What have I taken on?!

[skwdenyer]

Hi

This is probably a little outside the realms of many DIY projects, but here goes. I have a 2200 sq ft, ground floor space (approx 28ft x 78ft). It was, we think, originally built as a dairy or laundry. It has glazed-brick walls, and a roof supported by steel pillars up the centre.

The floor we "inherited" was T+G chipboard over battens. It sloped, badly. We took up that floor, and found what seemed to be a "concrete screed" floor - basically quite a dry mix of concrete, about 4-5 inches thick. . That floor sloped, too.

The sloping was for drainage. The centre of the room (the "long" axis) is a few inches lower than the sides, and one end higher than the other, to allow water to drain.

Anyhow, we dug out the old screed (kango hammers and shovels - about 50 tons in total), and now we have a concrete floor which still slopes, is very uneven (height varies by +/- 4 inches in place), and appears to have been laid in several sections and patched up over many years. That "floor" is also the "slab". Being Victorian, there is no DPM under the slab. There are also various drains (gullies and manholes) in that floor (quite a few of which were simple screeded over before...).

OK, so far, so silly. We can't dig out the slab and start again, for all sorts of reasons, mostly to do with foundations and so on. The finished floor surface we'd like to use is EcoScreed with embedded under-floor heating (pipes).

So the question is how to get from where we are to where we want to be! We've had one "expert" who suggested the following:

- levelling the existing concrete with compacted sand (a lot of it!)
- laying slab insulation (Celotex or similar)
- laying builder's plastic, overlapped and taped
- lay UFH pipes in proprietary plastic "grid" system
- pump EcoScreed over UFH pipes to 50mm or so depth

The net result of this is to create a floating screed, sitting on the insulation.

The main thing I'm concerned about is whether compacted sand at varying thicknesses (0.5 - 5 inches) is a sensible and/or sufficient way to proceed, or whether I'm going to need to put in some concrete first to bring up the levels.

Any thoughts? Thanks!

I also have a problem of having to bring up the manhole / gully levels up, which will mean that they are "rigid" whilst the screed surrounding it will be floating. Any ideas on how best to deal with that for reliability?

Thanks!!

 

[Symptoms]

skw - your 'expert' has given you the correct advice. Your floor is too far out to economically level it any other way ... compacted sand is the cost effective answer. Floating floor just means it isn't fixed or bonded to the sub-floor, it doesn't move up or down so there are no issues regarding the screed interface with the inspection chambers (manholes) & gullies - this is only true if all has been correctly installed. EcoScreed is designed to be either bonded or unbonded (floating). Maybe get the EcoScreed people in to give you a technical assessment.

 

[skwdenyer]

skw - your 'expert' has given you the correct advice. Your floor is too far out to economically level it any other way ... compacted sand is the cost effective answer. Floating floor just means it isn't fixed or bonded to the sub-floor, it doesn't move up or down so there are no issues regarding the screed interface with the inspection chambers (manholes) & gullies - this is only true if all has been correctly installed. EcoScreed is designed to be either bonded or unbonded (floating). Maybe get the EcoScreed people in to give you a technical assessment.

Hi Symptoms, many thanks for the reply, and sorry for the delay. When I used the term "expert" it was actually to denote the aggregate (no pun intended...) of a number of points of view, not a pejorative term - didn't really make that clear. One of those opinions was from the EcoScreed people (very, very helpful people in my limited experience so far), but the nature of their service to us in this situation is that they wouldn't be able guarantee the results, hence casting out for further opinions!

I understand the issue re floating vs bonded - what I was more concerned about was that the insulation might have some limited degree of vertical compliance, allowing the screed to move relative to the subfloor, whereas the gullies / manholes obviously don't move relative to the subfloor.

Do you have any experience as to what degree of compaction would be required to achieve the desired results? I'm presuming I would need some type of whacker plate, and I would imagine that I need to arrive at some sort of standard test for compaction (is a CBR test appropriate?).

Finally, what type of sand?!

Many thanks again

 

[Symptoms]

skw - the layer of sand blinding on your rough slab is to ensure that the insulation boards are continuously supported; you don't need to use a whacker plate (and in any case some of your thicknesses of sand, 1/2", won't take it) just level it with a board & float smoothish. Use sharp sand.

 

[skwdenyer]

Symptoms: many thanks for such a quick reply.

Obviously I was getting confused by the term "compacted", taking it to mean sand which was compacted over and above the force of gravity! I'd been assuming I would need a minimum, say, 2" of sand at the least specifically to allow for compaction. I believed I had read that uncompacted sand could settle by up to 10% (of uncompacted height) over time, which clearly would be bad news for me in terms of keeping the floating screed "slab" supported. It sounds like you think this wouldn't be a problem - do I have that right?

Since the underlying slab has no DPM, my current presumption is that putting large-overlap, taped building plastic over the sand but under the slab insulation would be sensible, as well as the same over the insulation to act as the "bath" into which to pump the screed. This is based on the assumption that PU foam (Celotex / Kingspan / whatever) doesn't much like getting damp. Would you think that worthwhile?

Thanks once again![/list]

 

[Symptoms]

skw - when I said "just level it with a board & float smoothish" I should have said "just tamp it level with a board & float smoothish". So you will compact it to a degree. I'd also go with your plan for the poly sheeting (but check with whichever insulation manufacturer you choose whether it's also need below their slab) but make sure you have plenty turned up at the walls, etc.

What fixing system are you using for your UFH (remember to test it for leaks before you screed ).

Unless you're going to drive vehicles over the finished floor I'd not worry about it moving 'cos most insulation slabs resist compression well and with tamped sand all should be firm. If you're worried about the inspection covers, etc. why not fashion some steel frames to border them, mastic the small gap.

Finally, what are you going to use the place for ... an open-plan living space or offices? What about internal walls?

 

[chappers]

i would gereally agree use sand to level up the slope compacted by hand will be sufficient why not get your screeder to do this it will be perfect. i would lay the membrane under the insulation, with the underfloor over that followed by the screed. Ensure you have at least 75mm of screed as it will be un-bonded. Are any of these inspection covers actually still part of your existing waste system

 

[Symptoms]

skw - a final thought ... use a laser level, buy or hire a good make and not a cheepo as their beams won't cast far enough. If you've a problem with the set-up height of the laser (in the centre of the room) excavate a shallow pit in the existing slab to accommodate.