Proposed workshop build

[DomP]

Thanks Fitz, that's exactly the issue. I'll take a look at the link as I want the raft to be my finished floor with 3 courses of brick then my studwork

 

[Jones]

Basically there's nothing you can do that isn't a bodge. Poured concrete is moderately waterproof so moisture won't move far into it. The dpm will stop moisture coming from underneath so it's only the edges you need to worry about . Keeping the edge dry is the most important thing, a gravel surround, good roof overhang and keeping vegetation down so you've got good air flow should achieve that. When it does rain water will penetrate the concrete a bit but if it can dry again that's not a problem.If the dpm is folded up it is exposed to uv and mechanical damage and won't last long. Bitumen paint, tanking slurry etc might help a bit but sealing the joints is not reliable.

 

[highwood122]

It does not give continuity if you follow Mike's design, moisture can still enter the slab from the edges and get into/through your brick course. Per Mike's diagram you then have a DPC on top of your brick course to prevent moisture entering the timber structure and you have another barrier between slab and flooring (500 gauge polythene to keep any moisture in the slab away from your floor.

If you use the concrete slab as your finished floor and you want it isolated from any moisture source you need another design. If you google 'concrete slab floor wall detail' you get lots of different design, for example this one, where you can see extra elements to give continuity of the DPM around the slab.

Fitz

what use is the 1200g poly at the bottom

 

[Jones]

what use is the 1200g poly at the bottom

It contains the pour and retains the water in the mix giving a better cure and makes it easier to float. If it's all you've got it keeps the center of the slab dry .

 

[DomP]

Basically there's nothing you can do that isn't a bodge. Poured concrete is moderately waterproof so moisture won't move far into it. The dpm will stop moisture coming from underneath so it's only the edges you need to worry about . Keeping the edge dry is the most important thing, a gravel surround, good roof overhang and keeping vegetation down so you've got good air flow should achieve that. When it does rain water will penetrate the concrete a bit but if it can dry again that's not a problem.If the dpm is folded up it is exposed to uv and mechanical damage and won't last long. Bitumen paint, tanking slurry etc might help a bit but sealing the joints is not reliable.

Would thompsons water seal work?

 

[Jones]

Any paint on product on the slab edge won't do any harm and will probably help a bit. Unless you put in a dpm lapped into the dpc though it won't be perfect.

 

[TomGW]

Some areas of Canada do a variation of the footings and block walls you mention heimlaga by pouring a solid concrete wall on top of the footings. When I was a pup out of high school near Vancouver BC I worked as a framers helper. We put in the footings and forms for modest sized houses without basements, for the concrete, poured in one go. Footings were of 2x6 or 2x8 held with stakes a couple feet apart and 1x2 were nailed across the tops. Rented plywood forms were set on top of the 1x2s. The outside sheets against a 2x4 also nailed to the 1x2s for locating. When ready the concrete trucks and sometimes a pumper truck showed up and the concrete poured into the forms which then flowed down into the footings. These were for foundation walls from 2 to 4 feet high. Once the tops of the foundation walls were troweled smooth and level we went home for the weekend. The following week the forms were striped, scraped clean and rental company picked them up. The 2x material for the footings were cleaned up and reused in the house. The only thing left of the forms were the broken bits of 1x2 across the top of the footing and through the walls. No termites so that wasn't a concern. Then the wood floor was built or the walls were built with the slab being poured after the plumbers had placed all their pipes. I like the method much more that the bricks on a slab method used by many on the forum as there is never any chance of water getting through any of the mortar holding the bricks.

This area also has frost like you do but the practice for piers is to use helical/screw piles going down at least 10' or until enough resistance is met. Plastic sleeves on the outside of the screw pile lets the ground heave by sliding along with the frozen dirt. Quick to install and no digging needed.

Sorry for the diversion. Back to the regularly scheduled progam.

Pete

Divided by a common language
In the UK & Ireland, in conventional construction, the initial strip of concrete at the base of the excavations is referred to as 'foundations', while the block/brickwork up to dpc or floor level is known as the 'footings'. This seems to be the reverse across the pond, giving rise to some confusion.

 

[Inspector]

Divided by a common language
In the UK & Ireland, in conventional construction, the initial strip of concrete at the base of the excavations is referred to as 'foundations', while the block/brickwork up to dpc or floor level is known as the 'footings'. This seems to be the reverse across the pond, giving rise to some confusion.

Yup that would be the case except for most places blocks/bricks are not used. More concrete instead.

Pete

 

[TomGW]

As Jones says , there's little you can do that isn't a bodge. However, I built a garden room/shed for my daughter and this is the bodge I used and 4 or 5 years later it's as dry as a bone.
I put a 25mm layer of insulation on top of the blinded, compacted hardcore with a sturdy dpm on top and turned up the formwork. Trimmed the dpm level with the top of the timber formwork after pouring and before rubbing up the floor. Then after removing the formwork I stuck the dpm to the edge of the concrete slab, built a row of concrete blocks with a sole plate on top. Next I stapled a wide dpc to the soleplate and stuck it to the blocks with blobs of Tec7 and to the turned up dpm with a continuous bead of Tec7. The dpc extends down to the bottom of the slab.
Then fixed (screwed) roofing slate to the soleplate before battening and cladding. The screws in the slates are hidden under the bottom of the cladding which provides a drip rail. The vertical dpc is both hidden and protected by the slates that are simply butt joined - I used man made slate as it's cheaper and easier to drill and cut.
Yes, it's a bit of a bodge but it is 100% effective and looks very smart.
You could actually make the soleplate 25mm wider to overhang the concrete block and stick 25mm insulation between the dpc and slate, to give a thermal break.

 

[DomP]

As the weather is looking good I intend hiring a mini digger next weekend to dig out for my type 1 mot sub base, I have a few questions however.

As the ground is pretty solid clay I thought 100mm mot, blinding sand, membrane followed by 100-120mm concrete pad (7.2 x 4.8m) my main query is should I spread the mot beyond this with shuttering above so there is no subsidence of mot when forms come off? The problem I can see with this driving retaining posts through the type 1.

Any thoughts?

 

[Molynoox]

As the weather is looking good I intend hiring a mini digger next weekend to dig out for my type 1 mot sub base, I have a few questions however.

As the ground is pretty solid clay I thought 100mm mot, blinding sand, membrane followed by 100-120mm concrete pad (7.2 x 4.8m) my main query is should I spread the mot beyond this with shuttering above so there is no subsidence of mot when forms come off? The problem I can see with this driving retaining posts through the type 1.

Any thoughts?

I'm just building a cartlodge at the moment and this is how we have done it. The blockwork retains the type 1.

 

[davethebb]

DomP, When I built my workshop (look for 4.4 x 6.8mt Workshop in a grade 2 listed property - I will update) I used scaffold boards as the shuttering for the sand, Type 1 and the concrete. I folded the DPM up the scaffold board and turned it back into itself about halfway up the concrete height. Just before the concrete was poured (I used fiber reinforced as it saves laying out the rebar (which is difficult to accurately do if you only have 100 mm concrete), I scooped away the Type 1 and sand about 4" from the scaffold boards - this creates a high strength perimeter beam and also encloses the sand and Type 1.

 

[DomP]

DomP, When I built my workshop (look for 4.4 x 6.8mt Workshop in a grade 2 listed property - I will update) I used scaffold boards as the shuttering for the sand, Type 1 and the concrete. I folded the DPM up the scaffold board and turned it back into itself about halfway up the concrete height. Just before the concrete was poured (I used fiber reinforced as it saves laying out the rebar (which is difficult to accurately do if you only have 100 mm concrete), I scooped away the Type 1 and sand about 4" from the scaffold boards - this creates a high strength perimeter beam and also encloses the sand and Type 1.

I have been lead to believe that fibre reinforced isn't for slab strength but surface strength, I aim to set my mesh towards the bottom of my raft. I also thought the MOT around the raft helped with drainage and aopporting the slab

 

[DomP]

View attachment 174618
I'm just building a cartlodge at the moment and this is how we have done it. The blockwork retains the type 1.

I intend a raft so no foundations for blocks

 

[DomP]

I've got a mini digger booked for Saturday but I'm having second thoughts on my area of 7.2 x 4.8m (just a shade over 30m² internal area) it looks quite square but that works well with sheet sizes. I'd go 8.4 long but without planning that's pushing it I think.

 

[DomP]

Well I got my pad dug out with a fair amount of help from my nephew who does landscaping. Just waiting for some time to go and pick up some boarding to make my form work with before getting a load of type 1 to whacker plate down. I'll book the concrete ready for a few weeks time when I know I've got some helpers.

 

[Jameshow]

Well I got my pad dug out with a fair amount of help from my nephew who does landscaping. Just waiting for some time to go and pick up some boarding to make my form work with before getting a load of type 1 to whacker plate down. I'll book the concrete ready for a few weeks time when I know I've got some helpers.

8.4m??

 

[Molynoox]

8.4m??

I think he is considering expanding from 7.2 to 8.4

 

[DomP]

For now it's 7.2m to stay within the permitted development of 30m² internal floor area....well nearly

 

[flying haggis]

Nothing to stop you putting down a bigger foundation so that you have a work area outside, that could be built over later............