Extra wide spacing between steep roof trusses?

[Nötegårdsgubben]

I'm considering building a carport in the countryside against the short side of an outbuilding. I happen to already have three nice roof trusses from an old henhouse with a pitch of maybe 40 degrees. Snow load zone: 2.5. Planned roof covering: corrugated sheet metal.

If the trusses are set with cc 1200, the carport will be only 3.6 meters plus the overhang on the last truss, which is a bit short. However, if you go up to cc 1800, you suddenly reach 5.4 meters, which feels quite generous.

Of course, I can make or buy an extra truss, but how big is the problem with increasing the cc measurement really?

 

[Åsa Lund]

Three rafters cc 1200 give 2400 plus overhang

 

[Rabbithole]

You must count N+1 to get the correct number, so for 4.8m it becomes (4.8/1.2)+1=5 trusses

 

[13th Marine]

However, you don't necessarily have to have a roof truss and attach it to the house.

 

[Rabbithole]

True, it is enough to screw 2 reglar to a wall

 

[Nötegårdsgubben]

That was exactly how I thought and calculated. Fasten the studs to the wall and have three loose rafters. Sorry if it was unclear.

 

[justusandersson]

When increasing the c/c spacing between trusses, three things happen. Regular decking boards are no longer sufficient as roofing material; instead, you need thicker dimensions. Thicker k-plywood or thicker grooved timber. The load on each individual truss increases, which may require thicker dimensions. The point load on the wall structure that the truss represents becomes larger, which likely requires compensation. There are many reasons to stick to a maximum of 1200 mm c/c.

 

[Nötegårdsgubben]

I understand that increased distances lead to increased loads, but I'm mostly fascinated that it's so common to recommend cc 1200 regardless of the roof structure.

My thought was even simpler than tongue and groove. The roof on the chicken coop today only has square rods between the metal roof and the rafters, and I have thought about something similar.

My reasoning, right or wrong, was that with a steep pitch on a metal roof, all the heavy spring snow slides off, so the potential loads become less than on a low-pitched roof, for which you have the same cc recommendation. So I was a bit surprised when I couldn't find anyone else with similar thoughts online.

If any construction happens, I'll probably end up copying one of the existing rafters myself—it might be fun to have done at some point—but I'm still fascinated that the answer so often ends up being 1200, regardless of the conditions.

 

[justusandersson]

The modular thinking in Swedish construction has been around for quite a while now. It started with the transition to frame constructions with mineral wool panels in the late 1940s, which required standardization. The Swedish government directed this transition through favorable loans. The reason is said to have been the desire to enable increased export of timber by reducing the construction of massive wooden houses, a parenthesis.

 

[Nötegårdsgubben]

An interesting parenthesis nonetheless, I think. But it doesn't stop me from wondering why one is often so rigid, almost always thinking that cc 1200 suits well for ceilings.

 

[kulle]

It's not always enough with 1200, sometimes you have to go down to 600 cc
If you go up to 1800, the ceiling will sag between the rafters after a year or so, unless you use very strong battens.

 

[Nötegårdsgubben]

I could also increase the rafter dimensions.

As mentioned, I'm not looking to promptly do things in the easiest way possible for me and only use the material I already have, but the advice is so rigid that certain factors can hardly be changed as it is the way it is/"should be" is a bit tricky.

I understand that the suggestion to decrease the truss spacing to cc 600 was a joke, but it's still interesting that the tip is 600 and not 1000 or even 900. The fact that modular thinking is so strong even in DIY constructions of simple sheds is fascinating.

Now, there will probably be a fourth truss anyway since even I feel that 1800 is a bit much, but if I landed at 1300 or 1400 to achieve what I consider a suitable length, I would have built it that way without hesitation.

 

[ArneTW]

Of course, it is possible to increase the distance between the rafters...
There are countless examples of sheds and barns built with slender timber and large distances between rafters.

One can talk about timber quality and experience all day, but what they probably did was compromise on the safety margin.

I would guess that today's recommendations have a safety factor of 2-5, but if you want to approach 1 instead, there is much to save on...

/ATW

 

[Mikael_L]

I understand that the option to reduce the spacing of the roof trusses to cc 600 was a joke, but it's still interesting that the suggestion is 600 and not 1000 or even 900. The fact that modular thinking is so strong even in DIY construction of simple outhouses is fascinating.

No, it was not a joke. It can simply be required in certain cases.
But the most common is exactly cc1200mm. This is the case in about 98% of instances.

And it's not surprising that it has almost 100% prevalence even among DIY builders. There are several good reasons.
Firstly, most people want to do it well and professionally, so they adopt established methods. Then you'd have to be a real masochist to choose anything other than cc1200 or cc600, because that means you'd have to cut every single insulation board, and at least I have more fun things to do than cutting itchy insulation.
Then practically all roof trusses you buy pre-made from a manufacturer will have cc1200, all building descriptions have cc1200.
And as soon as you deviate from this measurement, as mentioned, all sorts of materials stop fitting, and at the same time, all rule of thumb dimensions.

 

[RoBo]

Could also go up in bearing lath dimension.

As I said, I'm not out to immediately do it in the simplest possible way for me and only use the material I already have, but the advice is so rigid that certain factors can hardly be changed as that's the way it is/"should be", it's a bit tricky.

I understand that the alternative to reduce the rafter spacing to cc 600 was a joke, but it's still interesting that the tip is 600 and not 1000 or even 900. It's fascinating that modular thinking is so strong even in DIY projects of simple sheds.

Now, it will likely be a fourth rafter anyway, as even I feel that 1800 is too much, but had I ended up at 1300 or 1400 to get what in my opinion is the right length, I would have built like that without hesitation.

One reason why odd measurements aren't used is because insulation doesn't fit. It requires c/c 600 or c/c 1200.
Besides the dimensioning.