Shed design – size of rafters

Building a shed in the garden, most of the frame is up bar the front wall and roof. Trying to decide what size rafters I want 3×2 or 4×2 – the overall size of the shed is 3m x 3.6m so reasonably big. I’m doing a design with a central ridge beam (6×2) with rafters birds mouthed to the wall plate.

Roof will be 11mm OSB3 with probably EPDM (chance of shingles, low chance of slate) on it (tho for now it’ll just be a thick sheet of polythene whilst I wait for my bank balance to recover from the cost of wood! £38 for a sheet of 11mm OSB!!!). Peak loading will be me sitting on top of it with the EPDM roll in the middle / a tree branch falling on it (it’s under some trees).

I tend to overbuild things and the price difference between 3×2 and 4×2 is pretty small (about 30 quid). What are peoples thoughts…. here are some drawings of my current plans (blue is the space for the door, grey are bits that are already up, all of the frame is 4×2 with the exception of the roof beam (6×2) and the rafters in this picture are 3×2 – the overhang on the side isn’t representative, just a full 2.4m beam).

So what say the collective – 4×2 or 3×2?

I used 4×2 on mine. Shed is 12’x8′ but is a pent roof. Walls are 3×2 with 11mm OSB outside, 9mm inside

Mine is 9×9′ approx with a turf roof which is heavy.
I went with 2×7″ beams and they easily support the weight of the roof and two people walking about on it.
I would just fit the biggest you can, no downside really as far as I can see.

I guess the only downside is that it makes the weight higher (building it by myself) and will effectively reduce the strength of the roof beam.

I’ll be replacing the 12×8 pent shed next year, hopefully timber prices won’t be nuts.

A few considerations that I’ll be looking to do. A big overhang outside, somewhere handy to sit, work or to dry stuff under a covered area. I’ll be cladding vertical York boards, much sturdier than standard lapping cladding.

A double door and I’ll mibbe insulate too. Defo pent roof and EPDM covered.

The spacing of the rafters is probably more structurally important than the size, more rafters supports more because they spread the load more sort of thing.

I just made entirely out of CLS, which is 3×2 I think, built the frames flat on the ground then lifted them up and pinned with the first rafter, then added them one at a time, no heavy or awkward lifting, can walk about on it no problem.

Best tip I discovered was concrete studs, looks like a big screw with a hex head for a socket, they need a hole drilled but they partially self tap and the socket head makes it far easier, they grip like nothing else straight into concrete or slab, you can go within 20mm of the edge of a slab.

Yeah, this shed is half built and primarily for the lawn mower (we moved to the country in covid so i have a little ride on now). The base is on ground screws which saved a lot of shifting concrete up a hill and digging!

Rafter spacing is on 45cm centres.

Having built v similar, 4×2 will be more than up to the job

There are span tables if you really want to be certain.

Bear in mind the the biggest load a roof will often see is actually upwards, from wind suction in a gale. Extra weight is helpful in that case. In practice (without doing calcs or checking span tables I think 3 x 2 would work but I’d go for 4 X 2 to give you more fixing area at the connections.

Having done some back-of-an-envelope calcs (lapsed mechanical engineer….) on a couple of similar things recently:

1. Any wood chunky enough to be practical to work with is probably many times stronger than necessary. You can put your whole weight on a single piece of 3×2 over a pretty decent span without any issues, never mind spreading the load across several of them.

2. Bending / deflection is usually more of an issue than actual strength.

3. 1 & 2 nothwithstanding, 4×2 is about 75% stronger and stiffer than 3*2, not 33% as you might expect – the vertical dimension is much more important.

From a purely mathematical point of view, I’d hazard a guess that the 6×2 ridge beam is actually the weak point.
Probably fine in reality though.

I ended up going 4×2 – arrives tomorrow. Realised that I could have less rafters if I went 4×2 (tho I was probably over speccing even with 3×2!) so the cost difference was essentially zero, so if in doubt, go large or go home (and absolutely no doubt that’ll it’ll cope with a 1 in 100 year snow fall!).