Flat roof insulation? (external add on?) Anyone done it, have recommendations?

Werewolf

Werewolf

Werewolf

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Panting like a fiend
Hi

In an attempt to improve the insulation for the Wolf den i'm looking again at insulation, namely for the "flat" roof (really a sloped felt one*) on the extension, this roof has zero in the way of proper insulation as it was originally built by an idiot*, and the current "sloped" felt roof was built on top after we gave up on the flat roof aspect (after about 3 replacements the last builder looked at it and worked out the most efficient way was to effectively build the new roof as a frame over the flat roof rather than keep trying to keep it flat).

Now what I've seen is that there is apparently a method now for adding insulation to a "flat" roof, which from what I can tell basically involved lifting the felt, putting down thin plywood layer, then a foam insulation (kingspan or similar), then a top layer of ply, then reapplying a felt layer, but I've no idea of idea of how effective it it, or how much I should be looking at for it it.

So has anyone heard of it/have any ideas?

Cheers:)



*The full description would probably have an additional descriptor. If I could afford it, I'd tear it down and rebuild it from the foundations up, it seems it was a DIY job/cheap job with corners cut so much that they're curves (unfortunately most issues didn't turn up for years).
 
I think you are talking about a warm flat roof, also called a built up roof. I also need a flat roof repaired/replaced, so will be interested in the answers. Having talked to a few roofers about using an EPDM rubber roof membrane, rather than felt, most would rather use felt. Not sure why. There is some info in the video:

 
Adding internally was my initial thought but ceiling heights could well be a problem.
You could cut a hole in the existing ceiling and see what void space there is and add accordingly, even thin boards like those above have good u-values. The downside is having to take the ceiling down and reinstating. Still, more of a diy job I feel.
 
Aye ceiling height is a bit of an issue, as is the fact I can't really take the rooms in the extension out of use at the moment to do major works (that and I'm not capable of DIY'ing it, so it would require paying someone to do almost everything).
It's also complicated by the way our extension roof has been altered over the years, so the structure above the ceiling has some oddities (including the remains of the frame for a skylight or two), so the only option "internally" would probably be to lower it all, but it's already around 10-15cm lower than the rest of the house (2.25m vs 2.4m)*.

Cheers for the video casper, i'm going to have a look at that :)


*As I say, built by a bodger who took all the shortcuts :/
 
The flat(ish) roof on our dormer has 10cm kingspan between the joists and another 10cm running the other way on top if the joists. This is then covered with ply and firbreglass, what you propose is perfectly possible would add significant insulation and save you loosing internal space. I’d avoid a felt roof and go fibre glass or rubber.
 
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