Timber frame horizontal diaphragm

[Martin29]

Hi all.

My first post on this forum so any help would be much appreciated.

I have provided the structural design for a three storey block of flats with a flat roof. The building is a timber frame structure with metal-web joists at first/second floors and flat roof. In order to make the flat roof work as a suitable horizontal diaphragm, I have specified that 18mm structural plywood is fixed DIRECTLY to the top of the timber joists. As the flat roof is to have a slight slope, the Client has asked if they can fix the ply to to the top of the timber firrings which will then be fixed to the timber joists.

My view is that (unless the firrings are shallow enough that the nails/screws can go through the ply & firrings and into the joists) this is not suitable as the firrings would only be skew-nailed to the joists which would not provide a suitable fixing. Also, if the firrings are quite tall (say 200mm) they would be relatively slender and could move sideways under lateral wind loading. The only info I can find within the BS code states that the structural deck is to be fixed DIRECTLY to the joists. Therefore, my solution would be to fix a sub-deck directly to the joists, then install the firrings above with another deck on top of that. Or form the falls within the insulation above?

Any thoughts on this???

 

[jayrod12]

Agree with some of your concerns. Can they not frame one side higher than the other to create the slope with the structure versus furring it out with sub-purlins or sloped shims?

If you're doing pre-fabricated trusses, why not have them build the slope into the trusses directly?

 

[GeorgeTheCivilEngineer]

Agree with your assessment. All free edges of the board should be nailed to noggins and firrings would prevent that. Can they not make the required fall in the insulation layer?

 

[Martin29]

Hi both.

Thank you for the replies.

jayrod12 We did discuss sloping the roof joists but the firrings slope in different directions which would mean installing additional trimmers within the roof. So this was ruled out (by the Client) due to cost.

GeorgeTheCivilEngineer I did suggest making the fall within the insulation but again this was ruled out due to cost! (Can you see a pattern here!!!).

Although the flat roof is currently designed with metal-web joists (and not trusses as jayrod12 has mentioned), we could possibly use timber lattice trusses with a sloped top chord. They may need to increase the depth of the flat roof structure (currently 225mm joists) but certainly worth mentioning to the Client.

I will let you know the outcome!