Hello!
I am in the process of extending a catwalk in the attic to a full floor. The current catwalk sits on short joists placed on top of the collar beams to rise above the insulation; I plan to remove these and lay joists across the entire attic instead, so I can lay flooring everywhere.
I have a sketch of the attic showing all dimensions and loads. I was thinking of placing joists on top of the red-marked beam. According to the sketch, the load on those beams is 30kg/m2, and the utilization rate is 100%? That doesn't seem particularly good. There is a part of the attic with a dormer where the utilization rate is 75%, see picture 2.
I understand that a beam with 45x175 and that span can't handle much, but I'm not planning to store things "in the middle" but along the sides. On the part of the attic where there's a 100% utilization rate, there's now a wall underneath that runs in the middle of the beams. That should increase the beams' capacity.
How would it be if I lay a new joist on top, but also attach it to the trusses, and not just on top of the underlying collar beams? How much can the trusses themselves actually be loaded?
Ideally, I would want to lay a heavier floor of joists since there's 120cm between the collar beams, but then just the floor alone would weigh about 20kg/m2.
Grateful for advice!
I am in the process of extending a catwalk in the attic to a full floor. The current catwalk sits on short joists placed on top of the collar beams to rise above the insulation; I plan to remove these and lay joists across the entire attic instead, so I can lay flooring everywhere.
I have a sketch of the attic showing all dimensions and loads. I was thinking of placing joists on top of the red-marked beam. According to the sketch, the load on those beams is 30kg/m2, and the utilization rate is 100%? That doesn't seem particularly good. There is a part of the attic with a dormer where the utilization rate is 75%, see picture 2.
I understand that a beam with 45x175 and that span can't handle much, but I'm not planning to store things "in the middle" but along the sides. On the part of the attic where there's a 100% utilization rate, there's now a wall underneath that runs in the middle of the beams. That should increase the beams' capacity.
How would it be if I lay a new joist on top, but also attach it to the trusses, and not just on top of the underlying collar beams? How much can the trusses themselves actually be loaded?
Ideally, I would want to lay a heavier floor of joists since there's 120cm between the collar beams, but then just the floor alone would weigh about 20kg/m2.
Grateful for advice!
The collar beams are not meant to take up any larger vertical loads. If needed, you basically have to redesign the rafters. Up to 50 kg/sqm is probably not a problem. Older rafters might be oversized, but hardly yours.
Thanks for the response! What do you think about placing a 45x120 on top of the collar beam and attaching it to both the trusses and the collar beam with nail plates? That should reinforce the collar beam somewhat, right? I could also glue it.J justusandersson said:
50 kg/sqm is probably all that's needed. Then I'll aim for a slightly lighter floor!
Best answer
It's not the hanbjälkar that are the problem if you want to handle larger loads, but the truss as a whole. If you can manage with 50 kg/sqm, then there are no issues. If you want to reinforce the hanbjälke to counteract sagging in the middle, it's best to screw-glue a beam on or under the hanbjälke.