Loft conversion - structure of the new floor and ceiling

[evanjones]

Hello,

We're having a loft conversion - rear dormer. It's a 1910 mid terrace, currently 2 bed. There is no supporting wall in the middle of the property. The current ceiling joists span the full 6.7m front to back!

We have the usual issue of trying to make the most efficient use of height. There is 2.48m from the top of the 1st floor ceiling to the apex of the roof.

I'm pretty sure we are going to have to put an RSJ or something across the middle otherwise the necessary depth of wooden joist to span 6.7m will be ridiculous.

My question is: When that steel goes in, is it normal practice / possible to cut the current ceiling joists such that the steel can begin next to the ceiling? And rehang those joists from the steel? That would save us the depth of the ceiling joists (4") in headspace which could be very useful space.

If that is possible, what consideration needs to be made for deflection on the RSJ and it's proximity to the ceiling? Or is it so minimal as to be irrelevant?

Don't worry - I'm about to commission a structural engineer and we are doing a full plans application to Building Control before we touch anything. We're going through planning at the moment. I just want to be able to have an informed conversation with the guy doing the calculations for us.

Thanks for your help.

 

[tony1851]

My question is: When that steel goes in, is it normal practice / possible to cut the current ceiling joists such that the steel can begin next to the ceiling? And rehang those joists from the steel? That would save us the depth of the ceiling joists (4") in headspace which could be very useful space.

In practice, it would be diffcult to do that without causing damage to the ceiling.
You say there is no supporing wall below? That would be unusual having a bedroom 6.7 m long in a 2 bed house.
Is there a non-loadbearing wall there (eg lath-and-plaster)?
If so, can you put the steel beam under the ceiling and box it in?
Assuming the span is not too wide, you could use a 6" deep beam and box it in.

 

[evanjones]

Thanks Tony,

Yes, there is a lath and plaster wall there. That sounds like a good idea. The wall is around 20 cm wide. Will it have to go next to the wall? Or might it be possible for most of the beam to sit within the profile of the wall?

Gareth

 

[tony1851]

Next to the wall would be far easier.
It could be possible to disguise it within the top of the wall, but it would disrupt the fixing of the existing studwork; a way would then have to be found of re-securing the tops of the wall studs. This would entail bolting a piece of timber to the underside of the beam, and fixing the (cut down) tops of the studs to that.