After the good advice I was given re planning and a carport, here is the next question.

When I build the carport, it will have a side-to-side span of 5.5m which I really do not want to have a central support.

I plan to use 0.7 steel roof panels, nice and light. But the almost 6m span will need something strong to support the roof, and allow for snow (ha, if ever) and the dead load of the roof structure.
Cladco reckon their 170mm (i.6mm thick) Z purlins will span 6m. And I have seen plans that suggest 150x50mm timber would also be OK, though looks a little small to me.
However, I am wondering about 48mm scaffold poles. Possibly 2 welded together in an 8 as that would keep the profile low, and be affordable. I realise I beams would be ideal, but costwise would be rather hefty

Any other ideas/advice would be very welcome

A big no no on the scaffold tubes, just buy the correct kit as recommended

Shooter

I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large. And 150x50 timber is again flimsy at that length. Or maybe I am paranoid!

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Originally posted by cliftyhanger
I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large.!

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If you take a look at Building Regulations Approved Document A 1991, you will find a set of span tables at the back (later versions omit the tables, but I don't know when they dropped them).

A quick glance suggest that you are looking at SC4 timber, 220mm x 63mm, with 400mm spacing, for a 5.5 span - which coincidentally is the same as our conservatory and the same timbers that I used for a flat-roofed section of that.

[Edited on 7/10/20 by SteveWalker]

Thats a big carport, but they are great places. Good luck with the build

Why a steel roof though? Would you not benefit from the daylight of a clear one?

And I am interested why scaffolds are such a no, even though I agree they are not the right thing

Clear roof aesthetics are not great. Corrugated plastic looks very 1960's. and polycarb is no favorite of mine.
But the biggest issue is the "geography". The driveway at the pavement end drops buy almost 1/2m over its width, and although on the upper side it only drops 70mm from front of garage to the pavement, the other side the drop is again 1/2m. Upshot is the roof will be very visible to pedestrians etc as it will have to slope down from garage to pavement (well, 1m back from pavement)
It also means that to keep to no-planning rules, 2.5m on the downhill side means only 2m on the top. Take off 200mm for the structure and I (being a shortarse) will just about be able to clear my head.

But all that is moot. The wife likes the look of the sheet metal roofs, especially when edged with the large trims. However, tomorrow she is popping in to the neighbours for a cuppa, be interesting to see how that goes. The other option is OSB and fibreglassing.

Looks like z purlins are the way to go according to https://steadmans.co.uk/wp-content/uploads/2019/10/Purlin%20load%20tables.pdf

quote:

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Originally posted by SteveWalker
If you take a look at Building Regulations Approved Document A 1991, you will find a set of span tables at the back (later versions omit the tables, but I don't know when they dropped them).

A quick glance suggest that you are looking at SC4 timber, 220mm x 63mm, with 400mm spacing, for a 5.5 span - which coincidentally is the same as our conservatory and the same timbers that I used for a flat-roofed section of that.



[Edited on 7/10/20 by SteveWalker]

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Originally posted by ReMan
Thats a big carport, but they are great places. Good luck with the build

Why a steel roof though? Would you not benefit from the daylight of a clear one?

And I am interested why scaffolds are such a no, even though I agree they are not the right thing

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Originally posted by bi22le

quote:

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Originally posted by ReMan
Thats a big carport, but they are great places. Good luck with the build

Why a steel roof though? Would you not benefit from the daylight of a clear one?

And I am interested why scaffolds are such a no, even though I agree they are not the right thing

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I would speculate that the comment was based on a few points.

- The 6m span gap on 50mm dia tubes is a lot to ask, even if they dont fail, they would displace a fair amount causing failure at the connected ends
- They are design to work in compress tension and not so much bending (they are used this way, but with reduced distances) and as such a re made to that level. They may be seamed and of low grade.

As an additional to this though, I had my loft extended last year and they put a "tin top" on so they could work at a faster rate. This spanned my complete house as was obviously built to withstand wind rain snow etc. It had ALOT of support using special I beam style sections. Imagine two parallel poles approx 400mm apart with zip zag stays between them.

So the original suggestion is not mad, but you need depth to your support. look at 2nd moment of area equation to understand that depth is the most important thing for rigidity.

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quote:

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Originally posted by cliftyhanger
I am not sure what is the correct thing though.The Z purlins just appear flimsy, and a 6m span is large. And 150x50 timber is again flimsy at that length. Or maybe I am paranoid!

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Maybe we could all pitch in an have a go at playing engineers.

If we took a load of 0.25kN/m² for the sheeting and maybe 0.6kN/m² for snow and a span of 5.1m

We could assume that its simply supported so to get the bending moment its WL/8 (thats big W) or wL²/8

so thats (0.25+0.6) x 5.1² / 8 = 2.763 kNm per 1m width and 1.1052 kNm per 0.4m or joist.

we need now to work out a Z required.

From the internet it looks like for timber Z = M/permissable bending stress parallel to grain

permissable bending II to grain = grade bending II to grain and some K factors, by the look of it.

so if we're using C24

thats
grade bending = 7.5
K2= moisture, well its out side so thats looking like class 3=0.8
K3=duration maybe 1.0 long term?
K7=depth factor (300/h)^0.11, should we guess 150 deep as its a regular size? (300/150)^0.11 = 1.07
K8=load sharing, we got a load of these at 400c/c so 1.1?

So Z = 1.1052 x 10^6 / (7.5 x 0.8 x 1 x 1.07 x 1.1) = 156499mm³ or 156x10³mm³

47x150 C24 = 176x10³mm³ so they look like they're strong enough at 400 centres

But what of the deflection and the little bit of wind that might push down on it ?????

I'm sure we can work this out.

[Edited on 8/10/20 by pigeondave]

quote:

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Originally posted by pigeondave
Maybe we could all pitch in an have a go at playing engineers.

If we took a load of 0.25kN/m² for the sheeting and maybe 0.6kN/m² for snow and a span of 5.1m

We could assume that its simply supported so to get the bending moment its WL/8 (thats big W) or wL²/8

so thats (0.25+0.6) x 5.1² / 8 = 2.763 kNm per 1m width and 1.1052 kNm per 0.4m or joist.

we need now to work out a Z required.

From the internet it looks like for timber Z = M/permissable bending stress parallel to grain

permissable bending II to grain = grade bending II to grain and some K factors, by the look of it.

so if we're using C24

thats
grade bending = 7.5
K2= moisture, well its out side so thats looking like class 3=0.8
K3=duration maybe 1.0 long term?
K7=depth factor (300/h)^0.11, should we guess 150 deep as its a regular size? (300/150)^0.11 = 1.07
K8=load sharing, we got a load of these at 400c/c so 1.1?

So Z = 1.1052 x 10^6 / (7.5 x 0.8 x 1 x 1.07 x 1.1) = 156499mm³ or 156x10³mm³

47x150 C24 = 176x10³mm³ so they look like they're strong enough at 400 centres

But what of the deflection and the little bit of wind that might push down on it ?????

I'm sure we can work this out.

[Edited on 8/10/20 by pigeondave]

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I would use fully treated wood. I didn't and there was a lot of warpage even over two 2.5 metre spans and horizontal batons are a must. My span was 5 metres using 220mm floor joists. The whole carport was 6m long. Eventually I removed the central support and it was still more than strong enough, even jumping up and down on the roof it didn't budge.

Originally I used clear corrugated sheet for the roof but it was noisy and banged in the wind (it's always windy here), then went for a wooden roof but the felt covering leaked. If I was to do it again I'd have used galvanised corrugated steel. I took it down after about 8 years as it was right in front of my living room and spoilt the view.

This is the sketchup 3D model I used to work it all out before buying the wood. It worked very well and helped heaps.




[Edited on 9/10/20 by Mr Whippy]

Have a look at precast garages just to give you a idea of what holders the roof up
Jacko