Levelling sandy uneven screed subfloor

[nira49]

We recently lifted carpet and (somewhat cracked) vinyl tiles on the ground floor of our 1950's house as we want to lay celotex insulation and wood flooring. Little glitch: the vinyl tiles were stuck with bitumen on a 15-20mm screeded subfloor that is very dusty/sandy and cracked in some places (mostly near edges and hearth). There is no concrete slab below and from inspecting some of the cracks, the screed seems to be laid directly onto a sandy base, possibly the sand above the hardcore We cannot afford to replace the subfloor, so desperately looking for a way to level and seal this screed! Suggestions please...

We initially considered using a concrete patch repair compound such as U-can from B&Q, but then realised that this may just cause the screed around the repairs to crack again. Would self-levelling compounds for concrete floors work in this situation? What do we need to do so that they 'stick' onto our sandy screed? Another option we are considering is to hire someone to grind the screed sub-floor then proceed with laying DPM, insulation, flooring, but not sure if this is a viable solution or just wishful thinking...

View media item 65612
View media item 65613
p.s. - Here is the order of materials we plan to lay (from top to bottom)once the subfloor is level and sealed. This would lose about 7.5cm of the existing 2.4m floor to ceiling height, and we really don't want to lose too much more. Materials:
Engineered wood flooring (15-18mm)
Plywood (6mm)
Vapour control layer (tbc)
Celotex insulation (50mm)
DPM layer (bitumen paint or plastic DPM)
Existing subfloor with repairs

Thanks for reading

 

[dann09]

Do you live in the countryside? Even just after the war, it was rare for a thin screed to be laid on sand as the base floor.

However, i take it that damp is not a problem?
If so, simply make good to the screed and leave well alone. No grinding necessary.

search this forum with: "floating engineered floor" and the first hit, for Dec 1, 2008 should give you the info req'd. But do a little research.

In brief: lay a 3mm u/lay and float your engineered floor if Mfr's Instr's allow. Hence, read instructions before buying the flooring.

Use a local small shop supplier, and discuss the details that you've posted above.

 

[nira49]

Hi Dann,

I cannot find the thread you mention here, could you please send me the title?

I should have clarified that the floor is very uneven in places, with drops of upto 1cm within 50cm. You're right, damp is not a problem. However, a local supplier recommended putting in a DPM layer (to prevent warping, since the bitumen is mostly gone), then celotex boards (floating), plywood (floating) then floating click-lock floor.
Now >>>> the question is, how do we provide an even surface underneath so that everything stays in place once laid and does not squeak or bounce? We thought about using a self-levelling compound like f-ball p131 or green bag/ Ardex / Mapei (recommendations from various diynot forums) but most of these are meant for concrete floors that are 'clean'. Would any of these work on our sandy screed
View media item 65648
This probably was countryside when the house was built, but not anymore

 

[mikeey84]

HI Nira, in this case I think the best thing to do would be to prime the screed first with something like p131. If you call the Fball technical line, usually a very helpful person will phone you back, and when you explain your situation, will take you through the process, and what materials you need. Be warned- putting a liquid DPM down, with the necessary self levelling compounds and primers is not a cheap option, but as you are probably laying an expensive insualtion/engineered floor, you need to ensure its waterproof!

 

[nira49]

An update as we finally got this resolved (it's taken so long as there's been other building work going on). Thankfully, our builder confirmed that the floor was sandy but not damp. So, after an hour or more of meticulous vaccuming, he sealed it with PVA to settle the sandiness of the screed (Mikeey, P131 may have been a better choice but for some reason he said it wasn't needed in our case). He then patch screeded the worst dips in the floor and, the next day, laid a thin layer of self-levelling compound to level the smaller bumps - so the floor still has a slope, but it is 'correct' enough to lay a laminate/hardwood floor. All done and walkable in under two days!

View media item 70665
During the building work, we also realised that the floor itself is not that cold (the walls are), so we are not laying celotex anymore. However, we are laying a temporary laminate floor for the next year as we want to be sure about this

Sidenote: We are taking the same approach to our shed floor as that could do with being less sandy - it is not an expensive solution and this time we can go DIY as we don't need the levelling to be perfect

Thanks all for your help

 

[dazlight]

Just out of interest what screeding compound did the builder use

 

[nira49]

Dazlight, he used Cempolay, the basic variety. We had the storms just after the work was completed, and wondered if it was not better to go for the "deep" variety as that has tanking properties (especially since it's receiving a wooden floor) - ah well, we'll never know hopefully because there won't be a problem (and to be safe, we put a DPM layer before laying the flooring ).