Single storey extension. I have to attach roof purlins to inner leaf of aircrete wall for a single storey extension. The choice is - build the joist into the aircrete wall, or use joist hangers suitable suitable for no blocks above, e.g. strongtie safety fast joists hanger (this seems to be a fairly new product). To be honest, building in seems a more sturdy solution. Which method should I use ? Simon.
I am neither a builder or a structural engineer but, if it was me, I would assume that timber roofs are flexible structures that will move (if only slightly) with temperature, wind etc. I would always build a purlin into a wall so it was resting over the full thickness of the wall on the wall beneath without the use of fixings in aircrete! ymmv
Is building in still allowed under building regs? It isn't allowed for floor joists.
If not, that could explain the appearance of the joist hangers for the top of a wall with no building over. But there only appears to be one version out there. Floor joists, there is always building over. I'll have to check the regs. Simon.
What is the rationale behind that? Is it for insulation reasons?
When you say purlins/joists, do you mean rafters?
If so the usual thing to do is to put a wall-plate on top of the wall, and attach the rafters to that. The wall plate is mostly held by gravity but also tied down to the wall with galv strapping to stop and wind-lift.
Mentioning purlins makes me think that isn't your issue though.
Since when were floor joists only allowed on hangers? Which is better as gaps around the joists allow cold air into the floor spaces from the cavities. Cavity wall insulation stops it and insulates.
Correct. Make sure the wall-plate, which can be just a plank, is well secured down. High winds can rip a roof off and wind speeds are getting higher.
I mean purlins, yes, fixed at various heights up the roof slope. See previous posts for my complicated roof details ! Think a metal factory roof but in timber, with several purlins (parallel to the roof slope) carrying the main weight of the roof, thinner rafters over the purlins. Cheers, Simon.
All the hot air from the green brigade?
I suppose we can just throw up millions of wind turbines to act as friction brakes.
:-)
Yes, and air infiltration IIRC.
Ah, OK, got you - purlin to gable wall.
Beyond my knowledge, really - I think they are often just built in or supported on struts. Mine are corbelled but that was 120yrs ago, and not aircrete!
Build them in.
And use pre-stressed padtones to sit them on (cheap as chips)
What do your plans say - and are you talking about a floor joist, ceiling joist, roof rafter or roof purlin?
No matter, if possible, simply build them in and make sure that you put a bit of DPC or slate under them and compo all round 'em as well - BTW, make sure that the end don't protrude into the cavity.
Apparently, a single engineering brick is often used for this (75mm wide timber onto aircrete). Simon.
No, wind speeds are getting higher. Tornados are now more common.
Positive thinking.
On Thu, 25 Jun 2009 14:43:56 +0100, a certain chimpanzee, randomly hit a keyboard and produced:
It's still allowed, but not recommended. If they're built-in, you need to make sure they're well mortared in and sealed with a flexible sealant.
On Thu, 25 Jun 2009 15:04:07 +0100, a certain chimpanzee, Jim randomly hit a keyboard and produced:
Thermal and sound insulation. Thermal, as the gaps around their edges can allow air through into the cavity, so an issue on external walls. Sound for the same reason but, obviously, only an issue on party walls.
Seems more reasonable to build in for purlin fixings than with floor joists. I'm not sure about those joist hangers on a top row of aircrete. I notice you can also get strongtie safety fast rubber boots for buildng in to fix the air leakage problem. Simon.
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