How to prevent cracking in screed over UFH

BigGeorge

Am putting a screed - about 40-50mm -over insulation with UFH pipes in it. Looking to get any tips on how to prevent cracking of the screed when it dries out & tiles are laid on it. Have heard some horror stories!

FesterBeatty

For starters anything less than 75mm screed will curl at the edges when it shrinks while curing. Assuming its a non structural screed then you need a 75mm screed with 1 layer A142 reinforcing mesh.

Job done.

BigGeorge

Forgot to mention, its one of the self leveling liquid screeds that we're looking at - does that matter. Have heard these can be brittle, hence the cracking question

creedp

FesterBeatty wrote: »

For starters anything less than 75mm screed will curl at the edges when it shrinks while curing. Assuming its a non structural screed then you need a 75mm screed with 1 layer A142 reinforcing mesh.

Job done.

Are you saying that even with a 75mm plus sand/cement screed you should always have mesh? A bit late for me to know this as my builder recently put in 75mm sand/cement screed without any mesh. Is this going to be a problem?

Also downstrairs he put in 100mm 35N concrete (only 80mm in one area), again without mesh - will this also be a problem?

FesterBeatty

I specified this on a job recently. It wasnt put in and i got a phonecall last week to say the screed had cracked throughout. You might be lucky though, if the slab bay size is small...? But I would never recommend pouring a screed or slab without mesh...particularly when it contains pipes which are hot one minute (expansion) and cold the next (contraction) so theres quite a bit of movement there that in my experience always leads to cracking if unreinforced.

peterc14

BigGeorge wrote: »

Am putting a screed - about 40-50mm -over insulation with UFH pipes in it. Looking to get any tips on how to prevent cracking of the screed when it dries out & tiles are laid on it. Have heard some horror stories!

I have 75mm sand/cement screed with UFH and this is the program I am following..

. Screed Poured
. Leaving to dry naturally for 6 weeks
. UFH turned on at 25 deg for 3 days
. Then up by 5 deg each day until max
. Max Temp for 4 days
. Heating off and floor allowed to cool (4/5 days)
. Start Tiling
. No heat back on until 4 weeks after tiling completed

From reading different opinions this is the way I decided to go about things. I am sure people will have differing opinions.

creedp

peterc14 wrote: »

I have 75mm sand/cement screed with UFH and this is the program I am following..

. Screed Poured
. Leaving to dry naturally for 6 weeks
. UFH turned on at 25 deg for 3 days
. Then up by 5 deg each day until max
. Max Temp for 4 days
. Heating off and floor allowed to cool (4/5 days)
. Start Tiling
. No heat back on until 4 weeks after tiling completed

From reading different opinions this is the way I decided to go about things. I am sure people will have differing opinions.

Sounds like a reasonable schedule. As a matter of interest did you put in mesh with your screed?

Johnniep

BigGeorge,

Alot of the liquid screeds recommend a minimum of 25mm cover to any UFH pipes. If you take your typical 16mm diameter pipe on a very level floor (which is difficult to achieve) the absolute minimum you should consider is 45mm and if the sub floor is not level, the differences should be taken out in the screed.

I wouldn't typically use mesh in a UFH screed (subject to panel size) but have used a concrete with a fibre mixed in to reinforce it as it can be difficult to ensure the mesh has say 25mm cover to prevent the cracking, but with the fibre mesh you are taking this out of the equation.

Also it's worth noting that the thickness of your screed will also influence the reaction time of your UFH system and should be considered in conjunction with the heat source for the UFH.

Hope this helps.

creedp

Johnniep wrote: »

BigGeorge,

Alot of the liquid screeds recommend a minimum of 25mm cover to any UFH pipes. If you take your typical 16mm diameter pipe on a very level floor (which is difficult to achieve) the absolute minimum you should consider is 45mm and if the sub floor is not level, the differences should be taken out in the screed.

I wouldn't typically use mesh in a UFH screed (subject to panel size) but have used a concrete with a fibre mixed in to reinforce it as it can be difficult to ensure the mesh has say 25mm cover to prevent the cracking, but with the fibre mesh you are taking this out of the equation.

Also it's worth noting that the thickness of your screed will also influence the reaction time of your UFH system and should be considered in conjunction with the heat source for the UFH.

Hope this helps.

Thanks it eases the mind a little as fibre was mixed with concrete downstairs but I don't think it was upstairs. I'm hoping that its not such a big problem upstairs as hairline cracks shouldn't cause too much of a problem if covered by wood? Am I being very niaive here?

peterc14

creedp wrote: »

Sounds like a reasonable schedule. As a matter of interest did you put in mesh with your screed?

No I had no mesh... just the fibres mixed in along with the screed.

BigGeorge

Thanks folks for the advice - going to aim for 50mm of the liquid screed

Chief99

Hi,

I got 50mm liquid screed put in about 4 months ago and let it dry naturally (1mm per day recommended) and it worked out fine, only noticed one small hairline crack in whole house so happy with that.