Insulating between loft rafters and boarding on top

28 Mar 2017
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United Kingdom
Hi all, posting first time on here as I'm in need of some advice please...

Our loft has a boarded floor so that it can be walked on but there was a lot of debris falling from the roof which I believe was coming from the felt laid under the external tiles which over time has in some places fallen apart a bit. As a result it was also pretty chilly up there most of the time. There are also loads of cob webs etc. between the rafters. So I decided to get the roof insulated and then boarded on top of so that we can sort the temperature and debris issues and have a clean smooth internal roof and be able to go up there more regularly and store odds and ends up there without worry about dirt/dust.

For insulation, I've had 40mm multi-purpose Celotex insulation boards inserted between the rafters everywhere and to board on top of these I had bought some 5.5mm plywood sheets to cover it all up and make it clean. The plywood hasn't yet been fixed to the rafters and I have a few questions...

1. I feel like 5.5mm plywood may be too heavy to use just to board on top of the insulation and with the amount of plywood which will eventually be screwed into the rafter beams I'm concerned that the weight of all that plywood might cause issues. I've seen loft rafters boarded on top of with thin hardboard sheets (maybe 3mm) so am I better off using hardboard for overboarding or will the plywood be OK?

2. When the Celotex was installed between the rafters, I ensured a 30-40mm gap was left between the celotex and roof felt to allow air to flow (the rafters themselves are about 70mm deep) but other than that do I need to do anything else to ensure adequate ventilation? Am I right to have all rafter space filled with celotex or should I not put celotex in certain places at the top or bottom? At the moment, the only area which won't get celotex or boarded is a 2-3 inch gap between where the roof meets the floor all the way round.

Any guidance or advice on the above would be greatly appreciated!

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My worry is that the loft space will be sufficiently warm and humid that you'll get condensation appearing at the gaps. With your ventilation, if it's effective it should deal with that well.
I can't imagine 5mm ply would be significant in terms of the existing tiles, so just use whichever boards you can fit through the hatch.
Yes I've been conscious of the condensation issue which is why I tried to ensure the gap between the celotex and roof felt was left. The only other thing I can do is not put celotex along the bottom half metre all the way round but not sure if that will help or not.

Not sure what you're saying about the think it's OK to use and weight won't be an issue? The 1220 x 2440 sheets I bought can be cut length ways into two narrower sheets which will fit through the hatch fine.
A healthy loft is a drafty and cold loft. You should have just stapled a breathable membrane to the underside of the rafters. Job done and cheap.
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ok but now that I have the celotex up there is there any issue with boarding over with plywood?
It is yea, fair amount of gaps and holes existed in various parts of the roof so it should be well ventilated behind the insulation...

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