Shed build, many many questions...

[Torx]

Following on from this thread (thanks for the contributions), I've realised I'll be asking question after question so have decided to put it all together in one and hopefully document the build.

This is the first of two outbuildings I'm hoping to build in the garden, this one is basically a posh shed, a practise run for the second larger one which will be part home office / part summer house.

(Actually, I might need to temporarily partition part of this first one for a small office space if push comes to shove, so all the more reason to insulate it as suggested.)

I'm basing it around four 8' x 4' sheets, as it's convenient and it's roughly the right size for the space I've allocated for it.

The roof will be pent to keep things simple.

I've put a gully in just behind it, probably completely unnecessary but it was near enough to existing drainage that it was simple to do, and the pipe needs to run behind it to the larger building I have planned so I needed to get it in before I built this.

This is the base design (so far), using 2" x 4" pressure treated C24 timber with joist hangers where unsupported. I'll insulate it with used Kingspan / Celotex or whatever's cheapest, with 18mm exterior ply on top.

How deep do I need to dig / fill under the high-density concrete blocks with MOT type 1? I have a 6 year old who loves digging so free labour - no need to cut corners here. I was thinking I'd dig to about 350mm from ground level and fill/compact to 300mm. The ground is clay, rock solid. The adjacent garage has a 100mm slab put down by cowboys 25 years ago and hasn't moved (at least not noticeably) .

How do I work out how much hardcore I'll need? Will 1 tonne be enough?

Do I need mortar between hardcore and block?

The remaining 50mm I'd fill with loose gravel that I've got hanging around.

Some sort of weed membrane too maybe?

That leaves me with a 50mm gap underneath for foxes / badgers etc., to make their homes.

For the walls I had this in mind using 2 x 4" again (is treated necessary?) to allow for possible insulation in future or in the partitioned area if I decide to do that:

Are 19mm battens OK for between the cladding and OSB or am I better with 25mm to allow a bigger air gap and longer screws / nails for the cladding?

No clue how to work out roof pitch, this (about 4.5°) looks about right....

Oh yeah, permitted development. The red line is 6ft, and I'm 5.10.

For the roof material either a double layer of felt, or perhaps this stuff:

I think buying fancy windows would be pushing the budget, is there any reason I can't make simple non-opening softwood frames with double glazing panels only? Just buy the double glazed panels?

A quote for a similar building from a posh shed supplier (I've seen their stuff, it is good), came to £3800, so the plan is do better than that for a similar price...

 

[mikej460]

The foundations are largely dependent on the ground conditions e.g. any tree roots, water logging, clay? The normal rule of thumb is 100 - 200mm hardcore then concrete footings (if you have ruled out a concrete slab) then block or brick with a dpc.

My understanding is that the stud wall plan is incorrect, you need the OSB3 on the inside workshop stud work with insulation between the studs then breathable membrane on the outside of the studs then a venting gap created by battens then cladding. The principle being that warm, damp air moves out towards colder outside air and is carried away by vented space created by the battens, the OSB3 acts as a vapour barrier to stop any escaping into the building.

 

[John Brown]

Check out eBay or Facebook marketplace for Windows. I bought two oak framed double glazed units for £14.50, which I incorporated into my build.
I did give the builders a fiver to help me lift the larger one on to my roof rack, but still a total bargain.

As for overall cost, my 4.5m by 3m shed, with insulation and waney edge larch cladding, cost me the better part of £5k, and that was 2 years ago. I estimate timber is a good 50% dearer now, if not more.

 

[Jameshow]

I'd increase the floor beams to 6" deep and miss out the centre blocks and the doubled up timbers save a bit in costs and effort?

6" will easily span 8ft unless you 25st!

 

[Torx]

The foundations are largely dependent on the ground conditions e.g. any tree roots, water logging, clay? The normal rule of thumb is 100 - 200mm hardcore then concrete footings (if you have ruled out a concrete slab) then block or brick with a dpc.

My understanding is that the stud wall plan is incorrect, you need the OSB3 on the inside workshop stud work with insulation between the studs then breathable membrane on the outside of the studs then a venting gap created by battens then cladding. The principle being that warm, damp air moves out towards colder outside air and is carried away by vented space created by the battens, the OSB3 acts as a vapour barrier to stop any escaping into the building.

No roots etc., nearby.

That's a pest about the OSB, I wasn't planning to insulate the walls yet. Any detriment to put it on the outside?

Check out eBay or Facebook marketplace for Windows. I bought two oak framed double glazed units for £14.50, which I incorporated into my build.
I did give the builders a fiver to help me lift the larger one on to my roof rack, but still a total bargain.

As for overall cost, my 4.5m by 3m shed, with insulation and waney edge larch cladding, cost me the better part of £5k, and that was 2 years ago. I estimate timber is a good 50% dearer now, if not more.

Good plan, I'll do that. I do actually have one small mahogany DG window I could somehow incorporate.

I'd increase the floor beams to 6" deep and miss out the centre blocks and the doubled up timbers save a bit in costs and effort?

6" will easily span 8ft unless you 25st!

I'm trying to save every inch of height within my allotted 2.5m. Is that mad?

 

[John Brown]

I used rockwool insulation in the walls, and Celotex/Kingspan in floor and pitched roof. OSB inside, Tyvek outside. Even though I don't have heating, I've so far had zero condensation or rust problems.

 

[Torx]

I was copying Colin to be honest, though I thought the water barrier might be better done with something breathable.

 

[Fitzroy]

Repurposed windows off gumtree or similar is the cheapest option for definite, just depends on if you can work them into your design. If you build from cheap softwood you’ll need to paint. I built from Doug Fir which is supposedly semi durable, hence not painting. If found no need to open mine in the last 5 years.

OSB outside is great for the build as it goes up quick, however it is suboptimal from a condensation risk perspective. But if you go OSB inside insulating at a later date is slightly harder as you’ll have to remove it and replace. Perhaps not an issue if you screwed not nailed. At some point I will insulate mine ans I run the risk of interstitial condensation. However it’s not in constant use so I’m happy it’s a low risk.

Roof height was an issue for my build, so I decided to apply for planning permission. Cost a few hundred quid and a few hours for drawings but council were super helpful and I could build what I wanted. I live in a conservation area also!

I’d look at EPDM for the roof, it’s easy to use and worked out cheaper than other products on my calculations.

If you choose to OSB the outside 18mm battens probs ok as OSB also give some attachment depth, if you osb inside I think you’ll want 25mm.

My build was impacted by protected trees and I could not dig down more than 10cm due to root damage or something. Hence my shallow mot pockets, didn’t research beyond this.

Many of your decisions will be compromises between ‘by the book’ and cost/time to achieve. History will judge how successful your decisions were, but remember it’s a shed not a permanently occupied building providing a safe, warm, dry space for people.

Fitz

 

[mikej460]

That's a pest about the OSB, I wasn't planning to insulate the walls yet. Any detriment to put it on the outside?

If I were you I would install the osb3 on the inside and insulate with rockwool then put the membrane on. The membrane will keep the shed dry until you can get the cladding.

 

[Torx]

If I were you I would install the osb3 on the inside and insulate with rockwool then put the membrane on. The membrane will keep the shed dry until you can get the cladding.

I didn’t realise the membrane (Tyvek type stuff you mean?) was that effective.

 

[Torx]

Is there any point in putting a membrane in without insulation, or in readiness for insulating at a later date?

 

[Jones]

You should remove the topsoil under the blocks but don't need to go deeper than that, you'll find mortar under the blocks means you can level them easily.
All the joists need to be supported mid span unless they can do the whole span, a half brick will do but they need something.
If you build floor first then walls on top I would paint the board edges with bitumen paint,it's a real pain to replace rotted pieces with that build method.
Tyvek or similar will waterproof the shed but only the more expensive UV resistant stuff can be left uncovered for long.
Second hand windows and doors are often on Freecycle etc which will save cash.
Don't waste money on joist hangers, as you can get round to get nails in they're not needed, two 100mm galvanized nails into each joist is plenty strong enough.
You could build the studs tyvek the outside and just nail on bracing on the inside to prevent racking then insulate and board at a later date

 

[mikej460]

I didn’t realise the membrane (Tyvek type stuff you mean?) was that effective.

Yes it keeps rain out of the building but also lets water vapour (condensation) out

 

[Torx]

You should remove the topsoil under the blocks but don't need to go deeper than that, you'll find mortar under the blocks means you can level them easily.
All the joists need to be supported mid span unless they can do the whole span, a half brick will do but they need something.
If you build floor first then walls on top I would paint the board edges with bitumen paint,it's a real pain to replace rotted pieces with that build method.
Tyvek or similar will waterproof the shed but only the more expensive UV resistant stuff can be left uncovered for long.
Second hand windows and doors are often on Freecycle etc which will save cash.
Don't waste money on joist hangers, as you can get round to get nails in they're not needed, two 100mm galvanized nails into each joist is plenty strong enough.
You could build the studs tyvek the outside and just nail on bracing on the inside to prevent racking then insulate and board at a later date

Thanks. On the soil front, it’s clay, there is literally no top soil it’s just slightly looser on the top as I dug it over and rotorvated years ago before grassing. I’ve heard mixed opinions from ‘ooooh, that’ll sink you’d need a 200mm deep reinforced concrete slab’ to ‘that’s rock solid, plonk anything on top.’ I have to choose the right time to dig or it’s really hard work.

In the summer cm wide cracks appear in any ungrassed areas.

Here’s some photos from when I put a drain in:

I was planning to buy a tonne of MOT1 and divide it up amongst the 15 blocks which I think would make them about 15cm deep.

But as mentioned, the shoddy garage hasn’t sunk and I do have a habit of overdoing stuff.

 

[mikej460]

If I were you I'd hire a mini digger, one of the smallest ones that can fit through doorways should be enough. Much easier and more fun!

 

[Spectric]

With this design, once you have the base in place use wire mesh of some sort to seal all the edges to prevent rodents living underneath, mesh size will determine who can get in!

 

[Torx]

If I were you I'd hire a mini digger, one of the smallest ones that can fit through doorways should be enough. Much easier and more fun!

Already got one:

With this design, once you have the base in place use wire mesh of some sort to seal all the edges to prevent rodents living underneath, mesh size will determine who can get in!

Good plan!

 

[Torx]

Apparently there are 87 types of MOT Type 1....

https://www.huwsgray.co.uk/shop/catalogsearch/result/?q=mot+type+1
Does it matter? Hoping to get some delivered for next week so I'll need to order it tomorrow.

 

[Valhalla]

Apparently there are 87 types of MOT Type 1....

https://www.huwsgray.co.uk/shop/catalogsearch/result/?q=mot+type+1
Does it matter? Hoping to get some delivered for next week so I'll need to order it tomorrow.

Call me old fashioned, but I would build like Mr Chickadee......totally environment friendly

I am intending to build a workshop using similar methods to support the structure. If you decided to remove the structure in the future all you would have left are fillable holes in the ground......

 

[stimuli]

Hi Torx. In researching my own upcoming build, I've stumbled on this thread, and would like to share some of what I've put together. Please take it with a heap of salt, as I've likely forgotten more than I've learnt over this past year. And do ignore my advice where it conflicts with that of people more credentialed (I have practically none, so the bar is low).

OSB3 on the exterior as sheathing should be fine, provided you take steps elsewhere to reduce condensation risk; most OSB3 is at least a little vapour permeable, albeit much less than OSB2, etc. The main cause of condensation risk will be airtightness and vapour permeability of the building envelope, including the fitting of the insulation. Most often, insulation isn't fit - or foamed - tight to the frame. The gaps left, big or small, allow air and water vapour to exit the enclosure, and can cause condensation on the interior faces of external sheathing, which can cause damp problems you won't find until it's too late. It's especially a problem in roofs, I think due to mistakes or compromises in the buildup.

As noted by others here, a vapour barrier (VCL or vapour control layer) should go on the warm side of the insulation. Something to consider for your posh shed, and particularly for your garden office: You won't need to increase your buildings ability to transmit vapour to the outside (such as by sheathing the interior only), if you put adequate controls in place to reduce the transmission of vapour from the interior through the envelope. A plastic vapour barrier can be fitted to the floor, wall, and ceiling, and taped at the joints and openings using an appropriate tape. Provided your door and window openings are weather sealed, this can greatly decrease the transmission of vapour/ condensation from the interior to the exterior. Something people often neglect is that any openings made in the vapour barrier - such as for electrics - need to be sealed properly. I don't recall how much moisture can be exchanged through holes made for downlighters, sockets, etc. but I remember it was alarming.

Another thing to consider is the cold bridging that could occur on the interior faces, where the timber frame meets the interior sheathing. This is when heat from the interior escapes quicker at points where the frame is in contact with the interior surfaces, and creates a cold spot where condensation can form on the interior. It happens most where many timbers are sandwiched together, or in corners that aren't insulated. One common way to overcome this is to use insulated plasterboard to sheathe the interior, or thin PIR insulation underneath sheets of plasterboard, where cheaper.

The purpose of some of the advice in this thread seems to be to reduce the risk of interstitial condensation, which is important for the longevity of your build. But, I'd say it's also important to look at vapour control more broadly, and find ways to stop it getting into the building fabric in the first place. Unfortunately, most of the stuff I've mentioned will inevitably add to cost, but should also improve the performance of your building.

Hopefully I've not bludgeoned you with my essay. Good luck with your build!

PS Yes to the weed membrane. Pin it down with plastic pegs.