Replacement floor - screed or concrete?

[HarryM]

I live in Cornwall and have had a replacement floor installed in the dining room (about 13 sq. metres). The old floor was excavated, a DPM laid, 4" of concrete, and about 2" of screed. Unfortunately the guy made a hash of it and the screed crumbled and had to be removed.

I have had several quotes to replace the screed but they all differ as to what should be done, and none inspire total confidence.

Most builders say they would lay a screed with a 3 to 1 mix. But one said that would not be strong enough and suggested 5 to 2.

Yet another, who comes from out of County, said that the building sand in Cornwall is cr*p. He says a 4 to 1 mix would be best as adding more cement only weakens the screed.

He also advised that the concrete underfloor is unsatisfactory because the guy did not excavate properly up to the edges of the walls and the concrete has hardly any depth at the edges. In fact in some spots the concrete ends before the edge of the walls. So he recommends laying a concrete floor rather than a screed because a screed won't be strong enough.

This seems sensible, but builders merchants advise me that if I'm going to lay concrete I should use ballast, not sand. But the bulk ballast they sell looks extremely rough with big lumps in it, and not suitable for a smooth finish.

The choice available in bulk in my area is grey (granite?) building sand, the grey rough ballast, sharp sand (which I haven't seen), or the more expensive yellow local sand (presumably from a beach).

I think it would be best to go for a concrete floor, but I would welcome advise on what sand/ballast to use, and what would be the best mix. Also would the finish be smooth enough for tiling?

I'm being very cautious about this because I don't want to have to dig the floor out again.

Thanks in advance.

Harry

 

[HarryM]

Can anyone also advise the volume of sand contained in a bulk bag?

 

[noseall]

if what people are saying is true and the screed sand in your area is crap, then it would be wise to lay a concrete trowel floated floor.
this involves using sand and gravel (ballast) mixed with cement. 6 parts ballast to 1 part cement. you simply mix the two together with water, lay it to the floor area and tamp it level. this in turn works the fat to the top, allowing you to trowel float a pretty good finish later on.

in our area the sharp sand (screed sand or grit sand) is very good. this means we can lay a concrete sub floor with a 50-75mm screed on top.

the softer yellow sand is for bricklaying or rendering applications and most definitely does not come from the beach.(too mutch salt)

you will probably need 4 tonnes of ballast and about 25 bags of cement. (25kg bags)

 

[HarryM]

Thanks noseall,

I have worked out the volume of materials required as 0.85 sq. metres, so I intend to get at least 1 sq metre to make sure there is enough.

But the ballast down here looks very rough. It's not river pebbles as you would get elsewhere, but a mixture of grey sand and large chunky bits that look like the waste from a quarry. It also looks very dirty.

Is this OK for a smooth finish? Do the chunks sink leaving sand on top so it can be trowelled? Or would I be better off using sharp sand? I assume it would need to be a wet mix rather than the dry mix used for a screed.

Sorry to ask such newbie questions!

 

[noseall]

i'm not familiar with the type of aggregate you're talking about and haven't seen it. i would imagine that if its for sale as a concreting ballast, then it must be ok.
the slump consistency should be such that its easy to work with.
not too runny, so that its slopping out of the wheel barrow but wet enough to allow you to tamp flat easily.

a, sand/aggregate/cement/water mix, when agitated or tamped, allows the fat to work to the surface. this affords the user a finishing material that can be troweled later on.
you will not be able to actually trowel finish the floor till several hours after the floor has been tamped.

 

[HarryM]

Thanks, that's been very helpfull.

Just one more question if I may. I want to get it clear in my mind before I start.

You say several hours before trowelling. Will the mix have gone off enough by then to walk on? Or do you use a board as support?

The floor has a slight gradient from one end to the other where there are doorways. I was proposing to use screed battens in the usual way to get this right. Any problems with this?

Harry