Workshop construction advice

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19 Jul 2019
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Hi, I am looking for some practical advice on the "best" (Some subjective balance between cost and strength) way of constructing my new workshop. It will have a 12m horizontal span with a 15 degree roof slope so just over 6.2m from ridge to wall. I am considering four main options (there may be others i should consider so feel free...)
1. Steel portal frame with timber or metal purlins between frames to support a kingspan style insulated metal roof, walls will be waney edge larch on the outside then vapour barrier, then insulation then ply / OSB. I think it will be 5 bays so 6 frames.
2. Studwork style wall, but otherwise as per option 1, with pre made modern roof trusses with 600mm spacing, may or may not need purlins for roof may just fit directly to trusses
3. Studwork style wall, but otherwise as per option 1, with hand cut roof, so much wider spacing than option 2 with purlins to support roof sheets, my dilema is I don't have a clue how to calculate the feasibilty of getting long enough and strong enough timbers for the rafters/trusses - anyone know how to calculate the strength of a scarf jointed rafter? (I'm assuming I can't get long enough pieces in a single piece at a suitable width to provide the strength)
4. Insulated Durisol blocks (similar to ICF polysyrene things but more sturdy in construction process and easier to assmble and also being fundamentally wood, - but obviously filled with concrete- much easier to fix to) with either cut roof or pre made truss roof as per 2& 3 above
I may end up going to a QS / Structural engineer but I'm keen to get some more practical based input from all the knowledgeable people on the forum fisrt so that I am as informed as I can be before I start paying for advice theat I might not fully underatsnd or might have actually questioned if I'd known more first.
You havent said how long your workshop is. Portal frame is generally the mosyt cost effective for larger buildings but it also depends upon any specific foundation requirements. If using timber purlins you are OK up to about 4metre portal frame separation. Above that go for steel purlins ( as wooden ones become very expensive)
For the walls there is no reason why you cant also use metal insulated panels and then overclad with your larch rainscreen
Have you tried contacting some of the agricultural shed fabricators as many of these manufacture standard size portal frame kits and some also supply cladding .
I have seen quite a few insulated block types which are quick and easy like lego. Gablock is one the other was some kind of hemp composite i forget the name of them.
Otherwise SIPS are well proven by now and can be built really fast.

There’s no need to be jealous! 😂
This is my retirement plan to finally have space for everything I’d like to do and for storing all my agricultural and cycling stuff as well!
You could consider using sip panels (structural insulated panels) very strong and do big spans
I would avid anything that uses new roof Trusses, put a hand cut roof frame in and use the space you gain for storage. A bit more time but a lot more space.
I would avid anything that uses new roof Trusses, put a hand cut roof frame in and use the space you gain for storage. A bit more time but a lot more space.
My challenge is the timbers required to handle a 12m span at a 15degree pitch. All the tables I can find end up with the equivalent of multiple braced modern trusses. I was hoping to do something like a traditional king post design, so plenty of space for long awkward things! But this requires joining timbers in order to get the required length for tie beams and rafters and I can’t find any span table that will help me calculate the required timber sizes. Any ideas?
Steel portal fra would seem to be the way to go, cheap,quick and the structural engineering is well understood. If you want to go all timber there are balloon frames barns in the West Coast of America which use small timbers to build massive structures , though getting a structural engineer who understands that would be tricky.
Agree, I think a phone call to an agri building supplier would be well worth it. Steel will surely be much more cost-effective.