I am in the process of planning a "attefallshus" and would need some help with the dimensioning of the roof structure. Through various calculation programs, I have determined that the ridge beam should be 45x195 in c24. The transverse roof beams are placed 600 c/c, calculated to 45x120 in c14, with a utilization rate of only 37 percent. My question is whether it is reasonable to reduce the width of the roof beams to 28 or 22 and also increase the height to accommodate more insulation. I have tentatively thought of 22x195. The smallest dimensions in the programs are 45, and I understand that if you reduce the measurement and increase the height, there is a risk of them tipping over. But that should be counteracted by noggings and by securing the sheathing properly, right? For aesthetic reasons, I would prefer to reduce the c/c measurement to 300 as the beams will be visible in the eaves. The roof is 27 degrees, the snow zone is 2.5, and the roofing will be either sheet metal or shakes.
Is it possible to do what I am thinking, or is there something I have completely missed? Is it possible to have roof beams that are 22x195 and c/c measurement 300? Would it be unnecessarily expensive? Over-dimensioned? Under-dimensioned? Something else?
Is it possible to do what I am thinking, or is there something I have completely missed? Is it possible to have roof beams that are 22x195 and c/c measurement 300? Would it be unnecessarily expensive? Over-dimensioned? Under-dimensioned? Something else?
It's about the difference between theory and practice. A beam with dimensions 45x120 mm has a cross-sectional area of 5400 mm^2 and a moment of inertia of 648 cm^4. A beam with dimensions 28x190 has the same cross-sectional area but a moment of inertia of 1600 cm^4. So why doesn't everyone use 28x190 instead of 45x120? The answer is that it's difficult to build with the precision required. Try making a neat joint with two tongue-and-groove boards on a 28 mm narrow beam. Some prefabricated house manufacturers use consistently 35 mm wide beams, but that's because it involves factory production. Timber for on-site built houses should preferably not be narrower than 45 mm. Previously, 2 inches (i.e., 50 mm) was considered the narrowest conceivable.
What you've calculated is not over-dimensioned. The ridge beam is on the edge. I think C 14 is too low a grade for the roof joists, use C 24. Do not use C 14 at all except for noggings and similar. The eye is sensitive to deformations in the roof. Step up a notch, and it will hold up better.
What you've calculated is not over-dimensioned. The ridge beam is on the edge. I think C 14 is too low a grade for the roof joists, use C 24. Do not use C 14 at all except for noggings and similar. The eye is sensitive to deformations in the roof. Step up a notch, and it will hold up better.
Thank you for the answer! That it would be harder to nail on is an aspect I hadn't considered before. What can be considered acceptable regarding the deflection purely from an aesthetic point of view? I assume it depends on the span, but maybe there are some guidelines?
From a normative perspective, the requirement is usually 1/300 of the span, but there are situations where that is too much. Primary beams, such as ridge beams that support other beams, are particularly sensitive. An aesthetic assessment depends on the context. Beams that are part of pergolas look dull even with very small deflections. Beams over sliding windows are also sensitive. There, maximum deflection is often specified in absolute terms in mm. Sometimes other requirements, such as limited bounce, take precedence.