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Groundwork & Foundations

Replacing suspended timber floor with concrete - how to deal with deep floor build up

Anonymous user 09/03/2024 - 3.36 PM

Hi, The property is a 1920's bungalow which had previous damp issues (all now resolved). Most of the floor joists are either rotten or borderline so I'm planning to replace the subfloor with a concrete one, and have underfloor heating installed on top. There is some kind of coarse concrete slab on the ground which doesn't come all the way to the external walls (any ideas what this is and why it's there?). The build up from this slab to the top of the existing floorboards is about 650mm. This seems like a huge space to fill as I've only seen 50mm screeds and up to 200mm thick insulation. I found void former after some research, but this seems like something used mainly in large commercial projects. Even this has a max thickness of 300mm. Any ideas on how to go about filling the space up to the screed and insulation level? (or if this is the wrong approach, what would you recommend so that I can still have underfloor heating?)

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3 Answers

Anonymous user

Couple options I would go for. Either fill up 250mm with clean stone, 100mm compacted type 1, sand blinding, DPM, 100mm concrete, 150mm insulation then your 50mm screed or build the first 350mm up in type 1 but 150mm at a time and then compact before laying the next 150mm.

Answered9 November 2023

Anonymous user

Fill with type 1 sub base and compact in 200 mm layers

Answered16 November 2023

Clarks Building and Design

Rating: 5 out of 5

Heathfield

I personally would go for 300mm of good clean hardcore, max half brick in individual size, levelled and quickly whacked over with medium sized Whacker plate, then a decent sand blind, then a good C 35 Concret pad of 180mm throughout area, to lay a good grade DPM over whole area and up walls pass external DPM level, any cut sheets to be overlapped by a min of 300mm and also taped , fit a good sound 100mm celotex or the like, all underfloor first fix pipework laid followed by 70mm of screed with fibres to finish, this is a total of 650 mm and will be the most cost effective best bang for you buck that is fully Building reg approved and of a quality high standard, please note floor actual covering depth could also be tweaked into build up if desired .👍

Answered2 December 2023