My question concerns the dimension of the roof joists for a heavy/large skylight! (Apologies if I’m posting in the wrong place!)
It’s about a very small house, about 6 m x 2.5 m in total, with a 15-degree shed roof. Tongue and groove + breathable roofing membrane + tile sheet. The intended skylight weighs about 55 kg, with opening dimensions of 990 x 1440. The roof joists are 145 mm.
I don't want to increase the dimension of the roof joists, but I would also prefer not to reduce the size/weight of the skylight. The smallest I can consider would be 48 kg, and 990 x 1220. But there's not much difference in load, is there?
The exterior wall is 95 mm, with 12 mm plywood on the outside (for rigidity). Everything is screwed together. Under the top plate (along both long walls), a "bearing beam" will be recessed + just under the section with the skylight, a loft will be built, with recessed 95 studs for the loft joists. So, the whole house is a bit underdimensioned...
But, to the question!
Would it be "sufficiently" safe with this solution with 145 joists for 55 kg? Snow load zone 2 or 2.5. If not, does it help to put larger bearing beams on the long walls? I was thinking of taking 120 mm. Is it really so necessary not to notch the roof joists? It's about 25 mm.
It’s about a very small house, about 6 m x 2.5 m in total, with a 15-degree shed roof. Tongue and groove + breathable roofing membrane + tile sheet. The intended skylight weighs about 55 kg, with opening dimensions of 990 x 1440. The roof joists are 145 mm.
I don't want to increase the dimension of the roof joists, but I would also prefer not to reduce the size/weight of the skylight. The smallest I can consider would be 48 kg, and 990 x 1220. But there's not much difference in load, is there?
The exterior wall is 95 mm, with 12 mm plywood on the outside (for rigidity). Everything is screwed together. Under the top plate (along both long walls), a "bearing beam" will be recessed + just under the section with the skylight, a loft will be built, with recessed 95 studs for the loft joists. So, the whole house is a bit underdimensioned...
But, to the question!Would it be "sufficiently" safe with this solution with 145 joists for 55 kg? Snow load zone 2 or 2.5. If not, does it help to put larger bearing beams on the long walls? I was thinking of taking 120 mm. Is it really so necessary not to notch the roof joists? It's about 25 mm.
Hello, and welcome..
The binding answer to your question will come from a byggingenjör, who can take responsibility for the calculation.
The difficult question is what deflection the beams will have.
The sheet metal roof contributes to good snow runoff.
All notching contributes to reducing load-bearing capacity, but at the same time, it often provides good jointing.
The binding answer to your question will come from a byggingenjör, who can take responsibility for the calculation.
The difficult question is what deflection the beams will have.
The sheet metal roof contributes to good snow runoff.
All notching contributes to reducing load-bearing capacity, but at the same time, it often provides good jointing.
It depends, among other things, on the c/c you have between the roof beams. The roof window itself does not weigh more per sqm than a normally heavy roof does, but it may require the deflection of the roof beams to be limited. When I calculate a roof beam with c/c 1000 mm and snow load zone 2.5, a 45x145 C24 is not sufficient. If you use 45x145 C24 c/c 600 generally and glue-screw two 45x145 C24 to the beams surrounding the window, you will manage and keep the deflection under 1/300 on the entire roof.
Absolutely, but I'm not otherwise uncertain about the construction. Finding a structural engineer for just this small question will probably be difficult. They prefer to take on larger projects, I mean. Thank you for your answer anyway!GoForIt said:
Okay. It will be c/c 600 otherwise, but by the roof window, the distance will be c/c 1035. If I understand you correctly, it should be fine if I follow my drawing? So, two glued 145s by the roof window? + Also, I'm embedding a "carrier beam," but that doesn't relieve/help the point load from the roof window, right?J justusandersson said:
While I have you here
, will the roof be "stronger" with two noggings in each rafter bay?Thank you so much for your response! You're heaven-sent!
I actually don't understand that.V vagnen said:
No.V vagnen said:Now that I have you on the line here