Cavity ties and DPC in save bed joint

[MortarThePoint]

Can you put cavity ties in the same bed of mortar as a DPC and if so which one goes on the bottom.

 

We are using a slab floor product that rests on the inner leaf and the inner leaf is then built off it. The slabs are 375mm thick. There needs to be a DPC in the blockwork somewhere beneath it. When both leaves are blockwork that leaves a slight challenge as to hit gauge you need to have a coursing block (375mm slab and 75mm coursing block+mortar makes the required 450mm tie spacing). As the floor slab installation is likely to be quite an operation we'd like the bed of mortar beneath it to be just mortar. That leaves nowhere else below the slabs for the DPC to go but in the same bed as the cavity tie. If they can't go in the same bed we'll have to think of a different way.

[nod]

Can you not start above dpc with yours ties 

[MortarThePoint]

15 minutes ago, nod said:

Can you not start above dpc with yours ties 

 

The DPC can't go lower as the next bed down is only 225mm above top of trench concrete and has been laid anyway

[MortarThePoint]

This raises a question, I've been presuming periscope vents need to enter the void below DPC. Is that true?

[tonyshouse]

No, can go above, but beware of cold bridging 

[nod]

Periscope vents go directly below dpc 

[MortarThePoint]

Sorry, I was meaning inner leaf. Nice picture though ?

[MortarThePoint]

Is their no rule about ties and DPC sharing a bed joint then? @Brickie you must know

[Oz07]

I've never heard of a rule where you can't have both in same joint. The brickies will normally roll the doc out first then put the ties on top. As much as they are supposed to butter the doc felt down it rarely happens so the tie's wouldn't be doing much if below the felt

[MortarThePoint]

9 minutes ago, Oz07 said:

I've never heard of a rule where you can't have both in same joint. The brickies will normally roll the doc out first then put the ties on top. As much as they are supposed to butter the doc felt down it rarely happens so the tie's wouldn't be doing much if below the felt

 

Thanks. It does make the joint pretty busy, so I can imagine issues there but the ties are only in a small proportion of the joint. Would be good to hear the 'it's done all the time' type comment though. The bed joint would only need to be shared on the inner leaf as the outer leaf can have it's DPC higher up. That will help with tie sloping outwards.

Edited May 17, 2020 by MortarThePoint

[Oz07]

It was done on my last build. Wall hasn't separated at the bottom yet. I had to slit the cavity batts 225 vertically so the insulation overlaps ffl by 225

[MortarThePoint]

10 minutes ago, Oz07 said:

It was done on my last build. Wall hasn't separated at the bottom yet. I had to slit the cavity batts 225 vertically so the insulation overlaps ffl by 225

 

OK that's good to hear. The ties in question would only be 225mm above the concrete cavity fill. And so the next set of ties up only 675mm above concrete cavity fill. Concrete cavity fill is only one block course (225mm).

[PeterW]

This is why I hate concrete infill and now use / spec EPS below ground so there are no cold bridges. What’s the overall cavity insulation.? Is it fibre batts ..?

[MortarThePoint]

2 hours ago, PeterW said:

This is why I hate concrete infill and now use / spec EPS below ground so there are no cold bridges. What’s the overall cavity insulation.? Is it fibre batts ..?

 

Yes fibre batts in the wall. The floor beams have insulation moulded in but are very thick.

 

It's a pain concrete cavity fill has to come 200mm up the cavity and then there's a minimum 225mm to the DPC.

[Brickie]

On 17/05/2020 at 11:11, MortarThePoint said:

Is their no rule about ties and DPC sharing a bed joint then? @Brickie you must know

Not that I’m aware of. The dpc should be laid on a fine skim of mortar to give it a bit of grip & you then have virtually a normal bed joint on top. Personally,I put a fine skim on lintol tray damp too,for the same reason. 

[Mike]

On 17/05/2020 at 11:11, MortarThePoint said:

Is their no rule about ties and DPC sharing a bed joint then?

I was taught that to avoid that - theoretically the tie wouldn't be so well embedded, and there would be a small risk of damaging the DPC - but you could check the Agrément certificate for your wall ties and DPC and see what they say.

 

It isn't allowed for Ancon ties, for example (The first run of ties is to be laid as near as possible to, though not directly on, the damp-proof course).