Floor tiles cracking at wood/ concrete join.

[RRRRRR]

Hi- I'm retiling a 1930's kitchen floor which is concrete, with the middle section thick WBP plywood suspended. The old floor tiles cracked at the concrete-wood junction however.

Can anyone please tell me if there's a way to prevent this cracking again other than filling the wooden part with concrete etc.? Not familiar with it, but could something like marmox backing board overcome this if the floor was also strengthened?

Thanks.

 

[Richard C]

You have a floor consisting of two dissimilar materials which expand & contract at different rates, Marmox backer board on it’s own will not solve the problem. You need to use a decoupling membrane such as “Ditra” at least http://www.schluter.co.uk/produkt.aspx?doc=6-1-ditra.xml&pg=funktion
but if the floor area is large (what size is it?), you may have to resort to flexible joints to prevent future cracking. The suspended section of floor must be also very rigid to ensure there is no discernable flexing; a kitchen is a heavy load/use area & the minimum you should consider is 22mm but 25mm thick WBP would be advisable to avoid problems. The joist size/pitch & span are also important factors in deciding floor thickness or if other means of increasing rigidity is required; each floor will be different. It is also important to use quality trade tiling products of the correct type; avoid DIY shed products, they will generally lead to dissapointment.

You talk about a centre section, do you have 2 joint lines or one?

 

[RRRRRR]

Hi Richard- thanks for getting back and adding so much useful information.

The floor area is about 4m x 3.5m. The central wooden bit was joists resting on brick in turn resting on dug-back chalk. The ends of the floorboards then rested on a recessed lip of about .5 inch at the wood/ concrete junctions (2 joint lines). This is where the cracking is. Still awake!? You're doing well!

A 22mm board was used to replace the floorboards, resting on the lip and on the old joists. To complicate things, this wooden area becomes an 'L' shape below the entrance door with no lip in the concrete to rest the wood edge on.

You are right in that I can see the floor flex, and grouting has come out, so as you suggest, more joists I guess. I'm just wondering if a reinforced wooden option will be less work or more than moving mains water/ CH pipes, then concreting..... Thanks.

 

[Richard C]

The floor area is about 4m x 3.5m.

In itself, the floor area not overly large & should not be a problem as far as expansion/contraction is concerned.

The central wooden bit was joists resting on brick in turn resting on dug-back chalk. The ends of the floorboards then rested on a recessed lip of about .5 inch at the wood/ concrete junctions (2 joint lines). This is where the cracking is. Still awake!?

Yes

You're doing well!

Thanks

A 22mm board was used to replace the floorboards, resting on the lip and on the old joists. To complicate things, this wooden area becomes an 'L' shape below the entrance door with no lip in the concrete to rest the wood edge on.

Tiling over different floor materials can be problematic as there will be differential expansion/contraction; hitting the central portion from both sides makes things worse & an “L” shape floor just complicates it even further. A sketch with dimensions would be useful but if I’m visualising this correctly, I’m not sure a decoupling membrane would help.

You are right in that I can see the floor flex, and grouting has come out, so as you suggest, more joists I guess.

You must get rid of any discernable flex or it will fail again & the price of tiling materials that will take more flex just escalates beyond what is reasonable IMO; even than it may fail! To stiffen the suspended floor, you can fit sister joists & noggins.

I'm just wondering if a reinforced wooden option will be less work or more than moving mains water/ CH pipes, then concreting.....

You don’t necessarily have to move main/CH pipes but you might have to re-route or maybe not. As long as the existing pipe work is sound, just create a clearance duct in the initial concrete pour around the pipes; then fill the duct with Vermiculite to cover the pipe & screed it over.

 

[RRRRRR]

Hi Richard- greatly appreciate you taking time out to write this. Think I'm going towards concreting in. Just wondered- do you know if this would be building control notifiable and if it would need insulation as well as basic stuff like DPM etc? Thanks.

 

[Richard C]

You don’t need to bother with Building Control unless the area of the thermal element (the floor) you’re changing exceeds 24% of the total; & I’m sure it doesn’t does it Getting them involved will only cost you money & possibly more trouble than it’s worth. Installation guidelines from Celotex;

Over slab installation guidelines
• Install a damp proof membrane over the top of, or below, the slab. The damp proof membrane must provide continuity with the damp proof course in the surrounding walls.
• Level the surface of the slab; it should be smooth and free of projections.
• If required, use a thin layer of sand blinding on a rough, tamped slab to ensure that the insulation boards are continuously supported.
• Use the Celotex Insulation Saw to cut and fit insulation upstand to floor perimeter, to meet a minimum R-value of 0.75m2K/W, (i.e. Celotex TB4020). The upstand depth should be equal to the sum of the slab insulation and the screed thickness. The upstand thickness should not exceed the combined thickness of the wall plaster and the skirting.
• Lay the insulation boards directly onto the prepared slab with all joints tightly butted.
• Lay a polythene vapour control layer (VCL) over the insulation to minimise the risk of condensation forming at the insulation/slab interface and to prevent liquid screed migration.
• Apply a sand/cement or self levelling screed over the VCL and Celotex insulation boards to a minimum thickness of 65mm. Use scaffold boards or other protection to prevent wheelbarrows and other traffic damaging the insulation.

I would still advise a decoupling membrane over the whole floor area before tiling though as you will effectively have a separate screed slab as the centre section. I would also apply a liquid DPM to the sides of the existing screed to avoid any possibility of damp wicking up the joint line.

 

[RRRRRR]

Hi Richard- great help again! Could I possibly bother you for one last thing? I'll then leave you in peace.... You mentioned creating a clearance duct and pouring concrete around, and filling with vermiculite before screeding over. I'm aware of e.g. plastic pipe ducting, but are you describing a 'boxing in' method, which could potentially mean I can leave the current lead mains in place? I can't see how the vermiculite could be put in otherwise. And if so, how would I cap this chamber pre- screed? Thanks again, and I'm sure I won't bother you any more after all your excellent help.

 

[Richard C]

which could potentially mean I can leave the current lead mains in place?

Before I go any further & explain how to do it, are you seriously saying you still have a lead main water supply pipe in your property? If so it’s got to come out as anyone who drinks water in your property on a regular basis is effectively being slowly lead poisoned not quiet as bad as being shot but the effect is more or less the same in the long term

 

[RRRRRR]

Well there's actually a join to copper as it comes into the kitchen underfloor, so it's lead back to the property boundary through about 8m of garden. Well it knew it wasn't ideal, but I didn't realise the risks were so great since just about every 1930's house has this, and we have hardwater, not soft. Well I guess the floor will have to wait then if it is much more of a risk than I knew, and I'll have to find someone to do this.

 

[RRRRRR]

Will probably replace pipe anyway, but just found this on Interweb, which appears to be reassuring-

In the UK corrective treatment has been promoted, as opposed to the widespread replacement of lead pipes, as an appropriate first stage of achieving the new European standards for lead in drinking water. In consequence, about 95% of UK's water supplies are now dosed with ortho phosphate and it has become apparent that substantial compliance has been achieved with not only the interim standard of 25 µg/l but with the standard of 10 µg/l as well in many areas. The Government will assess the extent of compliance in the next few years and then consider, whether, and to what extent,  a  strategic programme of lead pipe replacement is needed to achieve compliance with the 10 µg/l standard .

 

[RRRRRR]

Meant to say whole article for fuller picture is on this link-
http://www.ciwem.org/knowledge-networks/panels/water-supply--quality/lead-in-drinking-water.

Think I'll contact my local water board for replacement advice/ assistance. Thanks.

 

[Richard C]

Sorry if I may have alarmed you but I wouldn’t use water delivered through a lead pipe for drinking/cooking & my first priority would certainly be to get it changed; without testing you’ve no idea how much lead you & your family are ingesting. Unfortunately your link didn’t work so I don’t know what it recommended but here’s one I found; http://www.nsf.org/consumer/plumbing/index.asp This reiterates my feeling that it may be more of a risk than your link appears to be implying & one which should not be ignored.

With regard to the pipes just make up some timber shuttering around them so you’re left with trench approximately 2x the area of the pipes in the screed after removing the wood fill the trench with Vermiculite over the pipes to around 15mm from the FFL & then level off with render.

 

[RRRRRR]

Hi Richard- Thanks for getting back.

This link should work now!
http://www.ciwem.org/knowledge-networks/panels/water-supply--quality/lead-in-drinking-water.aspx

Sorry, but I disagree that, 'it may be more of a risk than your link appears to be implying' but this is probably because you couldn't get the linked information, just the part I quoted! Even so, in the part I quoted it states, 'substantial compliance has been achieved': I.e. incomplete compliance, with obvious implications of possible concern.

Now you can see the full report (I hope!) you'll see it states, 'replacement of all lead pipes must be the long-term aim', so I do in fact agree that it is definitely, 'one which should not be ignored'. I think it is also important to read the whole report to get the full context, risks and implications rather than me picking bits out of context, in order to get a balanced view.
I have seen that my local water board's site they have useful advice on lead pipes and implications for the water type in my area, and appear to give a balanced view, and will even test water in the house if anyone has a concern. So obviously the professionals are the ones to contact over any concerns which may or may not be justified if anyone else reads this.
Thanks again for your help.