My First Workshop WIP Thread.

[North Wood]

Good afternoon,

New to this forum, so firstly hello! Been snooping around here for the last week or so gathering information and reading WIP thread so heres my one!

Current workshop spec, which will be built in the back garden :

- Length 3.6m (possibly 4.8m), Width 2.4m, Eve height 2.4m and flat roof pitch to around 10deg.

- 100mm deep concrete slab with 200mm around the perimeter foundation, kingspan and chipboard floating floor inside.

- 3x2 kit, fully insulated, ply or OSB lined internally.

- shiplap cladding, 2x1 battens for cavity with breathable membrane on kit.

- Hardwood windows and door.

- Yet to decide on a roofing solution, considering either a rubber roof or some kind of profile sheeting.

I will try and attach a cross section diagram I've quickly drawn up (currently have limited internet access), my main questions would be:

- Do you think the DPC placement (highlighted in green on diagram) would be adequate or should in addition i add one below the concrete slab so as to reduce moisture build up in the slab?

- Any thoughts on the current spec?

Pointers, good or bad would be great!

Thanks.

 

[mindthatwhatouch]

Welcome.
Certainly heading in the right direction.
Maybe 150mm above ground level, and a shingle trench around the perimeter below the cladding.

If you want to put membrane below the slab then you will need a separate slab to the foundation supporting the structure.

Could look at decra/ metrotile roofing.

 

[Mcluma]

Have a look on my build wip

bike-tool-shed-t89083.html

especially with regards to the detail of the rebated feather edge

and make sure do you a large roof overhang....

 

[North Wood]

150mm would be even better i suppose, the idea of gravel around the perimeter had popped into my head... suppose anything to aid drainage is only going to be a positive.

In term of the membrane below the slab i was meaning incase the whole thing... concrete slab, foundation up the outside then secured under the 2x1 in a similar position as the DPC in the attached diagram.

I've read your thread chris, very nice build. The finished roof structure looked very impressive, like those big mortise and tenons as well!

The feather board detail worked well but not sure if it would add anything to mine, obviously it was a key detail on your build to stop water from collecting on the top of your foundation due to the step in from the edge, since my shiplap would already be past the outside edge of the slab i wouldn't be facing the same issue.

I did notice you placed membrane under the concrete slab but not up the sides, what was your reasoning behind this? I've seen it before on various builds, i could understand if it was running up the side then under the wall covering so as to effectively enclose the whole concrete slab to prevent water ingress, surly leaving the sides open would mean water would still get into the slab so why lay membrane down to begin with?!?!

Not trying to doubt or knock anything thats been built just wondering if i should be doing the same as well!

Thanks.

 

[Mcluma]

150mm would be even better i suppose, the idea of gravel around the perimeter had popped into my head... suppose anything to aid drainage is only going to be a positive.

In term of the membrane below the slab i was meaning incase the whole thing... concrete slab, foundation up the outside then secured under the 2x1 in a similar position as the DPC in the attached diagram.

I've read your thread chris, very nice build. The finished roof structure looked very impressive, like those big mortise and tenons as well!

The feather board detail worked well but not sure if it would add anything to mine, obviously it was a key detail on your build to stop water from collecting on the top of your foundation due to the step in from the edge, since my shiplap would already be past the outside edge of the slab i wouldn't be facing the same issue.

I did notice you placed membrane under the concrete slab but not up the sides, what was your reasoning behind this? I've seen it before on various builds, i could understand if it was running up the side then under the wall covering so as to effectively enclose the whole concrete slab to prevent water ingress, surly leaving the sides open would mean water would still get into the slab so why lay membrane down to begin with?!?!

Not trying to doubt or knock anything thats been built just wondering if i should be doing the same as well!

Thanks.

Its always important to keep water away from your foundation or walls, so the gravel around the building is a good point, but better is a very large overhang like with mine of around 400mm, now with the extensive rain there is no water agains the walls, and with proper guttering!!!!! there will be not water dripping of your roof

so there will only be water coming up from underneath the slab, therefore the damproof membrane, you do not want water to come through the concrete. why no membrane up the walls, well in my situation it was not possible, as i build on the slab. and as the water will be kept far away from the slab there is no change that water will overflow the surrounding area's and flow back over the slab

my slab is all around 2 inches higher than the surrounding ground

just keep asking away, there is a lot of knowledge here on this forum

 

[Paul200]

A lot of knowledge and a lot of people building things! I'll follow yours if you follow mine :lol:

There's a mass of good info on here North Wood and I certainly wouldn't have started my project without first having read through some of the excellent WIP's. Good luck with yours.

Cheers

Paul

 

[North Wood]

Already been following yours paul, What a location to be building in, beats my back garden hands down!!

Water rising up into my workshop is my main concern, hence why I'm needing some advice on the damp proofing side of things. 150mm rise off ground level, shingle around the concrete slab and ensuring plenty over hang on the roof has been noted.

The only location i can build on is in the back garden which is slopped, obviously the ground will be dug back and slab constructed level etc but is there any negative impact in having a (sometimes) damp concrete slab? Its not going to be swimming but when it rains heavily constantly the proposed site is notably wet to a degree (Compared to your site chris by the look and sounds of it), if that makes sense.

The actual workshop (kit, flooring etc) would have a 1200ga DPC between it and the concrete slab (as proposed in the diagram), so would it be safe to say that will provide ample protection?

 

[Paul200]

:lol: Yep - the location's tremendous, shame about the house! I thought we'd come up here to retire but no, we fell in love with a 'project'. We could end up living in the summer house :lol:

Dpm's and concrete are not really my thing but from experience I try to get everything up off the ground where possible. I recently watched an American video that showed a workshop build based on concrete blocks sitting on individual beds of gravel dug into a lawn. If the blocks are not level you just shoogle the gravel up a bit until it's right! Not sure about the longevity of it but that's what our potting shed will be sitting on soon!

Scotland's a big place - roughly where are you located?

 

[North Wood]

Haha and a fine home it would make!

Up the north end of Scotland.... and by that i mean north!! so I'm not sure the gravel block idea would last the first winter... possibly even the summer! :wink:

I've attached a diagram of my initial design, Upper part of the build is all the same spec but instead of building onto a solid slab i would lay a rough concrete foundation followed by a perimeter course of 6" blocks and then a DPM enclosed insulated concrete slab (which would double up as the finished floor) inside the block work.

My reason behind wanting to change it to a concrete slab design (as shown in my first post) was:

- Require less digging down into what i suspect might be rocky ground.

- Less build time, i.e digging, wait for foundation to set, then block work and finally slab.

- Initial costing showed a slab would work out cheaper then extra foundation, block work etc.

Dont mind spending money to do it right but not wanting to spend tons of money over specifying the build, it is a 2.4m x 3.6m shed/workshop after all, not a house! If a concrete slab with DPM in the correct place will do the job then may as well save the money for another part of the build... or filling it with tools once complete!

 

[Setch]

Another reason to place dpm under the slab is so all the water in your mix doesn't Wick at into the ground, reducing the cure time and weakening the mix.

 

[Graham Orm]

Already been following yours paul, What a location to be building in, beats my back garden hands down!!

Water rising up into my workshop is my main concern, hence why I'm needing some advice on the damp proofing side of things. 150mm rise off ground level, shingle around the concrete slab and ensuring plenty over hang on the roof has been noted.

The only location i can build on is in the back garden which is slopped, obviously the ground will be dug back and slab constructed level etc but is there any negative impact in having a (sometimes) damp concrete slab? Its not going to be swimming but when it rains heavily constantly the proposed site is notably wet to a degree (Compared to your site chris by the look and sounds of it), if that makes sense.

The actual workshop (kit, flooring etc) would have a 1200ga DPC between it and the concrete slab (as proposed in the diagram), so would it be safe to say that will provide ample protection?

That moisture will permeate the workshop and cause everything in there to have a high moisture content. You must put a membrane down to avoid this.

 

[North Wood]

Thanks for all the input guys, Sorry about all the longwinded questions. Just want to get the spec sorted then i can get down to the building part and let the progress pictures do the talking!!

That moisture will permeate the workshop and cause everything in there to have a high moisture content. You must put a membrane down to avoid this.

Absolutely, that's why i was proposing to fit a 1200ga plastic DPM running between the top of the concrete slab and workshop above (as per first diagram).

Was just unsure if i should run in additional to the above DPM an extra layer either -

1) Along the underside of the slab but not up the sides.

2) Along the underside then up the sides to a height of the finished external ground level.

3) Along the underside then up the sides to a height above the bottom of the cladding, In effect wrapping the whole underneath of the slab in DPM.

4) Not bother running any DPM under the slab.

 

[mindthatwhatouch]

Your drawing is the correct option.
A strip foundation (blocks on a concrete slab) involves more labour but is cheaper on the materials.
If you want it pays to put some Polystyrene insulation vertically against the inside wall to prevent the cold bridge.
Foundation minimum 50mm either side of block, dig down to solid ground.

Option 3 doesn't work as the slab is free to move around or slip, It would need to be a raft foundation to resist cracking.

 

[North Wood]

Your drawing is the correct option.
A strip foundation (blocks on a concrete slab) involves more labour but is cheaper on the materials.
If you want it pays to put some Polystyrene insulation vertically against the inside wall to prevent the cold bridge.
Foundation minimum 50mm either side of block, dig down to solid ground.

Option 3 doesn't work as the slab is free to move around or slip, It would need to be a raft foundation to resist cracking.

Cheers for the input,

I take it you're referring to my second digram. Labour cost doesn't matter as i'm building it myself (with some help from friends no doubt, proved i supply beer!) but i did do an initial costing and doing it with a strip foundation and blocks looked to be more expensive then just running a concrete slab.