Floor screed depth

[RMS]

Hi all,

I have recently had the floor screeded in my new extension. Weeks later after the kitchen was fitted large cracks appeared and the screed was hallow in parts.

The builders have now taken up most of the floor up to the kitchen and installed a new screed.

The screed is a sand and cement mix and they have glued the floor with SBR. The screed is 35mm to 40mm max in places. I this depth sufficient?

Any feedback appreciated.

 

[Chukka63]

You should really have a min of 65mm and could even lay mesh to minimise cracking.35-40mm is too thin,hence the cracking.

 

[roughcaster]

Just to add to what Chukka said,, a floor screed is only as strong as the bond to the concrete underneath. All the floors that i've worked on, had runny cement grout, poured/brushed onto a wet floor, and the screed was laid onto it while the grout was still wet, giving a good bond. We only grouted small areas of the floor at a time, ensuring the grout didn't dry in to quick.
A rough/tamped finish to the concrete itself, also ensures a good bond for a screed. You dont want a smooth/ish finish to concrete, if you're going to have it screeded on top,,, that can cause the screed to lift, and become "bossed"/hollow.

Roughcaster.

 

[RMS]

Thanks for the replies.

The concrete below was not smooth and had a rough finish. i'm just very concerned about the screed depth, especially after the first screed did not bond and started to lift.

Where the new screed meets the existing screed still feels hallow in places. The contractor feels that this depth is adequate.

I'm also concerned that if I accept this an go ahead with a tiled or glued wood floor that the floor will not be strong enough.

 

[roughcaster]

If you think the floor screed is still not right,, tell the builder. Although 50/60mm is the ideal thickness for a floor screed,, 30/40mm,, grouted/laid properly, in a domestic kitchen should not be a problem either. As long as it's left to cure for 2 or 3 days before it's walked on,, and even then, put something onto it to walk on, cardboard, ply etc.

Micilin,, in the link below, stated that he's screeded down to 3mm before without a problem,, (now that is thin), but as with most things, it's all in the prep,, and one other thing,,, if the screed mix itself is too dry, it wont bond to the concrete well either, (semi dry is ideal).

//www.diynot.com/forums/viewtopic.php?p=1364356#1364356

Roughcaster.

 

[Chukka63]

todays preferred thickness is 75mm.the thin amount you need screeded may be better laid with asphalte but i dont know much about asphalte or costs.

 

[jeds]

30 to 40mm is normal for a bonded screed on a stable sub-base. 60mm+ screeds are normally only specified where bonding is unnecessary.

 

[Chukka63]

ive never seen 30-40mm screed specified on any job ive been on for 30 years.

 

[jeds]

Chem bonded screeds can be a few mm up to about 25/30mm. Cement bonded screeds are 30 to 50mm,. Anything above 50mm can be unbonded but 60/70mm is more usual.