Pour slab to dpc?

[23vc]

Hi guys
Sorry a basic question I know but I'm planning my own extension so I have a lot of these questions. I am looking at getting brickwork to dpc myself, which I know needs to be 6 -7 bricks above the top of the footing, question I had was if I pour the slab at that point and tamp it level with walls, that means the slab is level with the dpc so the finished screed is say 65mm above that, is that right? Or I thought could build a single brick course above the oversite and lap the dpm onto dpc at that point, but wouldn't normally expect to see bricks on inner leaf at that level? Hope I'm making some sense, obviously I could pour the slab after the walls are up but I'd find it easier to level it off the walls before building up. Any advice appreciated as ever! (I definitely want a screed)
Cheers
John

 

[23vc]

To ask the question a better way, could someone explain in idiots terms at what point is the easiest time to pour the slab, and where it then sits in relation to the dpc and screed?

 

[CJRatch]

Build your brickwork up to the desired DPC height. You want 100mm insulation, 100mm concrete and if you desire a 65mm screed although you can do it thinner. So measure from your DPC height 265mm down, fill your hardcore to that level when compacted, scatter sand to remove any sharp edges, lay the DPM out, push it tight into the corners and lap it over the DPC course. Lay the 100mm insulation inside the DPM, pour the 100mm concrete ontop of the insulation (With steel inbetween the concrete) and you will be left with 65mm for screed. The DPM will still be on show at the moment so be careful not to damage it while building the walls up. Once all water tight add screed to finished DPC level.

Oh also, building control may also want 1" of insulation around the perimeter. When you lay the 4" insulation fit the 1" around the perimeter upto the DPC height then pour concrete.

 

[23vc]

Cheers for the reply, so when you pour the slab your not levelling it off the top of the brickwork your pouring it to 65mm lower? Sounds harder to level it as I was hoping you could rule it/tamp it level with top of brickwork if you see what I mean?

 

[cjard]

Why screed on top of the slab? Surely just flatting the concrete off with a roller striker or power float will suffice? In this case, you can get more insulation in for your proposed buildup, and if you're having underfloor heating you'll want as much as you can get

Note: 100mm of polystyrene may not be enough, thermally, to meet building regs. There are many factors, including the lambda value of the insulation, the perimeter and the area of the floor and the other elements that make it up. To avoid an expensive mistake, use a calculator such as http://www.changeplan.co.uk/u_value_calculator.php (you can get lambda values from the manufacturer website, perimeter and area you should know already) and ensure the building inspector is happy with the proposal before pouring concrete.

 

[Tigercubrider]

If using the dpc level walls to establish the level then just use a big 2x4 and screw a bit of ply to the side so that you end up with a squashed T (or L) - both upside down from the letter that I used.

Make the 2x4 longer than the exterior dimension and the ply the same(or slightly shorter) as the internal measurement. You could also add a piece underneath so that you get an I shape
Set the drop to whatever level you need

Move it side to side while moving left or right

 

[CJRatch]

Cheers for the reply, so when you pour the slab your not levelling it off the top of the brickwork your pouring it to 65mm lower? Sounds harder to level it as I was hoping you could rule it/tamp it level with top of brickwork if you see what I mean?

So long as your concrete is 4" with steel it doesn't have to be perfectly flat as your screed will be the finished floor level. So long as its something like.

Why screed on top of the slab? Surely just flatting the concrete off with a roller striker or power float will suffice? In this case, you can get more insulation in for your proposed buildup, and if you're having underfloor heating you'll want as much as you can get

Note: 100mm of polystyrene may not be enough, thermally, to meet building regs. There are many factors, including the lambda value of the insulation, the perimeter and the area of the floor and the other elements that make it up. To avoid an expensive mistake, use a calculator such as http://www.changeplan.co.uk/u_value_calculator.php (you can get lambda values from the manufacturer website, perimeter and area you should know already) and ensure the building inspector is happy with the proposal before pouring concrete.

I was referring to 100mm foil backed kingspan, xtratherm or similar. Along with an 1" insulation around the perimiter building control will be happy.

Its a good point regards ditching the screed all together its perfectly fine to just use 4" reinforced concrete its how level you can get the finished floor. Using the walls either side as you mentioned and tamping it down across then floating it off should leave you with a good finish and save on screed money.

 

[cjard]

I was referring to 100mm foil backed kingspan, xtratherm or similar

Jolly good - the slab is one of those places where people look at the price of a kingspan sheet, the number of sheets required and think "holy crap.. i'll use Jablite EPS"

As its going under a floor, take a look at www.secondsandco.co.uk - be sure to work out the price per cubic metre of the ugly pallet of misshapes youre buying, as theyre priced randomly, but you can get some really battered stuff for a third the price of normal kingspan - it's even cheaper than sand at that price, so rather than blinding with an inch of sand I blinded with an inch of kingspan, then DPM, then more kingspan on my last job. Even if the boards ar a bit wonky, the concrete will take that out again, and you get a level floor..

Someone mentioned using an oldschool tamp to get the concrete flat - sure can and it does work but it's bloody hard work compared to dragging a spinning roller backwards over the slab and the finish isn't as good. If youre not into powerfloating it for a smooth top, you can always smooth over and fill in any rough patches later using patches of self levelling latex screed. Depending on your floor covering, it might not matter.

If youre going for underfloor heating, now would be a good time to plan it as ideally the pipes should be laid (cable tied to the mesh youre throwing down for the slab) and pressurised before the concrete is poured. Plan your heat source now too, so you can get the right pipe spacing

 

[garyo]

Last time I did this (for a 5M by 3.5M extension) I used a 2x4 to tamp the slab at the DPC level, and it worked surprisingly well - well enough to take engineered wood flooring without issue. I did the following:

- Pour slab, 100mm PIR under slab, 25mm upstands up to DPC level,
- Use 2x4 to tamp flat
- Leave a few hours to go green (can't remember how many... 4 maybe...)
-Lay scaffold boards on concrete block across the corners so that I could reach all parts with a 12" trowel
- Flatten/polish off manually with trowel. Crawl around boards to reach parts that I couldn't reach from outside (my Dad was in the circus)

One thing I'd say - you need to be clear about your finished floor level and how this relates to your uPVC - sometimes you'll have a 30mm sill and a 70mm door frame, which is quite a step. There are ways of addressing it, just give it some thought.

 

[garyo]

Why screed on top of the slab? Surely just flatting the concrete off with a roller striker or power float will suffice? In this case, you can get more insulation in for your proposed buildup, and if you're having underfloor heating you'll want as much as you can get

I've always fancied using one of these, but as a serial DIY'er rather than a pro I don't have the scale or funds to justify the trips to the hire shop when manual tamping and chucking man hours at the problem gives a reasonable result.

 

[23vc]

Brilliant thanks. Reason I wanted to screed is I'm not confident getting the slab good enough but I'll check out the roller options as like you say I can always self level it.
My other concern was rain coming in before the roof is on if it chucks it down and the slab is pretty much at door threshold height, have you guys not had that issue?
Cheers
John

 

[noseall]

Err, is this proposed floor marrying into an existing floor at any point?

If so, allow for a screed. You will do this by setting the sub-slab height lower by whatever floor screed thickness (min 50mm or min 75mm un-bonded) you choose. All the other ingredients are worked out from the existing floor down.

 

[23vc]

PS I've seen a lot of new builds recently where the floor looks bang on before the walls are above dpc, I guess they may be power float / rollering it , looks like a screed but it can't be as you'd never screed before your weather tight?
Cheers

 

[23vc]

Yes it is noseall but only at an existing door opening not a knock through. Thanks

 

[noseall]

PS I've seen a lot of new builds recently where the floor looks bang on before the walls are above dpc, I guess they may be power float / rollering it , looks like a screed but it can't be as you'd never screed before your weather tight?
Cheers

These are likely to be power-floated. You don't want to be going there unless you are confident working with concrete and a beast of a tool.