pouring a concrete slab to an extension before or after weather tight.

22 Jan 2013
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United Kingdom
Hi all,

I’m currently self-building an extension to my property. I’m adding an 8m x 6m single storey extension with steel work to possibly take a second storey in the future (when I win the lotto!) I’m wondering can I put the concrete floor slab, insulation and screed in once the roof is on and weather tight. I was thinking of overlapping the DPC with the DPM and taping the seams when it comes to it. I already have my hard core down as the clay was pretty messy with the dog forgetting to wipe his paws! My main concern is as I’m doing it weekend to weekend if it rains once the DPM is down I will be creating a massive swimming pool with floating insulation lilo’sl. I have a soil pipe which runs through the site at an awkward height meaning once the insulation is needed to go in the soil pipe will have to be moved meaning I will have to move out as the toilet will be relocated. I also have an existing kitchen extension which will be demolished and the floor will continue through at this height giving me my finished floor level with the final screed. The steels will go in once the walls are up to height and the concrete will cover the base of the steel column support bolted down to the pad foundation rather than shuttering this off and back filling at a later date. So I’m wondering is this common practice with something of this nature?

Thanks all

Please see photos thanks


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The floor slab can go in later, yes. As long as you make provision for the DPM to lap onto the DPC with some slices of poly DPM left hanging out of the joint for future lapping.

How are you going to 'move' the soil pipe? Are you relocating it along with the bathroom and cloak suite onto a proposed outside wall? Bit baffled by this.
We don't let stuff like that hinder us from getting our slab down. We often have to run temporary waste pipes above slab level and out of say a doorway and into a relocated drain.
Also the screed can deal with any discrepancies once you demolish the old kitchen structure and give you a scar free finished floor.
Thanks noseall,
Im relocating the downstairs toilet and the toilet in the bathroom. They will both be located on the side wall of the property and a new soil stack teed into the sewage line. Can I use the wide dpc left sticking out of the brickwork as the skirt or is it better to better a skirt of the Dpm?


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Use a wide strip of DPM, it will be easier to manipulate. We tack ours to the wall once it is built.
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Bit late as your levels might be wrong but you could have forgone the screed and just made the concrete base as you ff. Infact stick an extra 50mm of eps down and you'll have still saved yourself money. If you can borrow or buy a bull float the finish you get would be fine for any floor finish, or you could get someone to power float it and use that as your finish.
I've just done an internal floor with a colour hardener and power floated that in, it looks quite good!

Depending on your orientation you may find it a bit dindgy at the back of the extension have you considered some roof lights?

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