they start the project.

Repairing an Old Flat Roof – Worth the Cost?

If your existing flat roof is leaking, then it may be possible to repair it rather than replacing the whole lot. The most common areas where leaks occur are:

  • Where the felt joins a wall and it has come loose
  • The drips edge – where the felt overhangs into the gutter
  • Damage caused by falling tiles or other third-party damage

If your roof is leaking due to any of these examples, then a patch repair may be possible, this will obviously cost considerably less than a replacement. If however your roof is leaking because it’s old and has suffered from cracks /UV damage etc, then doing a patch repair is not a long-term solution and at worst could be considered a bodge.

If the flat roof is suffering from leaks and you notice that rainwater is “ponding” on the top and the felt is sagging then, a patch repair is certainly a no-no and a full replacement is required.

Cutting Corners – Watch Out for This

One method some unscrupulous roofers use is to leave the original felt in place and just put the new materials on top of it. This is perhaps the biggest “bodge” in the whole roofing industry and can lead to a whole host of problems. It is essential that the old felt is removed so your roofer can inspect the timbers underneath and get everything in order for the new felt finish.

Another con is when the roofers cover the roof with a liquid tar or sealant, these are usually good for a year or two until the movement of the roof and UV rays cause it to split and leak.

Variations in Price

The cost of a replacement flat roof will vary wildly; some contractors will use cheap materials while others will use top of the range products. Some roofers have their own scaffold, so there is no need to bring in an external scaffold contractor. Ease of access is also a primary factor when pricing a job,  a flat roof that is three storeys high is of course much more challenging and costly to access than a garage at ground level.  How much decking needs to be replaced is also a key factor, the decking costs around £25 per sheet to buy, and there will be an additional labour fee to install it too. If you need new lead to overlap onto the felt from a wall then this will bump up the price considerably.

How Long Should The Work Take?

For a single garage, the work would be completed in one day if two men were working on the project, Even if additional timber work was required it is unlikely that the project would take more than one day.

For a double garage or a similar sized flat roof, the work would extend into a second day, although the roofer will make sure that the flat roof is watertight overnight. They would return on the second day just to finish off the edgings etc.

Unusually shaped flat roofs or where there is a lot of cutting required (around pipes, Velux windows and parapet walls etc) will, of course, require extra time and cost.

Questions You Should Ask

The flat roofing industry is not without its shady traders so here a few questions you should ask, make sure you get your answers backed up in writing.

  1. Will you remove the old felt?
  2. Will you make sure the water doesn’t pond? Will this cost more? How much?
  3. How much will any new decking cost per sheet? And how many sheets are on the roof?
  4. If the felt is butted up against a wall, will he use costly lead or cheap felt as an “upstand” ?
  5. What is the finish of the felt? Stone chippings or mineral felt finish?
  6. Is the insulated and how much does extra insulation cost?

Other Materials

Torch-on bitumen mineral felts are an extremely popular choice for roof coverings and if applied correctly will last 15+ years. Assuming rainwater isn’t allowed to pool/pond then they may last even longer.

Other materials to consider are Mastic Asphalt which is a tar-like substance which is layered by hand onto the roof in one seamless piece, it’s costly, very few people are qualified to complete this work but it lasts for many years, certainly longer than 20. Rubber based membranes such EPDM or even rigid fibreglass are another material you could consider. EPDM has become popular in recent years as a heat torch isn’t required during the installation process. This reduces the risk of fire and makes it easier to install.

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6 Responses to “Flat Roof Cost”

  1. Jamie LairdOctober 22, 2015 at 6:14 pm #

    Flat roof should have loose layer if not boards need primeing and for double garage your cheap couldnt do it for that

  2. alan obMay 24, 2016 at 2:08 pm #

    Don’t know when you put those numbers together but today they seem to be nearly twice what you suggest above for a double garage roof in the south east. Three quotes all small business around the 2k mark for 3 layer felt and new OSB deck

    • QuotationCheck AdminMay 24, 2016 at 2:53 pm #

      We created this page back in November 2014. The figures shown for the double garage flat roof do not include the cost of OSB board replacement which costs between £40-55 per sheet to supply and fit. Also we asked for a two layer system rather than a three layers, which would cost more. So for full board replacement and three layers I feel £2k isn’t too far off.

  3. MartinJune 19, 2016 at 3:01 pm #

    Part L no mention of cost to consumers or do most felt roofers just forget to inform there customers
    They should be informing building control at an extra cost
    Now who is the cowboys ?
    And surely a 3 layer system should be standard ?

    • GeorgeSeptember 16, 2016 at 7:18 am #

      PL is only triggered if more than 50% is to be replaced. I agree, no less than 3 layers should ever be used.

  4. AlexDecember 11, 2016 at 2:59 pm #

    This is a very informative article, even an industry person find it very detailed and honest, because usually contractors will not list out pricing publicly and loses the chance to close a sale without an initial approach with clients. Wish you all the best with your business.

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