Gents, I am building a two storey rear extension. The original building is an 1800s stone construction with a simple rafter and mid-span purlin pitched roof with raised collar beam (the first floor ceiling therefore follows the rafters up to the collar beam height, across, then down the rafters to the oposite wall). My rear extension meets the building at 90° and again will have the same construction and raised collar beam design. Ok, to the question(s)... The new gable wall of the extension supports the purlins of the extension roof at the gable end of the purlin, but how best to support the other end? Should I build up the rear wall of the existing house within the new roof space to support the new purlin or (as I would pref) construct timber supports down to the existing wall plate? This option would also give access to the loft space from the old roof area without the need for another loft access. But what size of timber is required? I have tables for rafter and purlin sections/spans, but not support of the purlins. This must be a common situation (or at least it was before trussed rafters took over). Any pointing in the right direction would be a help, thanks.
How many supports would be fitted - one for each rafter? Nothing special needed - 4x2s would surely be perfectly fine. Possibly even 3x2s... Timber is very strong in compression. I am not a builder, chippy, or nuffink - take with requisite pinch of salt. (But I think I'm right)
Or, re-reading your post, is it just a single support at the end? If so, a 4x4 should be fine - but the purlin itself has to be hefty enough to cope with the span, of course. (Normal cop-outs apply...)
Yes, I'm just talking about the support at the end of the purlins where the new roof meets the existing building. I agree in principle with your statement, although "4x4 should be fine" might not convince BC. This must be a common situation that someone has actually dealt with first hand.
Hi, From your description, I take it that you are forming a valley to where both roofs meet. Re the new purlins, would they be positioned vertically, that is the rafters resting on the top corner?
Done it loads of times and always build up to purlins with concrete blocks, you can leave access at the centre and lintol across, build on top of the wall plate and between the rafters,cut the rafter feet off flush you will need this wall anyway if you have a raised ceiling, this will completely satisfy BC and is a much better job than propping, Hope this helps
Thank you doctor (sorry couldn't resist ;o) ) You are of course right, the wall does need to be raised to the purlin height/collar beam height doesn't it, or it would be a sorry sight from within the new upstairs room in the extension looking at the rafters of the main roof facing you. I just couldn't see it in my head, until I read that. It's all crystal clear now.