Reinsforcing a crack in screed before tiling

[Tim Watts]

Back to building matter after Xmas...

I have just dried out the screed in the conservatory, ready for tiling...

I have a fine (0.5mm) raggety crack across the width of the floor. It's clear why, in hindsite.

The builder used what seemed a reasonable technique of bedding 3 scaffold poles in the screed to use as ruling guides. 2 were at the edges of the room and one was in at the 2/3 width. This is 3" fibre reinsforced screed sitting on celotex on 4" concrete on vibro-packed Type 1 MOT.

What he did to finish was pull out the poles and add more screed to the channels and level and smooth off. This probably meant that the fibres did not mingle as well as they should, leaving a line of weakness. The crack is almost certainly due to shrinkage. There should be some continuous screed under this as the pole diameter is 2" in 3" screed.

However I am concrened that it could become the focus for stresses that may transmit to the tiles, especially as the screed sits on a slighty deformable substrate.

My solution is to bridge the weak line with 8-12" wide fine metal mesh of some sort, incorporated into the tile adhesive while laying the tiles.

My initial though was stainless mesh - but that's hard to source and expensive. Anyone see a problem with galvanised or otherwise zinc plated?

I'm thinking expanded lathing will be too bulky - so something more like bird-feeder wire mesh might be better. I do not think it needs to be at all heavy - after all the plastic fibres in the rest of the screed are doing a good job. It's really only to diffuse stresses that might occur and be more than the felxible tile adhesive can cope with.

Thoughts welcome :)

Cheers!

[Jim K]

merry twixmas Tim,

When I did a shower with aquapanel, the joints are left open (few mm) and t aped with fibre glass tape (like plateerboard tape) and filled over/through with tile adhesive....

When adding stud walls at say 90deg to existing stud walls I've avoided cra cking by picking off enough of the old skim to get the fibreglass scrim tap e onto both surfaces before skimming. Without that (done by builders- who e lse...) cracking follows closely...

SOooooo, how about some runs of fibreglass scrim tape bedded longitudinally (overlapping left middle right along the crack) in the adhesive when tilin g??

Cheers Jim K

[Tim Watts]

On Thursday 26 December 2013 21:06 Jim K wrote in uk.d-i-y:

And verily to you good sir!

You know - I think that could be a viable idea!

Glass fibre is pretty strong stuff. I wonder how it compares with fine steel wires?

It would probably be easier to work with as it will tend to lay flat and could be tacked down at the edges with a bit of copydex.

[John Rumm]

Using mesh over cracks is really designed to spread any movement that occurs over a much wider gap that the original crack - the hope being that 0.5mm of movement over 6" of plaster is less likely to crack than

0.5mm over 0.5mm of plaster. How well that will work with tiles is hard to say... if you bedded them on a flexible adhesive you would probably be ok - any crack would show as a slight widening of the grout line.

The other option would be to try an reinforce the screed so that the crack can't open. cutting 20mm deep slots across it with a diamond disc, and then resin bonding in Helibars every 10" or so would probably do that.

[Tim Watts]

On Thursday 26 December 2013 21:45 John Rumm wrote in uk.d-i-y:

Hi John,

Yes - it's difficult to predict hwo "flexible" adhesive will handle this.

That thought did briefly occur to me - it sounds like a job for my wall chaser (good depth control). As you've mentioned it, I might take that as a reason to consider it. Be a bit messy, but will probably give a better result.

I was thinking to cut the slots at slight angles to stitch the crack in a zig-zag fashion.

I wonder how many bars I should lay? A pair every 15cm perhaps?

[Tim Watts]

On Thursday 26 December 2013 21:54 Tim Watts wrote in uk.d-i-y:

Sorry - didn't see your 10" - no that sounds good...

[Jim K]

erm... if the slab and sub-base were to move, is it reasonable to expect a 3" screed to not crack? (whether "perfectly laid" or not [& then patched up])??

Cheers Jim K

[Capitol]

It's sitting on celotex, so it seems unlikely that the base has cracked also. Personally, I'd use a flexible ahhesive and reckon that 0.5mm didn't matter.

[The Natural Philosopher]

I had a whole section of wobbly screed. I poured half a gallon of PVA down it and left it. Then tiled it a few weeks later. Its been rock solid ever since

[Martin Bonner]

Trouble is, it comes in 50m rolls when you want a couple of metres.

I also like the idea of chasing in some reinforcement. Any solution that involves playing with power tools can't be all bad!

[Tim Watts]

On Friday 27 December 2013 08:02 Martin Bonner wrote in uk.d-i-y:

I'm less keen due to the mess! What John mentioned, I found on Screwfix:

It looks the biz, though expensive and leaves the problem of how to cut 6mm wide slots. If I went down that type of route, I would probably use 2-3mm steel threaded rod (to suit diamond blade slot width) and some cheap resin:

which I've used before for it's intended purpose. And space the stitches closer due to being thinner.

I've been thinking about this - we're probably not trying to provide a *lot* of strength as there is relatively little load on the screed and never will be. Not even sure if the surface crack is deep or not... It may be superficial...

Half of me says slap some mesh on in the tile adhesive:

and the other half says "go to town".

[Tim Watts]

On Friday 27 December 2013 01:34 The Natural Philosopher wrote in uk.d-i-y:

Did you dilute the PVA?

[Tim Watts]

On Thursday 26 December 2013 22:52 Jim K wrote in uk.d-i-y:

Hi Jim,

Slab is fine - this is just a shrinkage crack in the screed from drying. I have 1mm around the edges too.

What I can say is microfibre screed additive works - but you clearly have to be careful of situations like this (laying a 2" guide rail down the middle and patching later).

[harryagain]

I would be concerned that it appeared in the first place. The fibre reinforcing is supposed to prevent exactly this sort of cracking. Accelarated drying during the set is an extremely bad thing for cementatious products. Keeping damp is good. Also 3" seems a bit thin, especially on top of insulation. If the conservatory is large, the floor should be split into separate parts, the joints filled with silicon or similar.

Scaffolding poles as shuttering is an extremely bad idea, timber would have been better. Or self levelling concrete. Any expansion/movement joints are cut in afterwords with a disk cutter these days

I think you have a cowboy. I think it is his problem & he should be responsible. (Or possibly yours for drying out too quickly.) He should have covered the concrete with plastic sheet to prevent too quick drying. The concrete neesd to come up and be replaced/done properly. There is no proper means of fixing this crack, it will likely crack the tiles at some time in the future. Especially as it is on the soft substrate (the insulation). It will be a lot harder/more expensive to get it fixed in the future than right now.

[Tim Watts]

On Friday 27 December 2013 10:48 harryagain wrote in uk.d-i-y:

For the avoidence of doubt - it was not dried out quickly.

It sat damp for about 2 months before I got fed up with waiting (the weather was wet and the humidity remained high and cold) and bought the dehumidifier. And 75mm with microfibre is the recommended depth on top if insulation - I checked the beforehand.

Expansion joints would be tricky with underfloor heating - at least in retrospect. It's actually quite hard to see how he could have done it any other way with a non rectangular space. There are some screed rails used commercially which are like small I-beams that are designed to be left in place and have a series of round holes through the middle to allow the screed to pass through - but I've never seen them for sale for small jobs.

As for sorting it - piccies soon, but I think I'd rather add a little reinfrcing myself then I now what I'm dealing with.

[Tim Watts]

On Thursday 26 December 2013 21:45 John Rumm wrote in uk.d-i-y:

This is what we have: (1st is the worst, the rest are mostly hairline)

This is clearly the best approach - but I'm a little reticent to use an angle grinder as the clean up will take forever.

I wonder if there's another way to make short slots without throwing dust everywhere - it is only 3:1 screed, not concrete with rocks in. Wondering if my Fein with a carbide cutter would touch it. No harm in trying...

Is there any such thing as a hand saw with carbide teeth?

[js.b1]

I think this is minor.

It is very common for much thinner layers of SLC to have multiple fine hair line cracks - as long as it does not crackle underfoot then it is fine. 75m m is sufficient weight in itself to hold down well. A second pour over the poles cavity is no doubt a factor as you say.

I suggest you phone Mapei and ask them, I suspect they will say...

- a) ok, use Keraflex

- b) use Durabase WP slight-decoupling sheet over the crack

- c) least likely Durabase CI+ sheet over the crack which will accommodate much more movement by virtue of its 3D nature

NOTE, if this crack is a VERTICAL displacement crack, you have a problem as even CI+ will not handle that much at all (0.1-0.2mm perhaps).

Durabase WP...

- Keraflex floor with 3-4mm notch trowel (mosaic trowel)

- Wipe the Durabase WP with the FLAT back of the trowel to bed it in a flat bed

- Lay tiles similarly on an appropriate bed

Durabase CI+...

- Similar, you MUST fill all those holes with Keraflex

Mapei Keraflex being a flexible thinset cementitious tile adhesive.

The real problem is thin or poorly bonded SLC, any crackle at all and it ha s to come up because it has not bonded and those hairline cracks are the be ginning of the disaster if you tile - not the end of it.

You may want to check what primer they recommend (and use it). Possibly Eco T Prim which is a £30/5L of PVA (or so I suspect... not checked on that :-)

Re glass fibre mesh... You could use mapenet which is an alkali resistant glass fibre mesh, but pr icey. It is normally laid onto the top of SLC/Screed to provide extra reinf orcement (AFTER you have primed before pour... just to ram this home for hi story :-)

Frankly you want decoupling - and Durabase WPS should be quite adequate.

[Tim Watts]

On Friday 27 December 2013 14:51 [e-mail] wrote in uk.d-i-y:

One correction - it's not SLC by dry sand/cement/microfibre screed.

I was definately going to use a fexible adhesive as the UFH will cause some movement - the Durabase is an interesting idea. Ta :)

[Tim Watts]

On Friday 27 December 2013 14:44 Tim Watts wrote in uk.d-i-y:

Hmm - this

would probably do it if I had a body to take it :)

I have a

but that's a different blade fitting...

[Tim Watts]

On Friday 27 December 2013 10:48 harryagain wrote in uk.d-i-y:

This is a fair point... I will send the photos to the primary contractor (who subcontracted the base).

If I "fix" it without telling him, I kinda lose my rights to whine later.