Plugging hole before screeding floor

17 Jun 2016
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United Kingdom
Hi all,

I am preparing to screed my concrete subfloor, having pulled out the old one which was uneven and full of redundant pipework.
I am the top-floor flat and there is a riser for the soil pipe, mains water feed etc running inside the flats. There is a gap between the structural concrete subfloor and a lump of concrete blobbed on to these services.
Obviously, I do not want to chuck a load of screed down this gap, as it will simply fall down to the next floor or further.
A builder suggested stuffing some newspaper there and then expanding foam, trim and put the screed on top. I am concerned that it won't support the screed (70mm 1:3 cement:sand).
You can see that the gap is ~70mm on one side and ~100mm on the other. There's nothing to offer support in that void. The floor slab is ~110mm or more thick.
Timber framed walls that will surround the riser need to be built directly above those gaps (due to where windows are).

Can anyone suggest how to plug these voids so that the screed can go right over? Will expanding foam on newspaper really hold that chunk of screed??

Many thanks for any advice or views!


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What can you see at the bottom of the crack - is it the ceiling below?
The crack appears to go under a wall - where to?
Do you have any other cracks in the flat floors?
I can count maybe seven different pipes coming through the concrete - do you know whats what, can you ID the gas riser if you have one?

Can you step back and photo the whole length of the crack?
The void is just continuous, the flat below has a box around this riser space so there isn't a ceiling, just a void right the way down.
The wall you spotted is the external wall, it has a little notch that the blob of concrete on the pipes is notched in to, to help support it.
There's no gas pipe in this riser, that is located elsewhere. It's just mains water, tank water (more than one), soil pipe.

It's not a crack, it is the edge of the concrete subfloor, it was built with a square opening in it for the riser to run through. This gap was then covered with a big blob of concrete before the original screed was laid. In taking up the old screed the edges of this concrete blob got knocked off so there's a gap now.

Can expanding foam be used to plug this and take the weight of the part of screed covering it?
Or do I need to put down some kind of panel fixed to the floor?
You dont want any kind of flammable material in a chase such as yours.
Prep the edges inside the hole - use PVA and water or just PVA or any handy glue.
Squirt layers of expanding foam into the hole and allow to dry.
Remove some 50mm or so of foam and gently fill with
a very dry mix of sand & cement (small bags of mix are available at all plumbing suppliers).
Shape into a blob larger than the hole and gently fill the hole - leaving it proud.
Allow it to begin setting and then level off.

There are other ways but the above will do the trick.

Presumably you are going to box in the risers in your flat?

Whatever you intend to screed that surface with: first remove/brush/vacuum all dust, & then prime with a suitable primer for the screed.
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Thanks a lot for your advice vinn, really appreciate it, and I'll try what you suggest. I will box it in, and because of the location of windows, the framing for this box ought to be partially overlapping those gaps (which concerned me!)

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