What to do about cracked UFH screed


5 Dec 2012
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United Kingdom

Work is finishing on our new build extension - single room 6m x 7m. We have wet UFH (John Guest SpeedFit) with 3 loops in 75mm screed (4-1 sand mortar plus fibres) and we plan to glue a very good quality engineered oak board to it.

We left the screed 90 days to cure/dry, before commissioning the UFH. There have been two problems which have caused a number of 1mm cracks to appear:

1. Initially only one of the three flow valves was opened, meaning that for about 24 hours two of the loops remained cold while one heated up. This caused one third of the floor to expand, causing two cracks to appear in the cool section. These cracks go for about 2/3 of the 7m length of the room (same direction that the flooring will be laid), and run along the joins between the bands in which the screed was laid. They are wavy, with about +/-25mm deviation from a straight line. In a few places they have forked and rejoin.

2. I had been carefully warming up the UFH by 1-2 degrees a day as per instructions, but yesterday it somehow got cranked up by 6 degrees to 45 degrees. :mad: As a result, a number of new cracks have appeared, at right angles to the earlier ones.

My questions are - and sorry this is such a long post:

1. Are the cracks anything to worry about? Will things get worse over time? The screed itself seems to be sound, and the cracks have only appeared when something stupid happened.

2. What treatment, if any, should my builders use to repair them? Obviously I will make sure that everything else is done by the book from now on, though sometimes there is conflicting advice depending on who wrote "the book".

3. What is the very best flexible adhesive (regardless of cost), given any concerns about the cracks? BTW the manufacturer insists the boards should be glued down, and I do not want to use anything that would impair the efficiency of the UFH.

4. Any other Top Tips, except of the Captain Hindsight variety? :cry:

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Sounds like trouble. Do you have proof that the screed was mixed 4/1? or was that what the builder said? unless doing yourself or you can check material used i would never trust.

As a example i was out on site and there was the correct quantity of sand + cement for mixing 4/1. However they had 4 bags of cement left over. I calculated and it worked out a 10/1 mix !

I questioned how they got it wrong and they said " 4 shovels of sand and one shovel of cement" Well that is not accurate. I asked them to knock up one mix with borrowed sand for the next mix that was going to be laying. They used 4 humped shovel loads of sand against a half shovel of cement as that is all they could get out of a bag in one go . They could not see that they was actually adding just under half the shovel size of cement compared to the sand.

I proved my point when i asked them to load a measuring bucket and it clicked to them .

ANYWAY, im woried that you have cracks running in almost straight lines that fork and then return back into themselves. That would suggest to me that it is subfloor failure rather than shrinkage cracks. Difference being that one is hair line cracks and not all the way through and the other is a concrete slab that has broke in half and not attached to each other apart from the fibres.

I would recommend a site survey to get a pro to check out for you.
Thanks Matty

I'm pretty sure the screed mix was OK because they dumped the excess on our lawn (cheers lads) and it set so hard our builder had to attack it with a pneumatic drill to break through it to get to a soakaway pipe. The sub-floor concrete slab underneath seemed OK - mesh reinforced, 175mm thick, 150mm sand blinded hardcore, and all signed off by the BCO. Between that and the UFH screed sits 100mm of Celotex which again looked to be in order, though a bit battered by the time the screed was laid.

The forked cracks only splay apart by about 10-20mm - it's not like they shoot off across the floor.
i would speak to a company called Mapei. They make screeds and repair compounds etc.

They have a product called 'eco rip' or something like that . its a epoxy you poor down cracks and it bonds the concrete back together.

However i would speak to mapei tech people and seek there advice.
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I'm amazed at the lack of clarity or correct information across the board when it comes to piped UFH and the amount of builders who simply attempt it without knowing the rules !!

I don't actually know the correct procedure but have taken an interest purely from all the failures i have come accross as a flooring installer..

What i do know that does work is a screed called "Anhydrite". It is a pumped screed specifically designed for UFH but expensive. My knowledge of it is a bit thin as yet but i certainly wouldn't go ahead with any finished floor product until you have checked out the screed that you have down now.

Sorry i can't be of more help

Anhydrite is a pain in the bum. It is fairly new in U.K. It is used over underfloor heating as it 'flows' so minimizes voids. Due to this it can be laid thinner. It is also much faster to put down as its pump applied and almost self leveling.

That is the + points !

The negatives are that in the u.k we dont have any products to prepare it. Yes we can get them from abroad but costs a lot more. Basically you shouldnt use a cement based smoothing compound over the top. As i said just tho, we can import Anhydrite smoothing compounds which are designed to be used over them. The next issue is the drying time. It takes around the same time to dry as concrete but you cant speed up the process with surface DPM's. (however we are testing a surface dpm designed to work with Anhydrite at the moment) It also has a lattice that needs to be removed. The biggest down side is if you had a flood etc the Anhydrite will return to its liquid state. Basically it will break down.

Anyway, there is some knowledge for you to absorb :D [/u]

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