Insulating a sloped roof on 1st floor - sanity checking

12 Aug 2021
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United Kingdom
Hey everybody,

~1950's two storey hip roofed house, with part of the roof extending down through the 1st [UK] floor:


The original rafters are suprisingly small (45 x 70mm), despite some of them going nearly 8m to the apex. The main hip section of the house has been insulated for a while, but the upstairs section has none at all. I have a good understanding of engineering and we have a building inspector, but because these rafters are so small I would certainly appreciate a little bit of sanity checking to make sure what we're doing here is a safe approach:


To the underside of each rafter, a 120mm C24 is being mounted with square twist nails. The span here is around 3.3m. Based on the span tables the combined 190mm rafter depth is more than fine to span this distance, so the loss of them being 2 separate pieces should be covered. The base of the new timber is being seated via a birds mouth cut on a small stud wall, replacing a previous (load bearing) stud wall, effectively moving that stud wall back by ~1m.

This is being done one rafter at a time, ensuring they're supported where the previous stud wall was right until the last minute when the new timber goes in.

I have scoured the forums here looking for similar things, but most people seem to have much more reasonable rafter sizes - is this something anything you guys have encountered before? Thank you!
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The theory was putting too many nails in that small timber would be the situation that triggers a split. I.e. many smaller plates (6 on each side was the intent) vs fewer larger ones.

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