Part B and domestic extensions.

I had a call from an award winning architect looking for some assistant, with regard to a  problem which had arisen in terms of Part B of the Building Regulations on a domestic extension. The neighbouring property's architect had rasied an issue (as they do!) It certainly wasn't one of the normal issues I would be asked to deal with regarding Part B and domestic extensions.No this problem related to a timber framed extension which was built within 1.0m of the boundary. So what's the problem there you may ask?

Most architects and other designers (and builders) dealing with domestic construction seldom take much heed of Part B of the building regulations other than providing a fire alarm and means of escape windows, and maybe the positioning of the stairs and providing fire resisting construction to  to 3 storey houses.Most would not know what Appendix A of Technical Guidance Document -B is about, never mind Tables A1 & A2. Tables A1 & A2 relate to the required fire resistance of elements of structure, including walls, external and otherwise. So what has this got to do with our friend the architect?

The architect in question had designed an extension to the rear of a suburban terraced house, which was been constructed adjacent to the boundary wall (within 1.0m of the boundary) which was of timber frame construction. Appendix A requires 30 minutes fire resistance from both sides of the wall. ie both internally and externally. Most plasterboard slabs achieve the required fire resistance from the inside, where a conc block outer leaf is used externally with the timber frame, upto 4 hours fire resistance is achived from the outside, problem solved. But in this case, like many others, in order to safe space and get everybit of usable space in the extension, the cavity and block external leaf was ommited; the proposal to apply a render finish to the external face of the timber frame. Unfortunatley this construction, hasn't been certitifed as achiveing a fire resistance. I did find a soulution, in recommending fibre cement panel, which could be fixed to the external face of the timberframe structure, which is certified to achive the required fire resistance and got our friend out of trouble.

Had the wall been on the boundary the issue would have been more serious in that 1 hour fire resitance is required from both sides.