Summer house Timber construction questions?

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Hi Guys,

Been a while since I’ve used the forum, hoping it’s is helpful is it was when I did my renovations.

I am about to build a summer house/garden shed at the bottom of the garden. Plan to stay within the permitted allowances to avoid planning permission, and personally feel that 2.5m height will be sufficient for ceiling height in such a room. I guess the height internally will be closer to 2.3m, which again I feel will be sufficient.

Hoping for some advice on Timber for construction. The buildings frame will be built with timber stud work and clad in a combination of gravel boards and cedar…
My questions are:

1. What grade and dimension for the vertical wall construction? 4x2 or 3x2 is what I was thinking and C16? Baring in mind I want to insulate between the stud work.

2. What dimensions for the floor joist? This will be on a concrete slab, so less importance on the structural strength as the load will be supported. More about insulation?

3. What dimension for the roof? This is most critical. The span will be 3.4m. The roof construction will be ply on top of the joists and then felt or a rubber equivalent, so nothing too heavy. Again this will be insulated.

4. Out of interest, for such a building, do you think rock wool insulation will suffice? Or solid. My feeling is rock wool.

Many thanks in advance. I have plenty of other questions like vapour barriers before cladding etc and whether my proposed roof construction is okay, but that can wait.

Regards

Kyle
 
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How long is a peice of string?

Many of your questions are dependent on how much you want to insulate.

1. What grade and dimension for the vertical wall construction? 4x2 or 3x2 is what I was thinking and C16? Baring in mind I want to insulate between the stud work.

If you insulate internally then decent quality 3x2 at close centers can work (or even thinner if needed). Otherwise the thickness of studding is mostly determined by insulation requirments (deeper studs and less of them is better for insulation).

2. What dimensions for the floor joist? This will be on a concrete slab, so less importance on the structural strength as the load will be supported. More about insulation?

Why do you need floor 'joists'/battens, why not just have a floating insulated floor?

3. What dimension for the roof? This is most critical. The span will be 3.4m. The roof construction will be ply on top of the joists and then felt or a rubber equivalent, so nothing too heavy. Again this will be insulated.

Warm or cold roof?

Sloped or flat?

Probably 170mm deep joists as a guide.

4. Out of interest, for such a building, do you think rock wool insulation will suffice? Or solid. My feeling is rock wool.

=String lenght?

How much do you want to insulate and how much do you want to spend. Rigid keeps the thickness down when looking for high values.
 
Thanks Aron,

Fair point, I'm sure there are loads of variables in my choices, I was just after some advise and guidance. Thanks.

When you refer to a fgloating insulated floor. Do you mean insulating my slab? It sounds like you mean joists with insulation between them, but that is what I was siuggesting, so a little confused.

I am going for a cold deck, sloped (but almost flat) roof. I'm always undure whether insulation between the joists and play then roofing material is okay. What about condensation and ventilation?
 
On the floor slab I mean just laying rigid insulation over it and laying chipboard or something similar on that = floating floor.

With a cold roof you have to have ventillation above the level of the insulation (google warm and cold roof plenty of info).

Which one to go for really depends upon how deep the roof joists are and the level of insulation required.
 
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Hi Big all,

The width is 3.5m from back wall to front wall, which I mentioned with regard to the roof joist span. The length is variable and an odd shape, but essentially 6m. This shouldn't matter I wouldn't have thought, as nothing spans the length.

Aron, personally I haven't heard of such a 'floating floor' construction, but we are all still learning. If I can avoid the need for timber in the floor, i will.

I suppose for me, it will come down to cost. I.e rigid insulation with no timber joists vs timber joists and rock wool.

I've done a fair amount of research into cold vs warm deck roofing for my extension, but like you say, in this case I have limits on height, hence cold deck. If screwing ply straight onto joists and using either rock wool or rigid insulation, would you therefore have to drill holes or cut grooves into each joist or noggin to allow the ventilation to get around between the ply and void? I hope that makes sense, but I see the ply screwed to joists essentially creating loads of little air tight pockets.

Thanks again all.

Thanks
 
You should be able to google a number of cold roof designs.

Think of your loft, ventilation above the insulation, with gaps at the eaves.

You could use full fill mineral wool, with OSB, breather membrane, then battens/counter battens and roof felt/tiles. But you need to ensure no condensation will occur in the build-up, in a normal house there is a small risk, in a shed there may be no risk due to lower amounts of internal moisture.

For the floor you can look at polystyrene insulation to float your chipboard on, cheaper than rigid foam.
 
ok sorry missed the width and length lol

keep in mind sheet material sizes and also plank sizes when planning the exact sizes
for example sheet is in imperial 8x4ft 2440x1220mm
and planks are shorter as in 1800 2100 2400 and 300mm increments
as an example my shed is 10x12ft floor area with the frame sitting on the floor and the t&g shiplap on the outside covering the ply and most off the flooring timbers
now the planks will come out short as the floor is imperial but you can use the timbers at the corners to cover the end grain on both that edge and the edge around the corner allowing for the fact the planks are about an inch short for every 5ft on length so the corner timber makes up the difference by covering the studs by perhaps 15-30mm each end
 

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