single storey extension roof

[sm_jamieson]

I have a roof design to implement when my extension walls are complete. The design is about 6 purlins running at right-angles to the roof slope (shallow 12.5 degree pitched roof, more like flat roof design !), supporting standard-sized roof joists. On one side (a new wall), the perlins can be build into the inner leaf and packed as appropriate, which is easy. On the other side (the existing house wall), I have a choice of a wall plate bolted on parallel to the roof slope, with either joist ends cut to slot in, or timber joist hangers from the wall plate. On the other hand, what about masonry joist hangers which must somehow me positioned and packed to hold the joists level. I'm also not sure if the purlins should be angled to the roof slope, or on the horizontal - this may depend on the method of attachment. What's the best idea (the plans are vague on this point) ? Cheers, Simon.

[Roger Mills]

What size is the roof, and what size rafters are you intending to use? Are you sure that you need *six* purlins? With suitably sized rafters might it not be possible to use just *one* RSJ to support them in the middle? With regards to supporting them off the original house wall, bolting a horizontal wooden beam to the wall, and cutting them into that is likely to be easier than using masonry hangers.

[sm_jamieson]

Yep, all designed and signed off by struct eng. Design (by me with help from this group) minimized roof thickness. Roof about 4m in purlin direction,

5m in rafter direciton. Metal avoided for various reasons, including BCO familiarity. (!) Its just the purlin support that was in question. Beam on wall and cutting in I agree should be OK. Cheers, Simon.

[Roger]

The message from sm_jamieson contains these words:

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Seems a bit like overkill to me too even if purlins one and six are there as an alternative to supporting the rafter ends on walls.

FWIW my house is about 11 feet horizontally from ridge pole to wall plate with only 2 purlins between them and the widest span is about 14 feet. OK so the purlins are substantial tree trunks but they need to be as the roof is rough stone slates with an average thickness of over 3" (That's the average thickness of the slatebed, not the individual slates, most of which aren't much above 1" thick).