Sistering Rafters

[jamesvictorianDIY]

Help please, we're struggling to find answers. All advice welcome!

DIY Plan - Support the original rafters (in pic) and add new straight (as possible) timbers for plasterboard to fix to.

Problem: There is about an inch and 1/2 bow in the middle of the rafters (along and across).

Recommended approach (by a friendly roofer): drop the new rafters at the top to create a new straight line and add support timber underneath.

Our concern: If the new straight rafter line drops below the bowed rafters by a significant distance, we could lose space in which to fix the new and old rafters together (at the top like in 2nd picture).

Our query: Will this reduce the support the new rafters will provide?

 

[Dereekoo]

Why not just have hangers (metal or wood) at the top hanging down from the original rafters supporting the new ones

 

[jamesvictorianDIY]

Do you mean the ridge board at the top? I've not heard of hangers off of existing rafters. If you mean from the ridge board, the issue would be that the breeze blocks beneath sit quite far forward from the ridgeboard at the top. So a hanger wouldnt sit flush (just my thoughts thought based on very little experience so please do correct me if I'm missing something).

 

[catlad]

Why are you adding new rafters?

 

[cdbe]

If you have 1.5" deflection just make the sisters 1.5" deeper than the existing. Maximum deflection will be around mid span so your bits ending halfway up and halfway down will achieve nothing. Unless you are worried about shear (the existing rafters snapping at the ends) then the sisters can finish slightly short.

 

[jamesvictorianDIY]

@catlad We've just had new (heavier) tiles added to this section and they chap who fitted them suggested we add stronger rafters, as they felt a bit bouncy and are bowed. The existing rafters are 3" by 2"s, he suggested getting 4" by 2"s and sistering them.

 

[jamesvictorianDIY]

If you have 1.5" deflection just make the sisters 1.5" deeper than the existing. Maximum deflection will be around mid span so your bits ending halfway up and halfway down will achieve nothing. Unless you are worried about shear (the existing rafters snapping at the ends) then the sisters can finish slightly short.

View attachment 341644

Thanks @cbde, so we could finish short and the new "rafters" in this case wouldn't need sit on the wall plate on on a supporting timber at the top?

 

[ree]

OP,
To get a level plane for the plasterboard:
Pull two very taut strings - one from each corner to the opposite diagonal corner.
At the four corners use same size small blocks under each string to keep the strings below the present plane.

 

[Dereekoo]

Do you mean the ridge board at the top? I've not heard of hangers off of existing rafters. If you mean from the ridge board, the issue would be that the breeze blocks beneath sit quite far forward from the ridgeboard at the top. So a hanger wouldnt sit flush (just my thoughts thought based on very little experience so please do correct me if I'm missing something).

Forget my comments I thought you were trying to obtain a flush surface for fixing new plasterboards but it appears that you have structural issues hence your sistering of existing rafters , although having sistering rafters that are effectively cut in the middle (where the maximum bending forces are) does not sit well with my thoughts on effective sistering. What is the span and spacing of the existing rafters, I assume the original tiles have been replaced with concrete ones?

 

[cdbe]

Thanks @cbde, so we could finish short and the new "rafters" in this case wouldn't need sit on the wall plate on on a supporting timber at the top?

Yes.

 

[jamesvictorianDIY]

Forget my comments I thought you were trying to obtain a flush surface for fixing new plasterboards but it appears that you have structural issues hence your sistering of existing rafters , although having sistering rafters that are effectively cut in the middle (where the maximum bending forces are) does not sit well with my thoughts on effective sistering. What is the span and spacing of the existing rafters, I assume the original tiles have been replaced with concrete ones?

Yes we have added concrete tiles, and the rafters are circa 2.85m in length. I agree about the middle sistering issue. Ideally we would have completely re-roofed and simply added full length rafters from the outside, but we havent so we cant see a sensible way of doing this without pulling everything off.

 

[catlad]

I'm guessing slates have been taken off this old property and much heavier tiles have been put on. If you have taken away the flat ceiling and are now hoping to make it a vaulted ceiling , then I would advise you to at least sister each rafter with a new full length of timber, ideally 6x2 so you have space to fit some insulation.