It’s very important to get flat roofs right. Particularly in our situation – with a modern house like ours, there is a lot of flat roof (roughly 120m² or so) to go wrong. Keeping the existing – that we would have wanted to – was not an option, with the desire to raise ceiling heights and extend/remodel the house.

The existing flat roofs were poorly built, with limited amounts of insulation (if any) in between the joists and the external cover a rather unusual combination of 1969 woodchip insulation panel and a covering of the ubiquitous felt. It leaked, it retained water in puddles, and it looked terrible, and it was cold.

Replacing a Flat Roof

So what to do? Well, first of all, get the structure of the flat roof right. We took the decision to rip out the existing structure completely – the old joists would make useful firewood if nothing else – and build up from scratch.

We then took the decision that we wanted to maximise the insulation performance of the roof and go all out on reliability — after all, spending money on flat roofs is not something you want to do too often in your life if you can possibly help it. In many ways we were motivated by the terrible performance of the existing roof and the tangible impact it had on our young son’s shivering enjoyment of his bedroom — in addition to a fair few leaks.

removing a flat roof

Fully stripped off (the roof, not us) we installed new joists, covered with 18mm ply panels. We opted for a Sarnafil system, installed by Manuka Roofing, having tried out several of the options and met a few self builders and extenders who had used the system before. It promised first-rate insulative performance and ultimately long term reliability. The system consists, from bottom up:

  • Loose laid polyethylene vapour control layer
  • 120mm rigid foil faced PIR insulation board
  • Mechanically fixed (into deck) Sarnafil membrane, hot air (nice – no flames on site!) welded at joints and side upstands.
drawing of sarnafil flat roof build up

Drawing of a typical Sarnafil flat roof build-up

Plywood deck flat roof

OSB deck which will form the new flat roof

The single ply Sarnafil membrance is PVC, and has a BBA-accredited life expectancy of over 40 years. It’s dimensionally stable, fire retardant and – believe me, I’ve tried – can’t really be ripped or ruined. Handy for us, as we have a little area of first floor cladding that will need light treatment every five years or so, so accessing the roof without fear of compromising the membrane was key. It’s also got a lacquer to resist air pollution.

As with quite a lot of supply and fit packages that we’d bought in ourselves to the project outside the builder’s contract, there was a bit of preparation work required on our side to get things ready for the installers (with a cost implication). This was around forming upstands and a bit of time on site sorting out access, materials lifts, and so on.

roof covering Sarnafil membrane
Sarnafil roof membrane

Fitting took seven days for the team of two, occasionally three. The bulk of the work is not so much in the blanket covering, but more the detailing around upstands, our rooflight, and tower, and of course the gutter runs.

It’s also worth pointing out the way the structure is built up. What I didn’t mention above was the installation of firrings (essentially lengths of timber that taper to nothing on a low angle) – all cut on site by the builder – to establish a fall of around 3% on the roof itself to manage rainwater into pre-formed channels and into the downpipe system. That’s critical to ensuring water doesn’t puddle on the roof. With that, and a first-rate covering system, I think we’ll be dry – and warm – going forward. The total cost was c.£14,000 installed.

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