floor screed

[flandersfield]

Hi all.

we had a sharp sand floor screed laid 3 weeks ago fairly dry mix, 80mm in depth and was told to keep of it for a week which we did, today it is beginning to break up with soft patches which are just loose almost like dust 50mm in depth. should we be keeping of the floor longer? the mix perhaps not strong enough ? from looking around should be 3 parts sand 1 part cement. we are also due to have the floor tiled in three weeks around 42 days after laying is this to soon as i read drying takes 1 day per 1mm thickness. your help would be great

 

[mac391]

1, are you sure it was sharp sand and not soft sand ?
2, What sub-floor was it applied to ? soil, existing concrete etc ?

Generally a 80mm screed woud have stone in it to make it concrete, just sand/cement at this thickness is too weak.

Also, all screeds once dried have a dusty layer on top, it's called (laitance) and needs to be removed before any final flooring is applied. I'ts usually less than 1mm so your 50mm sounds scary.

A solid mix for screeding at this depth should be @ 1 part cement + 2 parts sand + 3 parts aggergate (stone)

Don't think you can tile on top until you know whats happened here..

 

[JamesFreakley]

Is sounds like the product has not been mixed correctly and that the sand has not bonded with the cement, this could be because the mix was too dry or the ratios of sand and cement were not correct.

I would not tile the floor until the screed issue is resolved.

Before the tiles are laid the floor does need to be dry, at best this will be 1mm per day however it could take longer, the best solution is to use a mositure meter to check the moisture in the floor before installing the tiles.

Hope this helps.

 

[flandersfield]

1, are you sure it was sharp sand and not soft sand ?
2, What sub-floor was it applied to ? soil, existing concrete etc ?

Generally a 80mm screed woud have stone in it to make it concrete, just sand/cement at this thickness is too weak.

Also, all screeds once dried have a dusty layer on top, it's called (laitance) and needs to be removed before any final flooring is applied. I'ts usually less than 1mm so your 50mm sounds scary.

A solid mix for screeding at this depth should be @ 1 part cement + 2 parts sand + 3 parts aggergate (stone)

Don't think you can tile on top until you know whats happened here..

Hi thanks for your reply, the screed was definately sharp sand and was laid on top of 200mm concrete base as it is part of a new extension. The two areas we had concern are where the old sink was , so there was the drains and in front of some new doors and it appears when the window guy came he was banging the newly laid screed and has broken the surface up which at that point had only been down two weeks, this has now been repaired by clearing out the loose and patching up, not sure this is acceptable.
Our tiler is going to lay a latex leveler and detrmax (sorry spelling probably wrong) it seems from other threads that an epoxy resin may be better than latex, Any further advise would be apprciated.

Thanks Graeme

 

[mac391]

Before you proceed the screed should be completely dry which will take approx 3 months.

Or,,, use an epoxy resin DPM . These are quite expensive but mean you can continue work immediately.
You will have to check wether your tile adhesive is suitable and will adhere to the dpm though, ring the manufacturer, most shops will tell you it will work or won't have a clue.

I can't believe another screed is required after it has just been done, it should be level enough but if it does need one i would use a water based and not a latex. Putting this down after the DPM also means you don't have to worry about your tile adhesive sticking, it will definately sick to the new screed.
The sub-floor should be primed with P131.
To be honest though i don't think your original 80mm screed sounds too good and will be a lot of money wasted if it goes wrong.

Good luck