Designing shed/workshop from scratch ... advice?

[DaKid]

Hiya,

I'm replacing a "package" 10' x 20' shed and will be building it from scratch. The quality of the shed was not high in the first place, and it's now leaking and generally damp, and is also not well organised. I'm moderately experienced in building larger projects (I've designed and built a 5m x 7m raised, roofed decking from scratch).

There is an existing 6m x 3m concrete base that will be good enough, and I'm intending to build this from predominantly 4x2s in order to keep the walls strong enough for mounting things on, as well as the extra affordance that will offer for in-wall insulation above 3x2s. I will be splitting the shed down the middle, in order to optimise wall storage, and also to clearly separate the storage area for bikes, garden furniture, garden tools from the workshop with power/hand tools and possibly cycle maintenance area.

The advice I would like is around the weather/damp proofing of the building. I'm pretty certain the slab was laid over DPM (I can see some of it at the edges, so I can only assume it's a "good" layer), but I'm not certain what's best to do above that. Does anyone have any advice or know of any good resources for how best to design the floor, walls, roof of a garden building to minimise heat-loss and - more importantly - to prevent damp inside the building. I have mains power to the existing shed, so I was hoping to have tubular heaters in each section to help keep it above the dew point in there.

Things I've been thinking about so far:

- Raising the building off the base (e.g. building on top of 2x2 runners)
- Bolting the main building to the base for strength, but then putting a DPM across the floor with insulation and flooring on top (e.g. celotex + laminate)
- Do I need a VCL on the walls?
- What cladding would be best ... is shiplap sufficient, and I presume I will want to make sure the outer extent of the cladding is beyond the base to prevent run-off?
- Would a breather membrane under the cladding be necessary?
- If I'm trying to keep to the 2.5m limit (it's only 1m from the boundary), is there any roof design that's optimal, considering I want >1.9m doors in the long edge of the building?

Thank you so much for any advice people can offer ... I realise I could just crack on and build it however and it would be fine, but there's a lot of unknown unknowns for me in this currently!

 

[Deluks]

How I would approach it, in a nutshell.

Build a timber deck and raise it up on slabs.
Make up your 4x2 walls with head and sole plates, 11mm osb on the outside, screw these into your deck.

Cover outside with breather membrane and batten. Cladding of choice, horizontal or vertical.


Screw down flooring of choice. Plastic vapour barrier all over the inside of the walls, insulation on the other side between the studs.

Clad inside.

Flat roof easiest. Just recreate the floor deck, but bigger.

You could put the OSB or ply on the inside instead of the outside, which saves on internal cladding, but fix your vapour control membrane first.

Pitched/vaulted roof for extra headroom or storage.

Insulate roof, vapour barrier on the inside.

Have your windows and doors to hand when you build the frames, to ensure that they fit.

 

[Deluks]

The other way would be to build a 2 brick high wall, and put your timber stud walls into that. Make a dpm 'paddling pool' between and over the bricks, then insulation on top of that and a floating floor deck.

 

[DaKid]

Thank you. Interesting that you talk about a timber deck on slabs ... are you saying put the slabs on top of the concrete base? What's the benefit there ... that the air gap would be better than a DPC then a deck directly on the concrete? I guess that the concrete slabs (by which I'm assuming 100mm dense concrete blocks?) would prevent moisture coming up from the base, although I guess 4x2s could also do that job as the air gap would prevent the build-up of moisture?

Would you put insulation in-between the joists of the floor, and then a vapour barrier above? Feels it would be easier to keep the temperature above dew point that way?

 

[Old Salt]

Here is mine
There is a dpm between the brick and wooden sides

 

[freddiemercurystwin]

I've always thought building a brick plinth was excessive for an outbuilding, a concrete block laid on it's side at 1m centres or thereabouts is all that's really necessary and then build a timber floor on top of those.

 

[Old Salt]

I've always thought building a brick plinth was excessive for an outbuilding, a concrete block laid on it's side at 1m centres or thereabouts is all that's really necessary and then build a timber floor on top of those.

Each to their own. I’m happy with mine and a timber floor would be useless in a garage/workshop.

 

[scbk]

Here is mine
There is a dpm between the brick and wooden sides

Looks good, what's the length/width/height? Is the framework mainly crica 4x2s?

 

[Old Salt]

Its about 3.5m x 7.0m. Takes a bmw 1 series with standard kitchen bases and worktop down the long side. Then a row of cabinets across the short side with desk and walk through space.

 

[Notch7]

Does anyone have any advice or know of any good resources for how best to design the floor, walls, roof of a garden building to minimise heat-loss and - more importantly - to prevent damp inside the building

Have a look at the thread “build a shed Mikes way” - it’s on the UKworkshop forum and Mike is an architect. He wrote it because so many people were asking about building workshops and were asking the same questions.

Build a shed Mike's way

I've put it off long enough! Hopefully, this will save me hours of repetition: Note that this only applies if your building does not require Building Regulations approval. If it needs to meet the regs, the base will need to be properly designed according to the local soil conditions and tree...

www.ukworkshop.co.uk

One of the most important details is using 50 x 25 battens on the outside before the cladding, that creates a 25mm cavity which means it doesn’t matter if the cladding is not watertight, anything that gets behind the cladding will just run out.

Also it means the cladding is ventilated at the back, massively increasing its longevity.

I built a 5m x 4m garden room like this, with celetex type insulation in floor, walls, ceiling. I don’t use the room all the time but without any heating it stays perfectly condensation free, things with bare metal on them never rust. The reason is because temperature fluctuates slowly and humidity is stable.

 

[DaKid]

That's really interesting. The only thing I don't quite get is how to attach the frame to the base if I'm not using the two courses of bricks. Just put a DPC strip underneath them and then bolt them to the base for strength?

 

[Old Salt]

Mine is bolted through the timber to the brick course with dpm between. The frame is mostly 4 x 2. Now has DG windows with framing to suit the opening. Insulated behind the ply panelling with an onduline roofing sheets also with ply boarding. Its been up about 4 years now.

 

[mrrusty]

The issue to watch out for with an existing concrete base is water finding it's way across the top of the base into the building - the cladding has to extend lower than the structure-base joint. If you can't do that, raise it up a bit.

A building that size will be so much nicer with a pitched roof - have you considered getting planning?

For a building you want to insulate, I would use SIPs. When you factor in the timber, osb and insulation the cost difference is miniscule https://www.simplysips.co.uk/products/

Here's a couple of outbuildings I have done recently - one block and trusses, the other SIPs

https://community.screwfix.com/threads/garage-work-in-progress.222282/

https://community.screwfix.com/threads/garden-room.256842/

 

[DaKid]

Ok, so I'm onboard now with the brick plinth concept, but what bricks would people recommend? Does it need engineering bricks for strength, or is a nice-looking facing brick going to be acceptable?