Extra wide spacing between steep roof trusses?

[Mikael_L]

Now I happen to already have three fine rafters from an old chicken coop with a pitch of maybe 40 degrees. Snow load zone: 2.5. Intended roof covering: corrugated sheet metal.

What was the center-to-center distance of the rafters when they were a chicken coop then?

You should be able to have the same at least, they are already tested, so to speak ...

 

[tobbbias]

If nothing else, it can't be that difficult to make 1-2 extra rafters in exactly the same pattern as the ones you have. You have the answer right in front of you like

 

[kulle]

If you want an oil-hardened board under metal sheets or tiles, it's also best to use 600 or 1200. The job simply becomes easier.

 

[Nötegårdsgubben]

No, it was not a joke. It can simply be required in certain cases.

Certainly, it can be required in certain cases, but now it was a rather specific case about which you responded with a blinking smiley. How, if I may ask, did you mean for it to be interpreted if not as a joke?

 

[Nötegårdsgubben]

What was the cc distance of the trusses when it was a chicken coop then?

You should be able to have at least the same, they've already been tested, so to speak...

It's still standing, but I'm not on-site, so I can't measure. I would estimate that there are two sections that are probably 1200 or thereabouts and a narrower one closest to the outbuilding. It's positioned against the same outbuilding that I plan to have everything against, but on the wrong side...

 

[Silver78]

Since most things are adapted for 1200 cc, I would have added an extra rafter. Cut insulation and other things. There are more fun things to do.

 

[Nötegårdsgubben]

I absolutely understand several of the advantages of standardized widths. At the same time, I must say there are some responses in the thread that I think skip over the specific circumstances quite quickly now; I am primarily interested in the roof's load-bearing capacity, not the possibilities of easily insulating or installing panel material. Isn't it fairly obvious that I'm talking about a simpler construction where I'm considering taking parts I already have and building something reasonably functional without relying on standard measurements, or?

As I mentioned, I'm genuinely surprised by the lack of threads where, for example, high cc measurements are discussed. Large spans, with calculations on what type of studs or beams are required for a certain span, are common, but other types of variations where one decreases one variable to increase another are rarer.

I can understand if one advises against low-slope roofs in areas with snow-rich winters, or that for such a roof under similar conditions, one recommends shorter cc distances – nothing strange about that. But that so few in the opposite situation would be willing to discuss the possibility of going the other way, I hadn't expected.

Then the project will likely become so large in the end that it might as well be done properly if it's to be done at all. I honestly started the thread partly because I couldn't find anything on the question no matter how much I googled, not because I had long-term plans to do it this way. So, if nothing else, maybe the next wild brain will find it easier to find arguments against a similar idea now.

 

[Mikael_L]

If you want, I can probably run a calculation in the "takstolsboken" program to see what the dimensioning will be with different cc measurements. However, the program only has the option to choose between 14° and 27° roof pitch, if I remember correctly.

Currently, I don't have it installed on any computer, so I first need to find the installation files and install it, then check.

Especially when machine halls are built with supporting metal constructions, it's not uncommon to have very large cc measurements between the trusses; it can be 4.5 or 6 meters... And they are either uninsulated or insulated with methods other than the usual MU-bales, so these constructions have a few more degrees of freedom.

Personally, I see the main reason for increasing the cc of the trusses is related to the desire for larger cc between vertical supporting parts of the walls, so the force is directed straight down without the need for transfers.

 

[13th Marine]

But it is the case that the truss is sized according to the snow zone, slope, etc., as well as a distance of 120cm c-c.
Of course, one can calculate with other c-c but this is not usually done for the simple reason that c-c 120 is usually preferred there.
If a truss from another snow zone were used, the distance between them could then be changed.

 

[Mikael_L]

A truss for cc1200 in snow zone 1.5 works just as well in cc600 and snow zone 3 ...

 

[witten]

My reasoning, right or wrong, was that with a steep slope on a metal roof, all the heavy spring snow slides off, so the potential loads are less than a low-sloping roof, for which you have the same cc recommendation. So I was a bit surprised when I couldn't google that someone else had similar thoughts.

Yes, that has been considered in the building code, but it might not be easy to google if you don't know exactly what you're looking for. The roof slope, among other things, determines the choice of the shape factor μ, which, alongside a couple of other variables, is multiplied by the basic snow load value. You can have several different values of μ on the same roof.

The standards that govern how snow and wind loads should be calculated for a building are actually quite extensive, and it is not entirely trivial, at least not for a building with a bit more complicated geometry.

 

[Nötegårdsgubben]

If you want, I can probably run a calculation in the program "takstolsboken" to see what dimensioning it would result in with different cc-measurements.
But the program only allows choosing between 14° and 27° roof pitch, if I recall correctly.

Thanks, but it's not necessary. I appreciate the offer.

The machine hall reference is interesting, it's not without reason that I've thought in that direction too. However, nowadays, these are often built with a lot of steel constructions in both directions. And as @justusandersson pointed out, large cc-distances generally mean that regular planking is not sufficient to keep deflection between the trusses down. In any case, I wasn't thinking of regular planking, but battens directly on the trusses, but the reasoning is, of course, the same.

 

[Nötegårdsgubben]

The standards that govern how snow and wind loads should be calculated for a building actually span quite a few pages, and it's not entirely trivial

It's outrageous that one can't just estimate a little. That lilla my has to go and ruin it like that!

 

[witten]

Little My is a true science nerd.

 

[Mikael_L]

The reference to machine halls is interesting, it's not without reason that I've thought along those lines too. But nowadays one often builds with a lot of steel structures in both directions. And as @justusandersson pointed out, large center-to-center distances mean that regular tongue and groove wood generally isn't enough to keep deflection between the trusses low. Now, I wasn't thinking of tongue and groove, but battens directly on the trusses, but the reasoning is of course the same.

Regarding the steel structures of machine halls, it's mostly because it becomes a lot cheaper to use slightly heavier steel profiles and space them wider apart than to place them at 600 or 1200 mm center-to-center in walls, and something similar in the roof. In addition, there are usually a lot of wide doors, and it becomes complicated and expensive to handle a lot of load transfers, when it's just as easy to upsize the steel so easily (and relatively cheaply).


If you go and look at old wooden buildings in agriculture, it's very common to see substantial center-to-center distances on trusses, as they didn't have to worry about the dimensions of insulation boards, and they often took down quite hefty logs from their forest and didn't bother to split them into narrower sizes just to then place them closer together.


Regarding the possibility of simply spacing trusses with larger center-to-center measurements, there's no structural rule or limit for this.
There are, however, material boundaries, where, for example, steel can stretch the extreme limit significantly more than wood.
But whatever you choose, it has different consequences, both above and under the truss itself. You must have a complete grasp of the construction before you can choose optimally. For many smaller buildings, up to large villas, it's often smartest to choose 1200 mm center-to-center, but not always.