# Forum Home Renovation Roofing  C section roof truss design

## .RC.

I am currently designing a shed lean to and I was thinking rather then build roof trusses out of pipe, I will instead use C section purlins, back to back, like you see in modern shed construction. 
The question is what size C section to use. 
The truss length will be 5.5metres and the span between trusses is 4.1m. 
The question is would C15015 back to back be adequate or do I need to go to C200 size.  Roof angle will be somewhere slightly more then 7 degrees.  And I do not want to use any angled supports from the legs to the truss. 
I already have C15015 for the purlins. 
Is there a calculator out there that does calculations for you?

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## cyclic

100 x 15 will span 5.5 m and carry a colorbond roof, so why bother doubling up back to back.
Obviously it is not being approved, so just build a flat frame using the 150. 
Marc put some tables up recently, have a look through Marc's recent posts.

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## .RC.

The wording in my original post might be a bit hard to understand. 
It is a skillion roof.  The whole size is about 12.4m X 11m.  There will be a total of eight posts and four posts where it is coming off the existing shed, so twelve total. The existing shed it is coming off is 12.3m (4 posts at 4.1m centres) wide. 
So the width between columns is 4.1m , and the beam length will be 5.5m, and there will be a total of eight beams.  The roof is going to be corrugated iron.    
It is what to use for the beams is the info I am looking for. 
For the purlins I was going to use C150 1.5mm thick, which I know is overkill, and they will be spaced around 1m or 1.2m apart from memory. 
A Lysaught publication seems to say I could use C150 2.4mm thick for the beams.  But that is not for a corrugated iron roof. 
Makes me wonder if I could go to C100 for purlins, which would be lighter.

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## Marc

Building Trusses in steel is not as easy as asking the span for single members. There are many considerations like how you are joining them, how are you erecting them, how are you bracing them, I'm afraid the best you can do is to take your drawings to an engineer. They have software for this things that gives quick answers and in this way you have it go through council if that is your plan. 
You can blow a lot of money unnecessarily by oversizing steel structures and for no real gain, particularly with roofs.

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## Marc

PS
Just read your second post, so no trusses, you are building a flat roof. 
Have a read here, plenty of useful info  http://www.bluescopesteel.com.au/fil...ablesjan10.pdf

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## .RC.

Thanks.  I think I am understanding the load tables more now.  Pity they are all in kilonewton metres and not kg. 
For purlins, if we take three lapped spans of z or C150 1.5mm thick.  An even load capacity of inward pressure with a span of 4200mm with one set of bridging it is 4.58knm.  If I am assuming correctly a rule of thumb is multiply that by 100 (98 would be closer) to get kg and it is 450kg capacity.  The actual roofing iron loading per purlin is approx 20kg.  Of course there is rain loading to go on top of that, but it would appear the 15015 is a bit overkill for the purlins. 
Z10015 come in at 2.56knm or I guess 256kg. 
The info also gives the equations to work out point loading.  So I should be able to work out what I can use for beams to hold the roof up, that is provided 1knm equals 98kg. 
BTW no requirements for council or anything for what I am doing.

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## Marc

Yes, multiply by 100. 
No, roof takes way more than it's own weight or the rain flowing on it. The wind is the major load to consider on a roof plus the safety of anyone walking on it. 
I am not a structural engineer so very reluctant to suggest the right size of steel for your roof. 
Let me tell you a short story. i once built a shed under a red gum of very generous proportions. I was reluctant to cut it so built the shed over engineering the roof rafters by 3 times, not only that but also bracing them to the wall frame.
The council inspector took it on himself to make fun of my frame and openly joking how this wog could build this way.
Not 5 years later the gumtree decided to let go of one of it's branches 20" in diameter and about 12 meters long landing on my roof. Had I built the shed to minimum standards I would be very dead. 
So when there are minimum standards there are also other considerations and you as the builder are the one needing to decide. 
That publication is very helpful and unusual for steel structures that usually are much more confusing and designed for engineers.

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## .RC.

I guess I will work it out then.

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