Reinforced insulated raft with UFH -Will I need de-coupling membrane and other queries

[Smallholdertoo]

Ground works for our new timber frame house starts in a weeks time. The highly insulated 1.5 storey house with a footprint of c.80m2 will be on an insulated reinforced  raft, 270mm deep concrete with wet UFH attached to the mesh. The concrete will be power floated to provide a surface for tiling. The water temperature in the UFH should be quite low due to the low heating demand  (2kw when outdoor temperature is -9C.) ASHP.

 

I have to order my windows and doors early next week and need spacers under the door threshold to provide for finished floor level.  We therefore need to work out an appropriate depth of these spacers to retain a reasonably flat threshold to comply with Part M. We intend using large format porcelain tiles (as yet not chosen) throughout most of the ground floor - except the shower room but am  unsure whether I need to allow for decoupling membrane  (as recommended by tilers). Taking account of this , how many mm  should we allow for the finished floor? The tiler has suggested 17-20mm.  

 

Also , how long will it be before I can get it tiled as it is a very thick raft? and should I put the UFH on first. 

 

I would appreciate your thoughts as to anything else I should be considering?

 

Many thanks in advance  

Edited May 3 by Smallholdertoo
Minor wording change

[nod]

17 -20 mil isn’t enough for large format tiles Even if you use a quite cheap tile Your still going to need 5 mil for Matt and at least 10-15 for adhesive Hopefully Hes misunderstood your question 

You will need to leave the slab a couple of months to dry out 

[JohnMo]

We just used flexible tile adhesive. Concrete expansion with low temperature UFH is very limited 

[Mr Punter]

Assume 10mm tile, 15mm adhesive, 15mm door mat, 5mm clearance, so 45mm to the underside of the door leaf?

[Smallholdertoo]

1 hour ago, Mr Punter said:

Assume 10mm tile, 15mm adhesive, 15mm door mat, 5mm clearance, so 45mm to the underside of the door leaf?

Thanks we will allow for an inset door mat within the top of the raft by the doors.

[Smallholdertoo]

1 hour ago, nod said:

17 -20 mil isn’t enough for large format tiles Even if you use a quite cheap tile Your still going to need 5 mil for Matt and at least 10-15 for adhesive Hopefully Hes misunderstood your question 

You will need to leave the slab a couple of months to dry out 

We are only looking for up to 600x450 so ok not large format and likely to be 9-10mm thick. I was hoping we won't need a decoupling membrane as low temperature and the tiles being porcelain rather than limestone or travertine!

[Smallholdertoo]

@Nickfromwales I would appreciate your thoughts on this matter

[nod]

3 hours ago, Smallholdertoo said:

We are only looking for up to 600x450 so ok not large format and likely to be 9-10mm thick. I was hoping we won't need a decoupling membrane as low temperature and the tiles being porcelain rather than limestone or travertine!

5 mil for matting 

and the rule of thumb is Same thickness of adhesive as the tile At least 10 mil tile 10 adhesive 

Your tiler should know this 

I wouldn’t get to hung up with the disabled access We’ve a 10 mill step on both our builds without issue from BC 

 

[Spinny]

Do you know how level the concrete will be ? 

Our builders said the slab for our extension would be level to within 6mm, but only had a man hand tamping and floating it. It was only level to within 24mm and the level was 11mm higher than it was supposed to be. We had to change the floor make up in the connecting part of the main house, and the flooring contractor will be using levelling compound across the top of the slab to correct the 24mm variation.

You probably have some very good pro's doing yours, but you will still want to check the actual level variation once it is poured and floated, just in case any parts need levelling compound, or different door apertures vary somewhat in level.

Just a thought.

[Nickfromwales]

8 hours ago, Smallholdertoo said:

@Nickfromwales I would appreciate your thoughts on this matter

As long as you leave for at least 60 days, preferably 90, for the concrete to 'go hard' then you'll be fine without the decoupling membrane. The important part is to use a good quality flexible adhesive, and prime before laying, and also as you lay.

 

I clean / decontaminate the surface of the slab with a 50/50 mix of primer / water, and scrub this into the slab with an old school janitors mop, until the surface is saturated. Then leave to dry after you back mop any pooled solution. Then I put 75(w)/25(p) mix or just neat primer onto each m2 with a sponge as I lay the tiles. I don't ever put adhesive onto 'dry' concrete, ever. 

 

Primer NOT PVA  

[Smallholdertoo]

4 minutes ago, Nickfromwales said:

As long as you leave for at least 60 days, preferably 90, for the concrete to 'go hard' then you'll be fine without the decoupling membrane. The important part is to use a good quality flexible adhesive, and prime before laying, and also as you lay.

 

I clean / decontaminate the surface of the slab with a 50/50 mix of primer / water, and scrub this into the slab with an old school janitors mop, until the surface is saturated. Then leave to dry after you back mop any pooled solution. Then I put 75(w)/25(p) mix or just neat primer onto each m2 with a sponge as I lay the tiles. I don't ever put adhesive onto 'dry' concrete, ever. 

 

Primer NOT PVA  

Many thanks that's great to know.  Would Mapei's Primer G work?.

 

[Nickfromwales]

20 minutes ago, Smallholdertoo said:

Many thanks that's great to know.  Would Mapei's Primer G work?.

 

My go-to stuff is,

 

https://www.solseal.co.uk/building-products/primers/ultratile-proprimer-advanced-polymer-primer-5l/?gad_source=4&gad_campaignid=18660036211&gbraid=0AAAAAC6dNVJMz5_DklvkI8Cb7s5WA0_vQ&gclid=Cj0KCQjw_dbABhC5ARIsAAh2Z-Szkw_J1yfigRCTCR-KcSqzIKRHp74qcZ292_5C6y0OlxwMnUEPySsaAu-HEALw_wcB

 

Just buy it wherever is the best price obvs.

[saveasteading]

4 hours ago, Spinny said:

Do you know how level the concrete will be ?   would be level to within 6mm

There are standard specifications, measuring  the gap under a 3m long straight edge.

You could ask the contractor if he is working to such a thing.

I don't know what 'level to within 6mm' means. If it means that finished levels would have a range of 6mm from lowest to highest then it sounds good, but not if the -3 and +3 are close together.

 

Concrete shrinks when curing.  Are contraction joints proposed?

 

 

 

 

 

[Spinny]

We had the suppliers come and measure up the openings for our doors and windows. They tend to allow 4-6mm tolerance all around the perimeter - doubtless reduces their risk.

 

Beware the laser level. One supplier measured at survey, then measured at fitting and declared our floor level to be 12mm higher than it should be. Wouldn't accept it when I explained that was impossible as the only change was they measured from top of joist the first time, and from top of floor boarding the second, and that was fully allowed for. I bought an old world water level - ultimately showed their measurements were wrong - used over a 9m distance, and comes in handy for finding levels between the inside and outside of a building.

[Nickfromwales]

18 hours ago, saveasteading said:

Concrete shrinks when curing.  Are contraction joints proposed?

Doubt that required at this thickness. Only ever seen this with a 100mm thick raft tbh. 

[saveasteading]

2 minutes ago, Nickfromwales said:

Doubt that required at this thickness.

It applies to any slab thickness unless there is lots and lots of mesh,  or other bars, carefully detailed and positioned. which will control the cracks to be multiple miniscule ones.

 

I don't think I have ever been involved in concrete that thick ( other than heavy Engineering (Bridges etc).   

[Spinny]

Our extension slab was about 7.6m by 5.5m and about 120mm thick with a single layer of reinforcing mesh held on concrete mesh men at about 50mmm up. So only half the size of yours and we had no expansion joint. Specified by our structural engineer. It has been down 2.5 years, still not floored over, with the heating pipes operational for 1.5 years. Looks fine with no cracks.

 

Plumber clipped the pipes to the mesh with zip ties. I went around after he had gone cutting off all the zip tie ends he left sticking up.

[Smallholdertoo]

20 hours ago, saveasteading said:

There are standard specifications, measuring  the gap under a 3m long straight edge.

You could ask the contractor if he is working to such a thing.

Concrete shrinks when curing.  Are contraction joints proposed?

Thanks , I will ask the contractor.

There will be no contraction joints in the raft- This is a highly insulated house so the UFH will run at a low but stable temperature .Once cured there should be little expansion and contraction

[Nickfromwales]

2 hours ago, saveasteading said:

It applies to any slab thickness unless there is lots and lots of mesh,  or other bars, carefully detailed and positioned. which will control the cracks to be multiple miniscule ones.

 

I don't think I have ever been involved in concrete that thick ( other than heavy Engineering (Bridges etc).   

There is no shortage of overzealous architects and structural engineers, so I’ve seen a few!! All domestic residences. 🤦‍♂️

[saveasteading]

5 hours ago, Spinny said:

Looks fine with no cracks.

Oh yes there are. But very fine and invisible, due to being constructed with care.

Reinforcement at mid depth, no added water, covered with polythene.