calculations and decisions

Sadly, at this stage, there isn't anything particularly permaculture about the workshop I am planning, except that I intend it to last a very long time! Once the roof is up, I can choose/experiment with solid cob, wattle and daub and rendered straw bales. I have made a 'freestyle' strawbale shed, and it's ease of construction makes strawbale very tempting, but I would like to make use of passive solar heating and I gather that solid cob is best for that, but then again, not much sun is going to hit the back wall if any ... but I digress, the current decision to be made is about supporting the roof:-
I am trying to figure out what size timbers I need for it's basic frame. It will only be small by US standards, 30m2 max due to local regs (UK). Regs also make building above 2.5 m high, prohibitively expensive so a proper timber frame is not really needed. It's got to be flat roofed. Sadly there is no source of round timber within sensible distance, so it's got to be standard flat roof construction, with rafters and EDPM. On the plus side, I can find no end of tables with detailed info on rafter spacing, span, loading etc. So the actual roof is fairly straight forward, it's just 'how to support it' that's the problem.
It seems anything goes on top of what folks in the US call stick framing, but I would rather not have too many uprights to work around and at 7m longest measurement, I am going to need some sort of support in the middle, where I really want open space. I am thinking some arrangement of posts and beams, but I can't find any usable info on dimensions thicknesses etc. Can anyone point me in the right direction, please?

Is 'Outdoor Structures' Time Life any good or is it only fences, pergolas and deck type things?

Hi Marion,

According to 'The Timber Framing Book', pine on end can support 600lbs per square inch, so a standard 4x4(3.5"x3.5" true dimensions) pine post, on end, can support 7350 pounds.

Annoyingly, the book does not seem to have a table of vertical strengths of other woods...

Googling about, I found a couple things:

1) A 'capacity of wood column calculator', allowing selection of various parameters including a variety of species: https://courses.cit.cornell.edu/arch264/calculators/example7.1/index.html

2) A PDF published by the American Wood Council with a LOT of information in it: http://www.awc.org/pdf/WSDD/wsdd.pdf

I'm pretty sure I'm unqualified to interpret these! Buuut I think the main takeaway is, vertical columns/posts of wood are REALLY strong. So strong that in a smallish structure it seems like the main concerns for the uprights will be making sure they are properly attached and braced to prevent lateral movement/wiggle, and well protected against decay/damage.

Hope that helps!

Hi Marion,
As always, visual aids help us to understand the question completely., but I can offer a little basic design advice.

Rafters typically run across the short side, so 4m? for a rafter span.

Here's a quick rundown of a design.

Posts 6x6, top plate - 4x6, rafters 2x8 on 24" centers, girts 2x6, 1x6 on 24" centers roof sheathing and a metal standing seam or pro panel roof.

I don't know your exact situation, but this is something to start from.

Thanks both. There's a lot of useful info there. Some of those calculators look promising, too.

I could do with a better drawing program, 'Paint' is the best I have.


I've got to the stage where I need detailed drawings, so I'll do them nicely by hand and scan them in when I'm done.
I think my favourite builders' merchants have more chunky timber than is in their catalogue, so I'll check with them first, otherwise, 8ft long 4x8's are the easiest to get hold of.
IDK why I was thinking of the rafters running longwise. 16ft rafters (4.8m actually) work out just right for the deepest section, supported 2ft in from each end. Unless it works out too much more than 2x8's at 24", I'll use 2x7's at 16". I don't trust 19mm OSB not to sag with 24" span, and I might even go with 2x8's at 16", the extra support will give me the option of a little light 'greening' later.