Hello,
I have an asymmetrical collar beam roof structure from the 1970s, see sketch. The proportions are somewhat exaggerated.
The roof surface on the right is significantly larger than the one on the left. I’m not completely satisfied with the structural design because of the uneven load distribution.
As I see it, the roof surface on the right transfers substantial horizontal forces into wall W, mainly through the collar beams. The roof surface on the left hardly introduces opposing shear forces to the collar beams, since it rests almost at the same level as the collar beams on wall W. This means the left roof surface likely puts the collar beams under tension rather than compression. In fact, the wall W supports the wall plate.
Therefore, I’m wondering if it might make sense, as part of the attic conversion, to install a central purlin on the right side with a post (marked in red, currently not present). The shear forces from the collar beams could then be transferred vertically through the new central purlin; supports are available. This would not cause any inconvenience and would be easy to install. (Alternatively, a post in the middle is possible since there is a load-bearing wall there, but that would be more disruptive.)
Thanks for your ideas, Peter
I have an asymmetrical collar beam roof structure from the 1970s, see sketch. The proportions are somewhat exaggerated.
The roof surface on the right is significantly larger than the one on the left. I’m not completely satisfied with the structural design because of the uneven load distribution.
As I see it, the roof surface on the right transfers substantial horizontal forces into wall W, mainly through the collar beams. The roof surface on the left hardly introduces opposing shear forces to the collar beams, since it rests almost at the same level as the collar beams on wall W. This means the left roof surface likely puts the collar beams under tension rather than compression. In fact, the wall W supports the wall plate.
Therefore, I’m wondering if it might make sense, as part of the attic conversion, to install a central purlin on the right side with a post (marked in red, currently not present). The shear forces from the collar beams could then be transferred vertically through the new central purlin; supports are available. This would not cause any inconvenience and would be easy to install. (Alternatively, a post in the middle is possible since there is a load-bearing wall there, but that would be more disruptive.)
Thanks for your ideas, Peter