1960s rafter/purlin roof - using binders to support a dormer

[mywetpants]

Hi

Haven't been on the forums in a while, but was wondering if someone could answer a simple question.

I'm planning for a dormer conversion, and my roof looks as attached.

I have three 2x5" binders that run from party wall to party wall. However, these sit above the ceiling joists (2x3") which go from wall plate to wall plate.

My question is, can a new floor be laid on top of the binders?

Because if I ran new I-beams, the new floor would have to sit above the binders raising the floor level.

 

[tony1851]

My question is, can a new floor be laid on top of the binders?

No, because they are only there to prevent excessive deflection of the ceiling joists, and are not adequate to support floor loading.

If you use steel beams (which is the correct way) you can bolt a timber piece into the web so that you can use joist hangers to support the floor joists. That way, you don't compromise the headroom.

 

[mywetpants]

Thanks Tony.

I've actually appointed a structural engineer to complete plans and calculations, who has suggested removing the binders.

Regarding the electrics, is it common to have a separate circuit breaker for the loft, serving a new ring main, lighting ring and electric shower radial?