Dodgy Decra (composite corrugated metal sheet "tiles") - replacement / repair / alternatives

[DanRobertsB]

Hi

I'm after some advice on replacing or repairing a Decra roof on a bungalow in south London. Thoughts welcome, including recommendations for who would be well placed to do the work. 

If replacing it with anything heavier, I expect a structural engineer will be needed to confirm whether the structure (see final paragraph) can take it. I'm doubtful it'll take much weight, at least not without reinforcements. And some of the building - the two longest sections - are only relatively lightweight timber clad timber frame (basically studwork finished with plasterboard on the inside, and timber cladding outside), though bookend on the short ends with masonry walls. 

As for replacement, I guess (structure willing), it could be anything from decra, metal sheet, EDPM, etc. 

One option would be to make it a warm roof, but I'm a bit sceptical - about 1/3 of the loft joists under the decra cold roof is well insulated at ceiling level (300mm of mineral wool), but for the rest, the vertical walls loft (and probably the whole wall down to the ground) would need insulating, there'd be increased heating costs of having to heat a large roof space (possibly justifying MHRV, etc). I'm also weary of spray foam insulation - I gather can be an issue for insurers and/or mortgage providers. But if that's ill founded prejudice, do correct me. 

Details below:

We have a large (c. 280m2) series of sloping cold roofs. These are low angle/gently sloped - I think most/all under around 15 degrees. These have Decra Classic (thin metal composite sheet “tiles”) which may benefit from some repairs (including re-sealing of a large c1.6 x 5.8m angled skylight) and need better insulation.

Parts of the Decra are in poor condition - cracked / banana’ing / patch repaired. However, we’re not aware of any obvious damp / water ingress over the past 18 months or so (though there may have been problems before that, given previous damp issues and repairs). It probably warrants at least some patch repairs. 

Angled roof over reception room. Note bending of Decra around chimney (suspect due to weight of leaves which we found there - now cleared)

 

 About half of these roofs (about 180m2) are cold pitched roofs built over a flat roof (which I think was probably the original external roof). For that 180m2 section:

• Loft is c. 150m2, with a further roof/loft extending over the kitchen (c. 30m2), and further over the car port (though the car port area does not need to be insulated).

• Construction appears to be uninsulated joists, topped with a flat roof made of a slab perhaps 75-100mm thick of compressed straw, covered with what looks like bitumen felt (or similar). 

• I think this may have been the original flat roof, probably constructed somewhere between c. 1955-1967. 

• On top of parts of this flat straw/bitumen roof, there's c. 50-100mm of glass mineral wool insulation (see photos); other parts there's nothing or boarded storage. 

• On top of this old flat roof, a lightweight timber frame was built, covered with a black (presumably waterproof) membrane and then sheets of Decra Classic (or similar) lightweight metal/composite roofing “tiles” (with what are I think GRP gulleys). I not sure there’s any OSB or similar between the membrane and the Decra - I suspect not. 

• Rafters are generally 1200mm. 

• Given the lightweight construction and wide rafter spacing, I expect it will only take a very lightweight structure (unless it’s substantially rebuilt/reinforced). 

• Appears to have poor thermal performance / need better insulation. However, given the unusual construction, it’s not obvious to me how best to economically deal with it (and to do so in a way that doesn’t create the risk of condensation issues). Would mineral wool laid out over the old flat roof work? Or PIR? Alternatives would seem to be open cell spray foam, or PIR warm roof - but probably only sensible if re-roofing (which I doubt we can afford). 

Loft hatch area showing straw

Section of strawboard lifted to show joists (uninsulated)

Showing 1200mm rafters etc

 

Some parts have up to 100mm mineral wool insulation above the old straw flat roof

To avoid a roof replacement, it might be that patch repairs (e.g. Decra can sometimes be bent back) and CorkSol SprayCork (or similar) might be useful to improve noise, weather sealing, aesthetics and insulation. But re-roofing might be more appropriate/necessary. Cost, however, is a limiting factor. 

The other half of the sloping roofs, also Decra/1200mm rafters, is also a cold roof. However, it’s now well insulated (c. 200mm mineral wool between the c. 220mm joists, and another 100mm laid crosswise on top). 21 solar panels have been installed on that part of that section of the roof, so it would be preferable to avoid having to remove and reinstall those. This area appears to be (like the roof over the kitchen/car port) is in somewhat better (though I suspect still pretty poor) condition. 

In terms of the angled roofs, the priority is the sloping section containing the large skylight, and perhaps also the sloping section with the chimney. 

There is a square sectioned wooden clad brick upstand (with TV aerial) in the former, which used to contain a water tank. It’s redundant and could (but need not) be removed if that’s easier. It may be brick beneath the timber (given that it's brick underneath, I suspect it is, so removing it is probably more hassle than it's worth). 

 

From outside:

From inside:

 

 

Insulation at the property is generally poor. We might ultimately insulate the walls, perhaps with external insulation. The walls are a mix - some are very thin timber frame with no insulation - just plasterboard and timber frame. Others are blockwork, others painted masonry. Whilst I imagine that would be ideally be done before any re-roofing works (?), the roofs are a priority in terms of insulating and weather proofing etc. Would it be possible to do any re-roofing (e.g. flat roofs) in a way that is compatible with a) any future external wall insulation and (b) any future re-roofing of the Decra Classic sloping roofs? 

 

 

[HughF]

No experience of or comments on the roof, but that's an interesting build...

[DanRobertsB]

12 hours ago, HughF said:

No experience of or comments on the roof, but that's an interesting build...

You haven't seen the half of it! (in both a  good and a bad way!) 🤣

[Timedout]

 First off I will declare that I am an advocate of Decra.

 

Decra will have been specified because of its low dead weight and excellent resistance to wind lift and water ingress. It was not to make it cheap, because it isn’t that cheap.

 

I have experienced it on buildings and it performs very well. 

[DanRobertsB]

I've nothing against Decra in itself - and it seems this roof structure was designed with Decra specifically in mind (or something else similarly lightweight; I don't think it can take much more weight). It's more that this Decra is in very poor condition. The other issues are that access is needed to clear leaves and for maintenance, and it doesn't seem to stand up well to that; and that very few people have any experience in dealing with it. 

Assuming it needs replacing (which everyone who's inspected it in person or seen photos and has offered and opinion says so), the options seems to be more Decra again (noting reservations above), or another very lightweight roofing system (?). Perhaps metal sheets / corrugated metal?

Without reinforcing the roof structure, and perhaps the timber frame building beneath it too (expensive, disruptive, perhaps not economically viable), OSB and EDPM would just bow given the width of the rafters (1200mm).