Waterproofing a building below ground level?

[perky416]

Hi guys.

So I want to build a large shed at the bottom of the garden (some of you may have seen my other post). Im going to build it out of block.
My only issue with it is, I need 2.5m clearance between the floor and the roof (enough room to swing a golf club basically).

My neighbours garages are exactly 2.5m tall, so I don't really want to go down the planning permission route and build it a foot higher as it will be an eye sore, and I have to go through the game of getting planning permission in the first place, so ideally id like it to be within permissible build dimensions.

With permissible build in mind, to get the 2.5m internal heigh I need, when considering the depth of the roof and its joists, id need to go down around a foot.

When building below ground level, what would be the best way to keep damp and moisture out? Ive never done this before. Are there any resources I could look up that explain what is needed?

I have a concrete garage than someone build below ground level, it has no DPM and the joins in the concrete leak like a sinking ship. Natrually I cant have this with my new build.

Thanks

 

[jeds]

Without knowing the details of your plan, the simplest way would be to lay a continuous dpm over the whole area and as far up the walls as needed. Also include drainage to the outside perimeter if possible.

 

[perky416]

Im thinking a single skin block wall laid on top of a raft foundation. Around 1.5 blocks high to be below ground level. DPM to be within the foundation. Initially I was hoping to run it up the wall however being single skin i couldn't think of a way to acheive this, so i thought about tanking slurry over the entire perimeter of the building from foundation level to a block above ground level.

 

[freddiemercurystwin]

What is the height difference between your floor level and the adjacent ground level? Presumably you're going to have to employ some drainage at the entrance at least as you'll have to step down to a landing to get in the door? Or are you planning on having steps down inside?

 

[perky416]

What is the height difference between your floor level and the adjacent ground level? Presumably you're going to have to employ some drainage at the entrance at least as you'll have to step down to a landing to get in the door? Or are you planning on having steps down inside?

Floor level to be 30cm lower than the ground level outside. I wont be able to have any drainage. There will he a slight step up into the door and then a step down to the floor on the inside.

 

[jeds]

I've done a couple of these - one middle of last year. My advice is allow for steps on the internal. Problem is, people want a very simple-to-build outhouse for storage and garden stuff etc. and then, later, they decide to add loads of insulation, heating etc, block up all the ventilation because it's cold, and suddenly you've got a (potentially) damp living area.

 

[perky416]

I've done a couple of these - one middle of last year. My advice is allow for steps on the internal. Problem is, people want a very simple-to-build outhouse for storage and garden stuff etc. and then, later, they decide to add loads of insulation, heating etc, block up all the ventilation because it's cold, and suddenly you've got a (potentially) damp living area.

This will be a golf simulator room. Not planning on having any insulation or heating, it gets hot enough when youre continually swinging a club lol.

Does the tanking slurry option with DPM in the foundation sound feasible?

 

[freddiemercurystwin]

This is quite hard to achieve successfully without a cavity wall IMO.

 

[perky416]

This is quite hard to achieve successfully without a cavity wall IMO.

What about a partial second skin? Say about a block above ground level, on the i side, with the dpm between the blocks?

Ive got to sell this build to the Mrs who will be covering half the cost, so really trying to avoid the cost of a second skin.

 

[freddiemercurystwin]

Yes that's exactly what I was thinking and actually knocked up a sketch showing that but there's still a slightly vulnerable bit (from rain ingress through the wall) bit where the render stops.

 

[freddiemercurystwin]

What I did do one one similar job which was effectively the same type of building (external store but adjoined to the rear of an extension, higher ground level) was to take the dpm back up the inside of the wall a bit, then protected it with a rigid board. A bit ad-hoc but BC were happy.

 

[perky416]

wow thanks for taking the time to put so much effort into your reply. Your diagram was my exact initial concept, however we want the patio doors to open flush with the wall on the outside, so the second skin will have to be on the inside.

The building will be on a raft/slab/mat foundation.

Do you think either of the options in the attached image will work? DPM in red. I haven't included DPC for clarity.

The one on the left, between the foundation and the floor screed seems easier to do it if will work.

Sorry for the late reply btw, mrs has given birth and we are contending with a newborn lol.

 

[freddiemercurystwin]

Obviously you have a step in the wall internally but if you're happy with that then either are fine, the left is certainly easier. The previously mentioned thoughts regarding the possible weak spot and the lack of a cavity remain.

 

[perky416]

Ive seen the step in many drawings but didnt realise it was for a purpose. Forgive my ignorance, but please could you tell me the purpose of the step?

 

[freddiemercurystwin]

You have a step on the inside face of the wall on your version yes?