No, I also don't think it could be that dangerous. However, I believe that if something ever collapses, it will, according to "the absolute law of everything's wickedness" or "if something can go wrong, it will" (yes, I work somewhat within IT), almost certainly collapse just as someone steps out of the car...Mikael_L said:
Yes, I should perhaps apologize, it naturally has no direct connection.Nissepisse said:
But it's a good example of what can happen if a construction isn't thoroughly thought out, and the potential risks that might be built into it.
Regarding the bridge, it was a perfect, beautiful, and sustainable construction in itself.
But then a boat came, which in fog and drift ice accidentally touched one of the supporting tubes a bit with a crane or maybe with one of the bridge wings.
It wasn't that this 27,000-tonner rammed the bridge.
There was a small dent in the supporting steel arch, and the bridge collapsed.
What I'm trying to say is that if you look for clever shortcuts in design, you take risks.
Just like you do when you stand on a plank and assess a roof's load-bearing capacity from that.
How much does the roof weigh, including soil, roof leak, sedum, grass, and whatever it might be?
What is the roof angle? How much snow load should the roof be designed for?
And, looking at the bridge analogy, what happens if someone accidentally "touches" one of the carport pillars?
But why put a roof over the car at all
I don't think so, you can say what you want.KnockOnWood said:
The goal hasn't been to find shortcuts. I started with the question of how much it can handle, but despite reaching page 2, I still haven't received an answer that is anywhere near what I was asking for....KnockOnWood said:
Because it mostly sits here, >90% of the time, maybe 100% during the winter, and I imagine cars last longer and better if they are under a roof protected from rain, snow, cold, and falling branches and other debris. A couple of 1000kr on a roof seems like a good investment.KnockOnWood said:But why even put a roof over the car
So, once again, is there anyone who can answer the question:
How much can 20mm råspont hold?
In other words:
What is the max load X [kN/m2] with given cc [m] on the roof beams and given roof angle (can be set to 0 degrees)?
or the other way around:
What is the max cc [m] one can have on the roof beams with a given load X [kN/m2] and given roof angle (can be set to 0 degrees)?
If you are keen on really learning something and ready to invest some time, this page might be a starting point: http://www.traguiden.se/TGtemplates/GeneralPage.aspx?id=202
Having some basic knowledge in statics and mechanics will naturally make it easier.
But there is quite little magic in this.
Calculating loads, forces, fractures, and deflection, etc., is just common statics and mechanics.
In wood construction, a lot of values for the load-bearing capacity of different types of joints, etc., are added. These are often empirically derived and tabulated.
And then there are some standards and limits to consider, such as serviceability limit states, ultimate limit states, temporary inconvenience, progressive collapse, etc., etc.
A bunch of new words and concepts for those who know mechanics and statistics but suddenly slide into wooden house construction.
Having some basic knowledge in statics and mechanics will naturally make it easier.
But there is quite little magic in this.
Calculating loads, forces, fractures, and deflection, etc., is just common statics and mechanics.
In wood construction, a lot of values for the load-bearing capacity of different types of joints, etc., are added. These are often empirically derived and tabulated.
And then there are some standards and limits to consider, such as serviceability limit states, ultimate limit states, temporary inconvenience, progressive collapse, etc., etc.
A bunch of new words and concepts for those who know mechanics and statistics but suddenly slide into wooden house construction.
Ok, thanks! But it sounds like a pretty long and complicated path. I already have an engineering degree behind me and had hoped to avoid another one just to build a simple roof. But I guess I'll have to study the subject further. Structural engineer might be my new future; there seems to be a lack in the field.
Otherwise, I think it's a pretty simple calculation in my case, if one has all the background information. A calculation that I also think should be used quite often by those who do it professionally or as a hobby. I actually think that to be called a "Carpenter," one should be able to answer my question since it's so fundamental. Someone should have been able to come up with simpler models that work as a basis without having to know everything behind them. A lot of wooden structures are built in this country, and the value of optimizing construction—not under-dimensioned so it holds, and not over-dimensioned so that unnecessary resources are used—is not negligible. Moreover, the dimensions and strength of the timber sold are fairly standard, so the possibility certainly exists. But, in the meantime, I'll dive deep into the study of wood strength theory...
Otherwise, I think it's a pretty simple calculation in my case, if one has all the background information. A calculation that I also think should be used quite often by those who do it professionally or as a hobby. I actually think that to be called a "Carpenter," one should be able to answer my question since it's so fundamental. Someone should have been able to come up with simpler models that work as a basis without having to know everything behind them. A lot of wooden structures are built in this country, and the value of optimizing construction—not under-dimensioned so it holds, and not over-dimensioned so that unnecessary resources are used—is not negligible. Moreover, the dimensions and strength of the timber sold are fairly standard, so the possibility certainly exists. But, in the meantime, I'll dive deep into the study of wood strength theory...
The question was about center-to-center (cc) spacing between roof beams with 20 mm decking. As previously mentioned, it is approximately 1.2 m. What the roof can then support depends on the dimension of the roof beams. There is excellent information about this in the wood guide that Mikael_L linked to, which often depends on the snow zone.
Sure, the distance between the beams can be increased, but then the thickness of the decking should also be increased accordingly. I believe there is 23 mm decking, which would allow you to have about 1.4 m between roof beams. I have a feeling that the choice of relatively thin decking and a spacing of 1.2 meters between beams is economically driven (decking cost + roof truss cost has a minimum).
Sure, the distance between the beams can be increased, but then the thickness of the decking should also be increased accordingly. I believe there is 23 mm decking, which would allow you to have about 1.4 m between roof beams. I have a feeling that the choice of relatively thin decking and a spacing of 1.2 meters between beams is economically driven (decking cost + roof truss cost has a minimum).
But that was exactly the answer you got at the beginning. 1.2 meters between the rafters. That's what a carpenter has learned and knows. He never stands out on a site and starts calculating what the sheathing can handle; he already knows that. A lot of this "knowledge" is ingrained in the craftsmen or is somewhat simplified in tables or so when it comes to more complex matters. As soon as pure calculations are involved, the task goes to a structural engineer, so the carpenter then only has a drawing to relate to.Nissepisse said:
I got the impression that you wanted to know how to calculate, and it's common statics and mechanics as well as the other aspects concerning wood house building, and most of that you can find in the wood guide. Then there are books and training courses.
But if you ask me about the distance between the rafters for the sake of the sheathing, I would answer 1.2 meters. With the addition that if it's not such an "important" building, you can probably increase it a bit, e.g., 1.4 or 1.5 meters if it is 21mm sheathing, but a maximum of 1.2 if it's 17 mm sheathing.
We are talking about a flat earth roof. The roof itself must be the heaviest thing found in small houses, right? Additionally, there will always be snow loads, and in this case, something you never usually account for: rain.
50 mm (50 kg/sqm) of rain doesn't come often, but when it does on this roof, it won't disappear quickly.
50 mm (50 kg/sqm) of rain doesn't come often, but when it does on this roof, it won't disappear quickly.
..but 50kg, 0.5kN/m² are not terrible loads even on the roof. The question "how much can råspont withstand" might not be very common, as for its normal usage it clearly suffices. But now soil of unknown quantity is to be added on top of the normal loads. Then one has to start understanding how wood behaves with long-term loads as well. It might work out fine.
In my case, I have thought of 3kN/m2 as the max load. A maximum of 10cm porous soil + some snow on it in a sheltered location in Skåne. I will have less than 1m cc below and consider that the wood paneling will hold. If it starts to sag, I will reinforce it afterwards from below.
But I feel the topic is barely covered. Just saying that 1.2m, maybe even more, will hold without evidence is not right. I posed the question as a general inquiry without any specific values and would have preferred an answer accordingly, maybe a general formula or table with the parameters: cc, load, roof angle, and thickness. There is surely a factor difference of at least 5-10 in the stress on the paneling between a 45-degree metal roof in a windless location in Skåne and a flat turf roof up in snow zone 4.5. Somewhere, there is a limit to what the paneling can handle, and where is the limit when it is okay to go down to 17mm? I don't believe for a second that every carpenter has this clearly in their muscle memory, as there are a few parameters that need to be considered.
Furthermore, companies like Byggmax (and possibly others) have reduced from 22mm to 20mm; is that still okay?
Then I can see a big advantage in assuming that the paneling holds for the worst roof in Sweden so that one can proceed with that and be able to stick to 1.2m to fit insulation and boards without thinking about it. But it also means many roofs are grossly over-dimensioned, and that also raises questions for me: who came up with this? how was it determined? where can you access it? (I haven't read through everything on the wood guide, I will do so)
I have also realized that my questions probably won't be answered in this thread, so I will leave it and read it when the ski season is over and new carpentry projects are on the agenda. Thanks for your interest anyway, and let me be enlightened in April/May
But I feel the topic is barely covered. Just saying that 1.2m, maybe even more, will hold without evidence is not right. I posed the question as a general inquiry without any specific values and would have preferred an answer accordingly, maybe a general formula or table with the parameters: cc, load, roof angle, and thickness. There is surely a factor difference of at least 5-10 in the stress on the paneling between a 45-degree metal roof in a windless location in Skåne and a flat turf roof up in snow zone 4.5. Somewhere, there is a limit to what the paneling can handle, and where is the limit when it is okay to go down to 17mm? I don't believe for a second that every carpenter has this clearly in their muscle memory, as there are a few parameters that need to be considered.
Furthermore, companies like Byggmax (and possibly others) have reduced from 22mm to 20mm; is that still okay?
Then I can see a big advantage in assuming that the paneling holds for the worst roof in Sweden so that one can proceed with that and be able to stick to 1.2m to fit insulation and boards without thinking about it. But it also means many roofs are grossly over-dimensioned, and that also raises questions for me: who came up with this? how was it determined? where can you access it? (I haven't read through everything on the wood guide, I will do so)
I have also realized that my questions probably won't be answered in this thread, so I will leave it and read it when the ski season is over and new carpentry projects are on the agenda. Thanks for your interest anyway, and let me be enlightened in April/May
These are definitely interesting thoughts you have.
But somewhat illogically, you try to jump between wanting rules of thumb and then how to calculate.
These are completely different things, and we have already presented the rule of thumb, so it remains to calculate the tedious way if you want to know/learn more.
Take these material data:
http://www.traguiden.se/TGtemplates/popup1spalt.aspx?id=1105&contextPage=4962
http://www.traguiden.se/TGtemplates/popup1spalt.aspx?id=1099&contextPage=4962
And then you apply an appropriate safety factor, considering it's not perfect wood (raw board is generally of quite poor quality) at about 2-3 something.
Then you just have to calculate with distributed load, deflection, and fracture limit.
You can get back with the results later.
But somewhat illogically, you try to jump between wanting rules of thumb and then how to calculate.
These are completely different things, and we have already presented the rule of thumb, so it remains to calculate the tedious way if you want to know/learn more.
If you already have an engineering degree, it might not be that difficult.Nissepisse said:
Take these material data:
http://www.traguiden.se/TGtemplates/popup1spalt.aspx?id=1105&contextPage=4962
http://www.traguiden.se/TGtemplates/popup1spalt.aspx?id=1099&contextPage=4962
And then you apply an appropriate safety factor, considering it's not perfect wood (raw board is generally of quite poor quality) at about 2-3 something.
Then you just have to calculate with distributed load, deflection, and fracture limit.
You can get back with the results later.
Mikael_L, please clear your messages, I've tried to send to you but the inbox is full....
I think you're thinking a little backwards. The roof will always be burdened since it's an earth roof that will likely be damp and heavy. So why not reduce the distance to 1 m or even 60 cc and use better dimensioning. You save minimal money with the argument of increasing the distance between the roof joists, but instead increase the risk of sagging on the board. Trying to retrofit additional joists afterwards becomes very complicated and time-consuming, which eats up the profits. Think the opposite instead.