Concrete subfloor question

[Nick2847]

Hi, novice here. Just bought a house and the kitchen floor has a sandy mix at the edges. This sandy material is damp in places. It's the gap between the external wall (conservatory on other side) and kitchen.

The concrete subfloor is about 2 inches thick, then I think it is solid concrete under that. Would this be correct? Or is this a layer of screed?

Are there any issues with this floor construction? Granted there doesn't appear to be a DPM but the concrete floor itself looks dry in the centre, just damp and musty at the edges.

Pictures show
A. edge of the concrete subfloor with sand between it and external wall.
B. The kitchen floor behind the units and dry overall.

Thanks for any tips

 

[Deleted member 174758]

This sandy material is damp in places. It's the gap between the external wall (conservatory on other side) and kitchen.

That immediately rings alarm bells for me. Properly mixed screed isn't loose or sandy and a concrete floor laid over a DPM or even a layer of screed over a DPM shouldn't be wet. I'd actually expect to see some evidence of the DPM bring lapped up the wall, but I see none in your pictures

The concrete subfloor is about 2 inches thick, then I think it is solid concrete under that. Would this be correct? Or is this a layer of screed?

I'd expect to see a concrete floor at 4 to 6 in thick. 2in is very thin and might crack. It is probably too thin to get rebar in there - rebar shoul be at least 1in in from the outer surfaces

Are there any issues with this floor construction? Granted there doesn't appear to be a DPM but the concrete floor itself looks dry in the centre, just damp and musty at the edges.

Without a DPM if the water table outside gets near to the surface, such as you get with prolonged rainy period in some soils, it is possible for water tabke to rise to the underside ofcthe concrete and the concrete sub-floor to wick up moisture.

I am tempted to think that your concrete sub-floor may well have a a DPM but that it may have been trimmed back too far (or not lapped up the walls high enough) before the screed was laid, allowing the screed to finish touching the bottom of the walls. The house looks to be fairly old, which suggests solid walls and no DPC/DPM in the walls. That means moisure can wick up the walls or soak through from the outside (in the case of thinner walls) and the moisture is then being drawn into the screed, hence the damp floor edges. There shouldn't be any sand in there at all.

Without putting a Tramex on the floor it is difficult to gauge how damp the floor is, But I wouldn't lay laminate flooring directly on it without sealing the floor with at least a liquid DPM, then installing any laminate on top of a foam (moisture resistant) underlay and ensuring the laminate terminates 10mm shy of the walls (picture 1 shows the laminate hard against the wall - laminate needs a 10mm expansion gap all round). Gaps like that are filled with cork packers, compraband, etc. I'd also consider chopping back the plaster at the bottom if the bottom of the wall where there is damp to maybe 50mm up from the floor to reduce and moisture in the bottom of the plasterwork

 

[Nick2847]

Thanks for the reply @JobAndKnock

I think the floor is concrete foundations then a 2 inch layer of screed. Thanks for the points on DPM. The adjoining lounge next to this room has the same concrete subfloor but a blue DPM can be seen around the edges. Same for the front room. The kitchen maybe with out one and I'll inspect more for any signs one has been trimmed back. A liquid DPM is therefore a good option.

The plaster seems to be chipped from the walls to prevent bridging and no moisture noted. Also the laminate to the wall is 2 small blocks for the fridge feet but I'll cut them down a little more to allow for a better gap.

The floor was changed by the previous owner a year ago. My main worry is the sandy material in the expansion gap around the edge of the floor. Is this just an old method of packing out the screed layer? Will look for any increase moisture when it rains to see if that is the culprit. What would be the best way to pack out the expansion gap around the screed floor edge, if any?

Thanks again

 

[Deleted member 174758]

My main worry is the sandy material in the expansion gap around the edge of the floor. Is this just an old method of packing out the screed layer?

TBH I'm unsure. These days they often use a fairly dense closed cell polyethylene foam for expansion joints including by the walls. A number of older buildings I've worked on have had a rubber or what looked like poured bitumen expansion joints. In some cases they just lap the DPM up the walls and pour concrete right to the edges, although brick and concrete do have different coeffcients of expansion, so not sure how well that works (probably OK in small rooms). The yellowness of the screed makes me think that whoever did the job skimped on the cement in the mix - every time I deal with screeded floors they are grey - and I have to wonder if it is sand at the edges or whether it's just the screed breaking up and looking like it is sand. Either way seems a bit of a bodgey way to do things.

Will look for any increase moisture when it rains to see if that is the culprit. What would be the best way to pack out the expansion gap around the screed floor edge, if any?

Also worth looking at gutters to see if there are splits or accumulated detritus causing over flowing, and down pipes to see if there's a leak, blockage or disconnected joint/missing shoe, etc. Then check the drainage if nearby to make sure it's actually draining

 

[Nick2847]

Thanks @JobAndKnock all great advise.

The whole house has previously had bodgey repairs so this is just another thing for the list! This room is next so I'll address the floor properly, just hoped it wasn't something that needed addressing straight away and turn into a big job.

I had a CCTV survey of the underground drains (that do run just in front of this wall) and all clear, but will check for leaks in the drain that is just on the other side of this wall so thanks for that advice.

 

[Deleted member 174758]

Yes, if you have damp in walls it's always worth checking the gutters, dowmpipes and drains because they are so often ignored. I'm not a screeder or concreter, I'm a chippy, but I do lay floors as part of the job, so I know more or less what I am looking for in a prepared surface. It's often a bit awkward with something like this to be 100% sure of how bad things are, but one thing is for sure - the floor should be bone dry or your laminate won't last long at all

 

[Nick2847]

Thanks mate, your help is appreciated and all taken on board. Some good places for me to start although I will be getting someone in as my DIY skills don't stretch beyond a bit of painting. Cheers.