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Wondering if there's any structural engineers on here who can help explain something to me please. We're planning a small single storey extension, and the roof design has two small rooflights. I would have liked to combine these into one larger one, but the structural engineer has said No, because of something to do with the roof providing a diaphragm which transfers lateral loads of the new walls back to the main structure. I suggested we could put a single beam to prop the new wall under the centreline of the larger rooflight, but apparently it's the diaphragm of the roof which is important. This seems a bit daft to me given that 1) the new wall is single storey and less than 3m long, and 2) the two existing walls which the roof connects to both have fully boarded floors which will provide their own diaphragm, not to mention the new steel beams going in.
Can anyone suggest if he's being overly conservative, or is this normal design? I feel like I've pushed him quite a bit and he's dug his heels in, so would be good to get a second opinion as to whether to keep pushing. Fwiw the architect think's he's being daft.
Cheers,
I can't quite understand what he's getting at there but it sounds like it's to do with resisting wind effect on the structure?
Not a structural engineer but in the building trade and can't understand how there would be no structural solution to wanting a single wider rooflight on there, there must be something else he can do like a windpost in the new wall.
I am a structural engineer, now retired. You're right that there are a few load paths there that wouldn't be there in a freestanding building. It's pretty robust looking, but I'll suggest a simple modification that, if I was advising you, would make a single big roof light viable.
I don't think the steel beams have any relevance, but I can't follow where they are from your diagram. To understand the diaphragm action, think about what happens when the wind blows on the purple wall. It's supported by walls at the sides. To support the top, in a freestanding building, the roof would work as a diaphragm and transfer load to the sides, in your case the load can go straight through the rafters into the existing wall (hopefully the floor behind is not too different in level). With a big roof light, that support is only at the edges, not in the middle.
The fix is to put some diagonals in below the roof light.
From a structural stability point of view I'd be happy with that. There may be something in the standards referred to by the Building Regulations that suggests otherwise - it's not the type of structure I worked on so I don't know, but your engineer should. The issue is that the standards are not mandatory, but if you don't follow them, you have to make a case to the Local Authority (assuming England & Wales), and they can be awkward.
Thanks Greybeard, that looks like a sensible option - although possibly the diagonals would need to slope the other way, to support the middle of the wall and send the loads around the skylight? Something our engineer was banging on about was the overall stability of the building - so not necessarily just taking side loads, but perhaps stiffening it all laterally. We'll see...
Which way the diagonals slope isn't crucial. I've put them so that the load goes down the middle rafters and back through the diagonals, so they are in tension. Your diagram has a more direct path but puts them in compression. It doesn't make a lot of difference on something like this but bracing is conventionally in tension to avoid it buckling. See what your engineer says.