Floor vibration damage ?

[Tim Streater]

The builder is about to put the insulation down on the base concrete of our new porch floor. This will be 150mm (looks like, I haven't measured exactly) Celotex and then whatever goes on top. AIUI, it'll be finished with self-levelling compound and then vinyl.

Now, we're putting the washer/tumble dryer stack in this porch. Is there any likelihood that vibration from the washing machine would, over some number of years, cause the Celotex to crumble, spread sideways a bit, and the floor to sag a bit?

[The Natural Philosopher]

No.

If you are worried, ask him to lay a rebar mat over the celotex before screeding.

[Tim Watts]

How thick is the screed on top? Will it have any fibre added?

I'm pretty sure no one has ever used 150mm celotex - are you sure? 75mm would be more usual.

[Tim Watts]

Although pumped screed (which may be the same product as used for repair work, eg Stopgap 300) is used industrially because it is easier to get a fast perfect screed over massive areas with less manpower. Indeed, I would have loved to get a pump in and add 15mm over my entire ground floor in one operation, but the costs are prohibitive for a small area like a house.

[Tim Streater]

Dunno. I'll have to enquire.

OK - just been outside to check (it's in the car port). 100mm it is. So much for my ability to judge thickness by quick eyeball as I walk past.

50% error :-)

[Tim Streater]

You mean I'll be walking on the Celotex? :-)

[Tabby]

Adding 1% plastic fibres into the mix increases its crack resistance a fair bit. If you've got a garden sherdder you can make the fibre from scrap synthetic carpet

NT

[Tim Watts]

100mm celotex is very generous - I'm only using 75mm for my shower (which needed digging out hence could be done properly). I've only got 20mm Marmox under my bathroom floor tiles over uninsulated concrete and even that makes a huge difference to comfort.

Usually you'd want 50mm reinforced sand/cement screed on top, not 15-20mm compound.

Reinforcing could be by either embedded wire (not necessarily as heavy as full concrete mesh - but maybe some lighter iron mesh - 3-4mm wire should be OK I think) - or reinforcing fibers which are added at mixing time. I have no experience with the latter.

Either way, 100mm insulation is compressible to some extent (we are talking about amounts in the millimeter range, but that's enough to crack weak rigid coverings) so you want the screed to sink en-masse under load and age (which you won't notice) - and not crack. The washing machine fixed load won't be a problem, but the dynamic load may be with all the vibration on spin.

Personally, I'd be happy with 3mm x 50mm spacing wire mesh in 50mm screed and it won't add any complexity or real cost to the job to incorportate.

[Tim Streater]

Thanks for that detail - I'll discuss with the builder.

[sm_jamieson]

The insulation can be under or over the main concrete slab. Personally, I prefer it under. I thought about the washing machine on spin with an unbalanced load when I decided this. If having underfloor heating, the insulation would usually be above the slab, with the pipes embedded in screed directly over the insulation. Fibres would be added. Simon.

[Tim Streater]

So far there's the sand, dcp on top of that, and concrete. The celotex goes on top of that, and then presumably the screed.

PS harry would you like to trim sigs in future?

[Tim Watts]

I hate dry screeding - it's very fiddly.

For a small area, like one that can be done from one point, I'd probably use a slightly wetter mix, more like concrete with no aggregate.

The way I finsihed a concrte slab of similar size was a tamping bar of 2x4 with a spirit level on top. I got a pretty flat and level finish in concrete alone.

[Tim Watts]

The celetox can either go under the slab, or between the slab and the screed. In the latter case, you need a thick preferably reinforced screed.

I'm wondering if the OP is referring to the former method, in which case levelling compund makes more sense - though it is possible to get a decent enough finish in concrete to tile to.

[Hugo Nebula]

100mm would usually be the minimum required to meet Building Regulations.

Steel reinforcement provides the resistance to tension that screed & concrete lacks. I would imagine that the vibration of a washing machine acts in compression only, i.e., the force of the weight of the washing acts like a little hammer at 1200rpm.

Fibres don't reinforce the screed as such, they just turn large cracks into many tiny ones. If the OP is covering the floor this isn't an issue to begin with.

Generally the minimum recommended depth of screed over insulation is

65mm. 75mm would be better.

[Tim Streater]

No, what we have down so far is:

1) rubble levelled out with sand 2) dpc 3) concrete

this leaves 200mm to bring floor level up to that of indoors.

I need to get it clear with the builder as to what he plans to put on top of the celotex (final finish is vinyl).

This is a side porch, added because:

1) previous owners converted garage to (in part) utility room, but no door to outside from it.

2) we thought to just have a door put in but then realised that wall faces north. In winter howling gale from N. Pole crosses North Kent coast, passes Canterbury, then hits us on top of Downs.

3) So, porch to protect door area. Planners said we needed pp anyway so we enlarged it enough to put washer/dryer in there.

[Tim Watts]

I think it is to do with my place being a renovation - the BCO is happy to see any reasonable worthwhile effort made for mine. I must admit - 100mm PU does seem a lot for a floor - but mental note made :)

Also noted ta - I haven't screeded my shower yet :)

[The Natural Philosopher]

Then definitely lay some rebar grid over the celotex ad screed 'around it'

Its more usual and cheaper to use extruded polystyrene sheet than celotex.

[Tim Streater]

Rebar. Haven't seen that since I had some foundations extended in my house when I lived in California.