Dimension of roof beams

[SnickareHobb]

Sitting and figuring out what kind of beams could be used to make a roof over two walls. Between the walls, it is 5.80 m (inside the rim beams on each wall).

The roof that is there today is old and low-sloping (3-4 degrees). I would like to double it to an 8-degree slope.

I have read this guide, and there I can find various dimensions, but it states that the maximum roof slope is 5 degrees: https://www.svenskttra.se/siteassets/5-publikationer/pdfer/svt_lathunden_2018.pdf

Can't a roof be built in that way with an 8-degree slope? Would I then have to use roof trusses instead?

 

[Mats-S]

5 or 8 degrees ... probably shouldn't make much of a difference. But if you want to be on the safe side, you can place the beams closer together (like CC60cm), the table in the link is calculated on CC 120cm. (Read the footnote under the table)
A bit more expensive then, but not more expensive than starting to make roof trusses

Thanks for the link, hadn't seen this cheat sheet before, SUPER GOOD

 

[justusandersson]

A simple shed roof can be built with rafters placed at the desired slope. The appropriate slope depends mostly on the choice of roofing material. With a span of 5.8 meters, glulam or similar is required if you don't want to build a truss.

 

[useless]

Can't you build a roof like that with an 8-degree slope? Do I have to use trusses instead, or what?

I would guess they say so to avoid having to account for the slope in their tables. With a larger slope, the beam becomes slightly longer, so if you're close to the limit, the simplest solution is to go up one dimension just to be safe.

 

[SnickareHobb]

Okay, that's possible of course. Interesting that the old roof beams are about 50x200 (unplaned). They have held for 50 years. According to the table, at least 90x270 is required. But there is of course some margin in the calculation. With 50x200, perhaps the roof bends down a bit more than recommended under heavy snow load.

 

[justusandersson]

50x200 is actually 2x8 inches. The difference compared to 45x195 may seem small, but the former has 20% higher moment of inertia. Add to that a high timber quality that approaches laminated wood in stiffness properties. The flatter the roof, the more significant the deflection becomes, which can then cause "dips" in the surface. A higher roof pitch is always better also from a construction point of view, as a larger proportion of the roof loads then appear as normal forces in the longitudinal direction of the beams.