Reinforcement of roof truss

[Grottan]

As an old designer, I believe your own suggestion is the best. Screw-glue a 20mm plywood on each side of every rafter.

Are you thinking of making a piece that fits between the braces and essentially replaces the splices in the middle?

The truss looked nice when I got a camera up and could inspect a little better - a bit of surface rust on the nails but otherwise like "newly made" 55 years later.

 

[qz-mackan]

I have a house built in 1970 where the garage has a relatively flat roof (about 4 degrees) where probably the passage of time and snow load have created a sag in the middle - where the ceiling hung down 120mm at most even without external load.

I opened up the ceiling and propped up the three trusses that had given in so it's not an acute situation, but I do need to do something about this.

The construction is simple and relatively weak - it looks basically like this:

[image]

The rafter (yellow) is dimension 145x45, the splice plates (green) are dimension 145x45, and the braces (blue) are dimension 22x95. The span of the truss between the supports on the walls is about 6m.

The lower splice plate is nailed with 12 4" nails, and the upper splice plate is nailed with 8 4" nails. The braces are nailed with two 4" in each beam. What has happened over time is that especially the nailed joint in the lower splice has started to give in, causing a sag in the middle - it was 120mm before propping and without any load other than raw board and tar paper.

Spontaneously, the entire execution of the jointing feels like a very weak point, but the question is how one can rebuild this so the roof holds up to snow load over time.

A working hypothesis is to remove braces and splice on one side at a time and replace these with a sheet of construction plywood that is anchored with glue + nails or screws and then repeat the process with the next side of the rafter.
[image]
A 1200mm wide sheet would essentially overlap the entire current construction, but the question is whether a 20mm K-plywood and the joints will be strong enough.

I have searched for information on truss constructions but have not found anything relevant for this construction - today there are entirely different requirements and possibilities to optimize material, splice with nail plates that are pressed into the material, etc.

I am open to all suggestions and ideas as long as it can be realized from below. Preferably avoiding any transverse beams as it entails a lot of other hassle.

I don't have much to contribute to this matter, but I talked to a truss factory some time ago, and they said I should use nails instead of screws in truss constructions because nails "bond better with wood than screws" and provide much better homogeneous durability.

 

[Utsliten och utdömd]

I would have gone with the plywood option, screw-gluing as long pieces as possible on each side of every truss. The cross-section of the plywood will be much higher than the short studs in the truss and has the advantage of being able to distribute the load better.
Use screws, the whole function relies on getting good pressure between the parts so that the friction between the surfaces distributes the load; the force should not actually go through the screw/nail.

 

[LaszloToth]

Why not just hire an engineer to figure this out and give you options?

 

[Dowser4711]

I think it looks reasonable, but an alternative to those screws that I've used myself when repairing a broken roof truss is to drill through holes, use threaded rods and large washers. (40-50mm galvanized washers are usually found near the galvanized threaded rods at the hardware store)

Remember to push up a bit extra before you start screw-gluing regardless of the method you choose, because it will sag a bit when you remove it no matter how stable a solution you build. So the "trick" is to get it to sag down to the original position, but exactly how much it will be you'll soon find out.

Good luck!

 

[Grottan]

Why not just hire a designer who can calculate this and give you options?

I also have a question out on that but haven't received any feedback at all, so you have to look for information elsewhere as well ;-)

 

[AG A]

Are you envisioning making a piece that fits between the braces and essentially replaces the splices in the middle?

The truss looked nice when I put up a camera and could inspect a little better - a bit of surface rust on the nails but otherwise like "newly made" even 55 years later

I would have removed both splices and braces. Then cut out and replaced with pieces of plywood, just as you planned.

 

[Grottan]

Thanks for all the answers, there's quite a bit to think about. It has held up well in its current form for many years, so if we can restore function with a better joint in the central seam, it's a more reasonable effort. After all, the trusses seem to be completely unaffected, so it feels a bit wrong to start tearing into them – centrally, however, it's clear that there's an obvious error.

I'll have to consider the options:

1. Screw in existing splice plates

2. Replace the central piece with structural plywood

3. Rebuild the entire truss

As long as the truss is intact, there will essentially only be tensile forces, so option (2) might feel doubtful since only 50% of the plywood would take up the forces. Therefore, I think (1) is a better option – perhaps replacing the splice plate with nailing plates – which would then be a plate of 140x400mm instead of a splice of 600x150.

Option 3, as mentioned, becomes a completely different project and more costly/time-consuming. Now that it turns out to be a truss, which I didn't initially believe, it feels a bit like fixing something that's not broken.