What kind of truss is this and can I open it up?

[andsj0732]

Hello See my sketch of our roof trusses construction on a house from the early 70s in Västra Götaland, with a concrete tile roof.

I understand that this essentially falls within the category of framework truss but I am curious about what it means that its rafters/upper frame are not in one piece but are two beams that connect below and above the collar beam. The joint is reinforced with nailed-on studs on both sides.

More specifically, I wonder what this means if you want to open up at the ridge using a ridge beam in glued laminated timber supported by posts connected to the collar beams of the preceding and following trusses.

I think that removing the collar beams in this case, regardless of successful support with the ridge beam, risks weakening the rafters'/upper frame's load-bearing capacity of the outer roof (concrete tiles). It could potentially "collapse" inwards at the weakened joint.

I also wonder what this might mean for the outer trusses' load-bearing capacity of the ridge beam.

What do I need to potentially reinforce to be able to open up at the ridge with a ridge beam? Is it even advisable?

In general, is this a common roof truss construction?

 

[justusandersson]

Joining the rafters in this way over the support beams or collar beams is quite common in older trusses. If you intend to remove the collar beams and replace them, for instance, with a ridge beam, it is less ideal. I suspect such an operation would require entirely new rafters, possibly in laminated timber, depending on size and snow zone. The ridge beam must rest on columns that can transfer the loads to the foundation. The number of columns depends on the length of the house and acceptable dimensions of the ridge beam. Whether it is too advanced an operation depends on how encapsulated the current roof structure is. The new rafters can be placed between the existing ones, but you need access.

 

[andsj0732]

It is quite common for rafters to be spliced in that way over support legs or collar beams in older trusses.(..)

Thank you for the excellent and helpful response! Then we should probably leave the collar beams as they are. For now.

May I take the opportunity to ask a follow-up question? Has anyone seen or experienced replacing the board/piece of lumber (used for splicing the joint between the rafter sections) with perforated plates/nail plates afterward? This would allow for a 1160 roof window that otherwise wouldn't fit as the truss bay is now 9cm narrower.

If it's even recommended to attempt, can/should/must you overlap several perforated plates to extend the covered area at the joint or use some other trick?

 

[petterovski]

If you're going to replace with steel, you could buy real flat bar like 150x5mm (you can get it in any length you want) and drill where you're going to put screws. Overlapping splice nail plate sounds like a bad solution.

 

[andsj0732]

Hi, thanks @petterovski have been looking for just that. Absolutely better than skarva. Where can you find such sheets?

 

[petterovski]

I unfortunately do not know. A local steel wholesaler in your area should have or be able to order what you need.

https://www.ahlsell.se/10/varme--sanitet/stal/stang/plattstang-s235jr/19040113/