Incorrectly constructed roof trusses (pictures available)

[Workingclasshero]

Screw you spikbleck.
Feltema/Bilbuttericks have cheap spikbleck.

 

[Autodidak1]

A bit weak trusses, but if they've lasted 35 years, they'll last much longer. It's not uncommon for old ones to sag a bit, the neighboring house (which is about 30 years old) with factory-made trusses (W) looks like rough seas, but it will last in the future as well.

But the solution to convert it to W-truss is good and will give you peace of mind. I would have used plywood and nailed it with a nail gun for quicker work. But screwed metal plates are excellent.

 

[daugaard]

as Autodidak1 writes: if they've lasted 35 years, they'll probably last a long time!

6 meters span is quite small, it's hard to see any problem with the trusses in the pictures. have you measured how large the sagging is on the inside of the roof boards? thinking that it might be the purlins and battens that have given way causing the sagging?

 

[heimlaga]

Nail plates are not particularly good.

It becomes much stronger if you use substantial pieces of 16mm plywood and nail with regular three-inch nails. Just as you yourself were thinking.

I have performed stress calculations on some roof trusses at different times, even though I'm not a real professional designer. I calculated by hand due to the lack of computer programs.
In principle, the brace that is furthest from the ridge is the most heavily loaded. The further out from the center you get it without it being too short, the stronger the truss.
With such a low roof pitch and heavy roof as you have, I would cover the outermost ends of the trusses with plywood on both sides, all the way in so that you can fit the end of the first brace between the two boards and nail it in place.
The outermost brace usually needs to be considerably thicker than the others. Then the loads decrease brace by brace the closer to the center you come.
I can't promise anything, but I would think that the lower and upper chords in your trusses are sufficiently thick.

Good luck!

 

[Autodidak1]

as Autodidak1 writes: if they've lasted 35 years, they'll likely last much longer!

6 meters span is quite small, it's hard to see any problem with the trusses in the pictures. Have you measured how large the sags are on the inside of the roof sheathing? I think it might be the strapping and purlins that have given way, creating sags?

Most often, as you say, it is the purlins that give way, but even the roof sheathing sags over time. Then the trusses also move. There is always a calculated deflection in all constructions, and from having "too much" deflection to the breaking point is a long way.

We demolished a house that was extended with a floor in the early 1900s and the joists were about 50x150, today the same joists are desired to be 45x220 (C24) with that span. It didn't sway much, but sagged a bit. But most importantly, it had held up without problems for almost 100 years.

 

[daugaard]

am quite convinced that this roof truss is not falling apart from what you see in the pictures. In my eyes, the roof trusses are not poorly constructed and do not lack a framework. Can someone here tell me where you see the faults?

 

[fahlis]

The rafters might be undersized. Possibly calculated for light roofing? The joint at the ridge looks a bit weak. It would likely benefit from a splice plate/gusset plate on the side facing the camera as well.

 

[f91jsw]

In my eyes, the trusses are not poorly constructed and do not lack a truss structure. Can someone here explain where you see the flaws?

It is not a truss. A truss consists of triangles, and all forces are (ideally) only compression or tension forces, except for the load on top of the roof, of course, which imparts some bending moment on the top chord. In the trusses shown in the pictures, most of the load is transferred down to the bottom chord, which receives bending point loads at the supports. This can be OK if the bottom chord is designed as a beam that can take the entire load, but evidently, that wasn't quite the case here.

If you want to get a feel for how trusses work, I can recommend the mobile game Bridge Constructor, available for iPhone and Android. The game is both fun and educational.

 

[Autodidak1]

Regardless of how the construction looks, there are no indications in the pictures that it wouldn't handle the loads. The construction in the pictures is not uncommon in older houses, I have seen similar construction on old villa roofs that also look completely healthy and stable despite not being standard W-trusses.

I wouldn't spend time reinforcing a construction that has stood for 35 years without visible damage. If one wants to reinforce it for peace of mind, it is obviously not a disadvantage. In that case, both metal plates and Plyfa work without problems (since it has worked without reinforcements for 35 years, the loads are not that high).

 

[daugaard]

It is not a truss. A truss consists of triangles and all loads are (ideally) only compressive or tensile loads, except for the load on top of the roof of course, which gives some bending load on the top chord. In the trusses in the pictures, most of the load is transferred down to the bottom chord, which gets bending point loads at the supports. It can be OK if the bottom chord is sized as a beam that can take all that load, but apparently, that wasn't quite the case here.

Shouldn't it have come apart a little at the bottom of the ridge joint if the trusses had collapsed? It seems to look tight and fine anyway.

It would be interesting to see a picture with a straightedge lying on the bottom chord to see how much it sags.

 

[Stefan1972]

The partner's parents do indeed have a smaller garage, but it looks the same, and the whole neighborhood looks like that. If you stand in one of their bedrooms and look out over the garage roof, it looks ridiculous. The ridge sags quite a bit, and the same goes for the roof. The area was built around 1975 and it has apparently always looked like that. With today's smaller and smaller amounts of snow, it should be a non-issue strength-wise, even though it certainly looks odd.

 

[Kicken]

Is it an optical illusion or are the upper and forearm side by side?

If so, it is difficult to get it right with plywood if you do as in the pictures because the braces will become slanted.

 

[f91jsw]

Regardless of how the construction looks, there are no indications in the pictures that it wouldn't handle the loads.

According to TS, the entire roof has settled, we should probably assume that.

 

[Autodidak1]

According to TS, the entire roof has settled, so we'll have to assume that.

I'm not questioning that the roof has "settled." But I don't believe the roof is dangerous in any way, which I base on the following:

In the pictures, it looks like the joints are intact and have not been pushed apart by loads
Fairly robust dimensions for such a small span
The construction is NOT unusual (but also not the most common)
It has held up for 35 years, why would it be dangerous now? Looks dry and in good condition in the pictures

This is my OPINION (guess?) without having LOOKED at it in REALITY without measurements and checks.

It is COMMON for roofs to settle over time. The trusses can compress over time and even unevenly. But most commonly, it's especially the battens that give way over time.

I amused myself by looking at the houses along the street. Of the houses that haven't replaced their roofs in the last 15-20 years, almost half have wavy roofs (probably battens sagging) and about 10-15% of them have a visible sag across the entire ridge due to the TRUSSES settling. These houses were built from the 1940s up to the 1970s.

But there is a SMALL risk that TS has problems with the roof structurally, but I don't think that's the case.

 

[Matstj]

I had roughly the same issue with a patio roof with concrete tiles that had settled about 20 cm and created backfall.
Removed the tiles and lifted the roof with a couple of jacks. Reinforced with an extra post and put the tiles back.
It went great.