If you could choose a time to renovate a house, you would start when I started: early May. Theoretically you have the worst of the heavy April showers behind you, and can build throughout the summer where, if the house were to be re-roofed (in this case a replacement flat roof would be added and left open on the odd occasion, it’s not going to suffer too much from the weather.
Well, it hasn’t been too cold, at least. Yet a month or so of significant rainfall, followed by dry heat, followed by a month of frequent showers meant that, as we stripped the roof off and windows out before replacing them with models fit for this century, a couple of the existing rooms and carpets ended up sodden through. Yes, we could have put a tarpaulin scaffold cover over the whole house, but that costs money and you would think, wouldn’t you, that we would get a dry spell in June?
Anyway, that gripe aside, progress is amazingly swift as we work towards getting the house weathertight. It’s important to get flat roofs right — particularly in our situation where there’s roughly 120m2 of it to go wrong.
Retaining the existing flat roofs was not an option as we were looking to raise the ceiling heights and extend/remodel the house. Not that we would have wanted to keep it anyway: the existing flat roofs were poorly built, with limited amounts of insulation (if any) in between the joists and the external cover was a rather unusual combination of 1969 woodchip insulation panel and a covering of the ubiquitous felt. It leaked, it retained water in puddles, it looked terrible and it was cold.
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 chose 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. 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 – was all the motivation we needed.
We opted for a Sarnafil system, having tried out several of the options and met a few self builders and extenders who had used the system on their projects. It promised first-rate insulative performance and ultimately long-term reliability. The system comprises, from bottom up:
• A loose laid polyethylene vapour control layer
• A layer of 120mm rigid foil-faced PIR insulation board
• A mechanically fixed (into deck) Sarnafil membrane, hot air welded (meaning no flames on site!) at joints and side upstands.
The single-ply Sarnafil membrane 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 an area of first floor cladding that will need light treatment every five years, so accessing the roof without fear of compromising the membrane is key. It has 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.
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 and our rooflight, and of course the gutter runs.
It’s also worth pointing out the way the structure is built up. What I didn’t mention 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 warm and dry going forward.