can I make a roof void a fire compartment?

[gongoozler]

Hi, first post!
Looking at a conversion of a large surgery into three ground floor only 2 Bed units. The roof is a complicated trussed rafter construction spanning over all three proposed flats. I would compartment the flats, but running up to the underside of the roof finish through the void would be a massive job, can't work out if it's OK to make the roof void a separate compartment (with detection) to get over this...anyone know??

 

[tony1851]

Unfortunately, the dividing walls would need to be carried up to the underside of the roof finish.
Approved Document B specifically states that the roof void over the top (ie any) storey of a compartment is included within that compartment.

 

[gongoozler]

Thanks for that, the trussed rafters are designed as a single large span, and so that would render the project non-viable, as each 'roof' would not be structurally independent....thus the roof would have to be replaced!

 

[tony1851]

Presumably the trusses run perpendicular to the party walls between the flats, then?

 

[gongoozler]

Yes, the plan shape is irregular, and you can't make it work with party walls/compartments along the line of truss orientation. In addition due to the shape the truss layout is complicated...so it would be a massive roof off exercise, it's a good roof too!

 

[tony1851]

If you could prove that failure of one part of the truss in a fire would not affect the stability of the truss-parts over the other flats, then it would be acceptable.
But you would need an assessment from an SE to verify this.

(structural timbers are allowed to pass through a compartment wall if they are well mortared in or otherwise fire-stopped).

 

[gongoozler]

Tony...many many thanks for your comments, checked it out with b.control today and an SE....all good, offer in and accepted. That keeps me out of mischief this summer then!