Will this floor patch give way ?

[Spinny]

Patching in floor at a doorway. All prepped and now the flooring guy says it won't be strong enough. Was using a strip of 20mm thick XPS backer board. Advice please - is he right or wrong ?

See sketch.

 

Backer board strip 200mm wide supported on one side on celcon blocks 100mm wide, and on other side by 30mm XPS resin bonded to blockwork. The XPS backer board is to have levelling compound applied over to about 15-18mm thick.

 

Flooring guy says it won't pass the stilhetto test, and the XPS must be replaced with solid board ?

 

Flooring will be LVT.

 

[Mr Punter]

Threshold details are a right pain.  I guess he is concerned that there may be cracking between the different substrates.  Who knows if he is right?  I guess you either do as he asks if he is willing to take responsibility in the event of it being defective or you leave it as is and you take the responsibility.

 

And maybe stop wearing stilettos around the house.

Edited yesterday at 13:20 by Mr Punter

[Nickfromwales]

The XPS board goes in the cavity, vertically, bonded to the inside edge of the outer leaf of block.

 

Then you remove the inner leaf for the full width of the opening.

 

Fix a piece of 30mm PIR, or more slices of XPS, as an insulation upstand. 
 

Then DPC, and then fill with a strong concrete mix with 10mm aggregate. 
 

Delete the cavity!

[JohnMo]

Sounds pretty weak. If you want a stronger version but also insulated use Compacfoam. Make it deeper than 20mm by doing a double layer glued and screwed together.

[Spinny]

I thought my test piece was fine - see photo with a wider span than needed and no leveller...

 

I thought people used this stuff under screeded underfloor heating pipes on suspended floors ??

 

I am thinking that with 15mm plus cementitious leveller on top it would have good compression strength and seems nice and stiff compared to thinner backer board. The stilhetto test seems pretty extreme (I havn't seen a man in stilhettos since a trip to Key West).

 

I could stick 10mm backer board to the bottom of a piece of 12mm ply I suppose ?

 

Never seen or used compacfoam so not sure what it is really like ? Polystyrene bears loads but not point loads.

 

Come on England ⚽

[Andeh]

Natural movements between the two supports would be more of a concern then the raw bridging abilit I think. Slight cracking and debonding on one side could be an issue?

[Nickfromwales]

2 minutes ago, Andeh said:

Natural movements between the two supports would be more of a concern then the raw bridging abilit I think. Slight cracking and debonding on one side could be an issue?

If there’s a grout line near the transition you’ll likely see issues there over time.

 

Delete the cavity, which is standard practice at openings. I may have mentioned doing this……

[Andeh]

We did, is just stick 20mm PIR against the external wall, and ??mm at the bottom of the cavity and then back fill with concrete.

 

That way you've mitigated the worst of the thermal bridging and have a strong support throughout.

 

 

Wish that's what we'd have done, but I was away with work and due to 2.4 x 8m sliders they backfilled entirely with concrete due to worries about the weight and movement destabilising. It's held firm, but we do have a sizable thermal bridge now.

Edited 14 hours ago by Andeh

[Nickfromwales]

Just now, Andeh said:

We did, is just stick 20mm PIR against the external wall, and ??mm at the bottom of the cavity and then back fill with concrete.

 

That way you've mitigated the worst of the thermal bridging and have a strong support throughout.

Indeed. 
 

I’m just waiting the hear the chink, from when the penny drops 👀

[Spinny]

Hi @Nickfromwales and all,

 

I get that you want me to fill the cavity with concrete. But have some concerns that mean I don't really want to do that. The DPC is above the bottom of the door cill and has been stepped down to run under the door cill. Note the door is only a single door so the reveal is 890mm wide only). So the cavity wall below the floor level is below the DPC. It has grey jablite type insulation in it. I was able to pull one piece out to have a look and found there was some moisture at the bottom of the cavity there when we had very wet weather. Clearly some moisture from the ground/hardcore outside the door can work it's way through the outer leaf to the bottom of the cavity when it rains a lot. The jablite was used at that level rather than PIR because it can cope with any moisture better. So I don't want to remove the inner celcon leaf (which would be complicated now the rest of the floor is in now anyway) because it would remove the cavity 'protection' from damp, and the jablite insulation. The inner leaf is also thermal blocks and provides a point of support for closing the cavity with a board over. The cavity is also probably 450mm deep there so would be quite a bit to fill with concrete and have dry.

 

The fundamental problem is the lack of any support on the outer leaf side of the cavity, and the fact this is difficult or impossible to access with power tools because there is only the 100mm cavity to fit any tool into. Therefore my proposed approach of bonding a support piece to the outer leaf, which also has to be insulative.

 

To deal with the stilhetto issue I am now thinking I could put 6mm No More Ply cement board over the 20mm XPS backer board to provide more rigidity and spread any point loads. (Not sure on the best thing for bonding the two together ? No more ply adhesive ? flexible tile adhesive ?). Then the. cementitious floor leveller should go on the top of the cement board quite happily.

 

New diagram attached.

 

On my test piece I bonded the XPS support strip to the concrete outer leaf with Rawlplug bonded anchor R-KEM 2. Chat GPT is suggesting there might be better alternatives to use for bonding XPS to concrete - thoughts. The biggest risk would seem to be this support debonding down the line. (I have some metal angle brackets but there is no way to screw them into the outer blocks.)

 

More thoughts comments please as I need to close this out nowish.