Building into a slope...

[Mr_Mickey_D]

Evening folks,

I've been a long-term lurker here, getting inspired to get back to my woodworking roots (apprentice cabinetmaker some 20 yrs ago..) and build some furniture for our home. My workshop is shocking though; its too small, lets in water, cold, dark, over run with mice and last winters weather made a right mess of the roof so i'm tearing it down and replacing with a new timber framed/clad workshop, sat on a 4" blockwork plinth, on a conc slab foundation. I'm comfortable with the timber framing work (thanks MikeG for the very helpful guide!) and poured a few slabs in my time, but i'm unsure on this part...

The site is sloping, with the current building built up on the downhill end to minimise the dig into the slope at the other end. I'm looking to improve internal head height without increasing overall height and would like to do away with the steps up into the building, so looking to dig the far end into the slope to a depth of about 750mm, to make the downhill end sit at (about) ground level. The block work plinth would be a single course high at the downhill end and built up to four blocks high at the "dug in" end to form a retaining wall. Retaining wall will be about 4m long, so i'll build up a pier in the middle. The length into the slope will be 5.5m.

I'll add a sketch when i figure out how to...

What i'm struggling with is waterproofing.
I was thinking i'd bring the visqueen from under the slab up the end/sides of the blockwork to above ground level, install a 4" perforated pipe along side the slab and back fill to ground level with a permeable fill material e.g. gravel (with a permeable geotextile around the pipe to stop fines blocking the perforations?).

Is this going to be enough to keep the blockwork & inside of the building dry? I've seen bituminous paint on the back of blocks used before, even a rubberised sheet instead of bringing the visq. up - should i be doing something like this too? Any particular products or specifications to be aware of?

Any other water proofing ideas you could suggest for me?

Advice gratefully received.

Mick

 

[cammy9r]

Hi. Why not dig out larger than the workshop then build the retaining wall, maybe with weep holes. Keep the French drain in front of wall then pour a slab to build plinth on. Give yourself around 700mm between retaining wall and workshop wall. Few extra days with a shovel will be forgotten quickly and you should never have to worry about the barrier failing and a damp workshop.

 

[MikeG.]

Right, this is important. My normal detail, and the one given in my "build a shed Mike's way" thread, assumes the top of the slab is above ground level. The DPM below the slab is not linked to the DPC. Normally, in an outbuilding like this, this works very well, but it isn't good enough for a situation where the ground level is above the slab. You will require a more orthodox plinth construction, allowing the DPM to be turned up vertically (and protected), and lapped with the DPC.

Retaining walls are a "thing" in themselves. If you are retaining 750mm of soil, your wall should be about 200mm thick, and that gives us a nice starting point for designing the DPM/ tanking. This equates to 2 courses of bricks/ blocks. The obvious thing is to have the DPM run up the cavity between the two. I've just been called away, so I'll try to come back to this later.

 

[Mr_Mickey_D]

Hi. Why not dig out larger than the workshop then build the retaining wall, maybe with weep holes. Keep the French drain in front of wall then pour a slab to build plinth on. Give yourself around 700mm between retaining wall and workshop wall. Few extra days with a shovel will be forgotten quickly and you should never have to worry about the barrier failing and a damp workshop.

If I had more space, I'd certainly do something like that. But garden space is at a premium and the passage between could be more workshop space!

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[Mr_Mickey_D]

Right, this is important. My normal detail, and the one given in my "build a shed Mike's way" thread, assumes the top of the slab is above ground level. The DPM below the slab is not linked to the DPC. Normally, in an outbuilding like this, this works very well, but it isn't good enough for a situation where the ground level is above the slab. You will require a more orthodox plinth construction, allowing the DPM to be turned up vertically (and protected), and lapped with the DPC.

Retaining walls are a "thing" in themselves. If you are retaining 750mm of soil, your wall should be about 200mm thick, and that gives us a nice starting point for designing the DPM/ tanking. This equates to 2 courses of bricks/ blocks. The obvious thing is to have the DPM run up the cavity between the two. I've just been called away, so I'll try to come back to this later.

Looking forward to your next instalment!

Ive just been looking at the vandex primary waterproofing/tanking stuff and the oldroyd membranes as a secondary barrier... Am I on the right lines here?

Mick

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[Mr_Mickey_D]

Trying to add my sketch again...

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[Inspector]

Before you get too far along you should explain what you have for soils and how much slope is above the site. Water in clay soil behaves much different than sandy/gravel and a 10 foot hill with a gentle slope sheds much less water than a 100 foot hill. Is the slope crowned away from the area, flattish to it, or are you in a bit of a gully? All factor into how you approach the type of foundation, waterproofing and drainage.

Pete

 

[Mr_Mickey_D]

Good point.

Soils are of the Brickfield 2 association (https://www.landis.org.uk/services/soil ... m?mu=71306) generally a fine loam with some seasonal water logging for the first 6 to 8 inches, a stoney clay loam for the next foot or so, then quite a dense block structured stoney clay loam (much greyer & mottled) as the lowest horizon I've been down to...

Essentially we are on glacial till between the mountains and the valley bottom. Wetness class III. Generally found to be reasonably permeable.

Above my property is a perm grassland field, with the top of the slope about 120m back. The slope varies from 1 in 5 to 1 in 4, prob 25 to 30m AOD difference between this shed and top of slope (including a rocky outcrop "step".

No other significant ground water considerations (eg tree roots, field drainage, buildings etc.)

Does that help frame your advice Pete?

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[gmgmgm]

It will be a continual nightmare having the retaining wall as part of your workshop- serious drainage considerations. I'd agree with trying (whatever it takes) to have a separate wall. Maybe you can do away with a retaining wall entirely (big cost saving) and just have a steep slope coming down to your wall?

Having a space at the back of the workshop could be handy- if you have it covered over (extend roof overhang) then you could use it for all sorts of storage of bulky items which can be outside. And long pieces of wood which are about to be used for projects.

 

[Inspector]

I'm not an soil engineer but brought it up more for MikeG and the like.

Sounds to me like it can get soggy for some time after rain while the water moves to lower ground. Having experienced failure of spray on tar like water proofing on our house I lean toward membrane applied from the lower edge of the footing up and across the top and up the foundation wall, with the dimpled sheeting on the wall to protect the membrane and facilitate the flow of water to the footing drains. We're having exactly that done to the house in a couple weeks. I would also make a poured concrete wall but that comes down to access to the site and personal preference. Form rentals are cheap around here but may not be in your world. The wall to footing joint should be sealed well and making a angled corner either with pressure treated wood or a cement fillet at the joint makes it easier to apply the membrane without gaps under it and not have water pool in those corners. I'll probably get dumped on for suggesting it but I'd run 2 drain pipes around the footings to daylight. The building is small so the extra cost of pipe is negligible and I would rather over build than have to dig it up again.

Now is the time to think about how or even if you will be insulating the foundation.

Pete

 

[MikeG.]

.......Maybe you can do away with a retaining wall entirely (big cost saving) and just have a steep slope coming down to your wall?..........

Which turns the wall into a retaining wall.

 

[Jamster21]

No comments on the structure of the retaining or otherwise wall - geotextile-wrapped or capped drainage pipe though yes and the other suggestion is to run the gravel to the surface not on a vertical fill but away from your wall at 45 degrees. I've seen this suggested as lecia fill in green building forums to add some insulative value too.

 

[gmgmgm]

.......Maybe you can do away with a retaining wall entirely (big cost saving) and just have a steep slope coming down to your wall?..........

Which turns the wall into a retaining wall.

I mean a steep slope coming down, then a flat path-like area at ground-level, then your wall goes up from there. So long as the angle is better than the angle of repose (IIRC) then the two are not connected and the wall retains nothing, I believe.