hello!
I have a small garage where the roof has sagged significantly.
Home-built trusses with a span of about 8.5 meters. The width of the garage is about 6.6 meters.
The roof is quite flat and has probably sagged about a decimeter in the middle...
I am considering inserting a beam under the ridge and then shimming under the existing trusses until it is straight.
Possibly also two additional beams roughly in the middle of each side of the roof, to ensure it's secure.
However, I've been thinking about using steel, glulam, or my latest idea, why not a timber log.
If I place posts on the inside of the walls, the beam needs to be about 6.4 meters, and glulam is usually 6 meters =(
But why not go to the forest and cut down a large spruce and trim a log to 6.6 meters?
How thick does the log need to be to correspond to, for example, a 90x315 glulam beam?
I have a small garage where the roof has sagged significantly.
Home-built trusses with a span of about 8.5 meters. The width of the garage is about 6.6 meters.
The roof is quite flat and has probably sagged about a decimeter in the middle...
I am considering inserting a beam under the ridge and then shimming under the existing trusses until it is straight.
Possibly also two additional beams roughly in the middle of each side of the roof, to ensure it's secure.
However, I've been thinking about using steel, glulam, or my latest idea, why not a timber log.
If I place posts on the inside of the walls, the beam needs to be about 6.4 meters, and glulam is usually 6 meters =(
But why not go to the forest and cut down a large spruce and trim a log to 6.6 meters?
How thick does the log need to be to correspond to, for example, a 90x315 glulam beam?
Member
· Norrbotten
· 3 390 posts
It's not a problem to get glulam in 8m lengths if you want.
A spruce log can indeed be stable, but it becomes very heavy, and its own weight will cause it to bend a bit. The fastening of a log is also a bit more "fiddly". If I were to estimate, you might need a cross-sectional area twice as large on the log as the equivalent glulam beam, because the glulam much better maximizes load-bearing capacity, both due to its standing form and glued joints.
I like old-fashioned solutions in the right environment. But in a garage, I wouldn't have bothered.
A spruce log can indeed be stable, but it becomes very heavy, and its own weight will cause it to bend a bit. The fastening of a log is also a bit more "fiddly". If I were to estimate, you might need a cross-sectional area twice as large on the log as the equivalent glulam beam, because the glulam much better maximizes load-bearing capacity, both due to its standing form and glued joints.
I like old-fashioned solutions in the right environment. But in a garage, I wouldn't have bothered.
For me, it doesn't matter if the log itself bends... you can simply prop it up so that the roof is straight.
A log with a 40 cm diameter on one end and 30 on the other might be something... because I don't care about cutting it like for a timber house, it can remain conical.
And you should let it dry for about a year before installing it, then its weight should decrease, maybe you should even let it "hang" at each end, so that the sagging happens before it's installed.
Am I thinking about this entirely wrong? In the house (timber house) I have a rafter roof as I understand it, where there are 3 logs, one ridge and one in the middle of each roof half, stretching from gable to gable with support in the middle.
So the spans are shorter, 5 meters maybe...
Sure, you can buy glulam...
but I guess it costs around 10000 if I want 3 solid pieces at 6.5 meters.
Then I'd rather cut down a spruce if it would work just as well.
A log with a 40 cm diameter on one end and 30 on the other might be something... because I don't care about cutting it like for a timber house, it can remain conical.
And you should let it dry for about a year before installing it, then its weight should decrease, maybe you should even let it "hang" at each end, so that the sagging happens before it's installed.
Am I thinking about this entirely wrong? In the house (timber house) I have a rafter roof as I understand it, where there are 3 logs, one ridge and one in the middle of each roof half, stretching from gable to gable with support in the middle.
So the spans are shorter, 5 meters maybe...
Sure, you can buy glulam...
but I guess it costs around 10000 if I want 3 solid pieces at 6.5 meters.
Then I'd rather cut down a spruce if it would work just as well.
Member
· Västerbottens län
· 18 024 posts
Now you probably priced it quite a bit around 1000 - 1500 each I believe more in.
Protte
Protte
Checked 115x405 and they were around 3000 something on the only site with prices... possible to find it cheaper but...
Byggmax 6 meters costs 1700:- 90x315
Byggmax 6 meters costs 1700:- 90x315
If a spruce log had the same strength value as glulam, it would need to have a diameter of about 30 cm to correspond to a glulam beam of 90x315. However, it does not, and since the specific spruce log's condition is also unknown, one needs to compensate for all uncertainties. It could result in a very large log. I don't think that feels particularly realistic. Moreover, I think it's starting at the wrong end. A common reason for garage roofs sagging is poor roof pitch combined with too weak rafters and incorrectly placed collar ties. A better method is often to pull the trusses together with a wire and possibly reinforce the rafters.
hmm if they had the same strength, then the log should handle more if it has the same diameter as the glulam's height...
but as you say, it's hard to calculate and not something I can do myself,
the ones that are there were likely home-built in the 80s, regular timber, and very few nails.
also, quite a small roof slope
so I have supports inside during the winter to avoid any risks.
tying together with wire and reinforcing the rafters can absolutely be an option, where can I read more about this method?
alternatively, of course, is to tear off the whole roof and put in new trusses, it needs to be renovated anyway before new roof sheeting can be installed.
the under-roof of masonite isn't the best either...
but then you have to pretty much empty the whole garage of stuff... and new ceiling, insulation, etc...
but as you say, it's hard to calculate and not something I can do myself,
the ones that are there were likely home-built in the 80s, regular timber, and very few nails.
also, quite a small roof slope
so I have supports inside during the winter to avoid any risks.
tying together with wire and reinforcing the rafters can absolutely be an option, where can I read more about this method?
alternatively, of course, is to tear off the whole roof and put in new trusses, it needs to be renovated anyway before new roof sheeting can be installed.
the under-roof of masonite isn't the best either...
but then you have to pretty much empty the whole garage of stuff... and new ceiling, insulation, etc...
I compared the bending resistance of the glulam beam with that of a hypothetical spruce log using the formula bxh^2/6. I simplified the calculation by considering a square spruce beam that fits inside a round log and arrived at a diameter of 29 cm. However, a spruce log can withstand much lower bending stress than a glulam beam, perhaps only half, depending on fiber density, amount of knots, etc. The example was mainly intended to show that this variant was not particularly realistic.
If the total span of the truss is 6.6 meters and you have 1.2 meters between the trusses, the trusses' rafters, in the worst case, would need to measure 45x220 and be of strength class C24. I have assumed that you live in Gävle and that the roof pitch is very low. I don't think you can go below 45x195 even if the roof pitch is 30 degrees.
The sagging of the roof is probably mainly due to poor collar ties (dimension, attachment, placement, etc.). With the help of a winch, you can draw the trusses together and attach a steel tie rod as close to the eaves as possible. The simplest form of such a rod is a regular flat steel that you screw into the sides of the rafters.
I completely understand that the idea of emptying the garage is repulsive...
If the total span of the truss is 6.6 meters and you have 1.2 meters between the trusses, the trusses' rafters, in the worst case, would need to measure 45x220 and be of strength class C24. I have assumed that you live in Gävle and that the roof pitch is very low. I don't think you can go below 45x195 even if the roof pitch is 30 degrees.
The sagging of the roof is probably mainly due to poor collar ties (dimension, attachment, placement, etc.). With the help of a winch, you can draw the trusses together and attach a steel tie rod as close to the eaves as possible. The simplest form of such a rod is a regular flat steel that you screw into the sides of the rafters.
I completely understand that the idea of emptying the garage is repulsive...
A spruce log with a 40 cm diameter is also not that hard to come by.
I just think that all barns/sheds, etc. are built with logs and spans significantly longer than my 6.5 meters.
However, the roof trusses are 8.5 meters if I remember correctly (it's gable-to-gable which is 6.5). I guess the trusses are built with 2"6, possibly 2"5 rough-sawn timber, so there's obviously the explanation for them sagging. But they also bow down inside the garage, not too much, but the ceiling probably hangs down 5 cm in the middle.
If I tension the truss as you described, it feels to me like the ceiling will bow down even more?
Sure, it's repugnant to empty the garage, but maybe that's the easiest way to go anyway.
Everything above the trusses has to be replaced anyway; there's masonite and thin battens that also bow when you walk on the roof. I'm guessing the battens are 25x120 rough-sawn, on cc40-50 I guess.
I will try to take some pictures of how it looks.
There are probably several solutions that can be used.
Maybe, for example, you could buy new trusses and place them beside the old ones, avoiding removing the ceiling that all the shavings insulation lies on.
Yes, as I said, I'll take some pictures and post them so you can help me figure out a good solution
I just think that all barns/sheds, etc. are built with logs and spans significantly longer than my 6.5 meters.
However, the roof trusses are 8.5 meters if I remember correctly (it's gable-to-gable which is 6.5). I guess the trusses are built with 2"6, possibly 2"5 rough-sawn timber, so there's obviously the explanation for them sagging. But they also bow down inside the garage, not too much, but the ceiling probably hangs down 5 cm in the middle.
If I tension the truss as you described, it feels to me like the ceiling will bow down even more?
Sure, it's repugnant to empty the garage, but maybe that's the easiest way to go anyway.
Everything above the trusses has to be replaced anyway; there's masonite and thin battens that also bow when you walk on the roof. I'm guessing the battens are 25x120 rough-sawn, on cc40-50 I guess.
I will try to take some pictures of how it looks.
There are probably several solutions that can be used.
Maybe, for example, you could buy new trusses and place them beside the old ones, avoiding removing the ceiling that all the shavings insulation lies on.
Yes, as I said, I'll take some pictures and post them so you can help me figure out a good solution
I'm a bit curious about the strength of round logs as well. I have an old timber house with a poor roof that I hope to save. From what I understand, roundwood has better strength than sawn timber. It's kind of grown to absorb bending forces. The sapwood is stronger than the heartwood. On a round log, the sapwood is situated where the load is greatest. The tree has also compensated for knots as it grows. So a round log has advantages. The disadvantages are that a round cross-section is not optimal for taking up load in one direction. Compared to glulam, it doesn't have the same knot problem as regular sawn timber does. The glue joints hardly add any bearing capacity.
I found a task from Finland that assumes roundwood has a strength class of C30, while unsorted sawn timber is at C24. Glulam is often classified as L40. Which I think is on the same scale.
but how can this then be translated into some sort of strength calculation?
the stock is round, and should therefore have an advantage, right?
the stock is round, and should therefore have an advantage, right?
Member
· Västerbottens län
· 18 024 posts
If the ridge has settled, then the rafters have pushed out the walls since the tie beam has come loose from the rafter and the braces from the ridge and the rafter are now pressing down the ceiling.
The root of the problem must be that the tie beam has more or less come loose.
Check if the walls are bulging, if so, help up the ceiling and pull the walls together with wire or flat bar or whatever.
Protte
The root of the problem must be that the tie beam has more or less come loose.
Check if the walls are bulging, if so, help up the ceiling and pull the walls together with wire or flat bar or whatever.
Protte
The figure is bending parallel to the fibers. The unit is MPa. But that isn't really self-explanatory either.
The quantities used to assess different aspects of strength, bending resistance, and moment of inertia favor height over width. The amount of material in a log with a 30 cm diameter could be enough for a beam measuring 10x70 cm, which would have twice the bending resistance of the round log. As for use in horizontal load-bearing structures, the log is probably not optimal. However, it is clear that a log used as a column has the advantage of not having a weaker buckling direction.