MortarThePoint

Good point, I do have a bonded out drawing I'd forgotten about, I'll take a look. A half brick periscope vent would be good though. Can you get them? Full brick vents have an area of 7000mm2. Based on those vents, NHBC's maximum 2m c/c is massively over the 12 needed by area (and therefore void volume) and the 13 needed by perimeter length. A half brick vent might have 3000mm2 and that would make for 30 vents needed which ties in well with the 2m c/c.

Something tricky here is that all this needs to get established before brick bond is established, so having the vent nicely in bond on the outer leaf isn't practical. If we had a wider slot in the inner leaf blockwork it would allow movement, but that would mean using narrower than header bricks either side of the vent. The vents look to be 205 wide. To allow 112.5 mm of movement hit bond(*) would need ((440-112.5-205)/2) = 41mm wide bricks either side of the vent on the inner leaf. (*) the semi hitting bond here is being accurate to within half a brick in Flemish which would allow some monk bond like fudges. Could take out two full blocks and use a 600mm lintel with more bricks but that will add quite a lot of work/expense. Can you get narrow periscope vents or periscope vents that allow sideways adjustment. Using some tilt I expect you could tweak about 10mm but not much more with the standard vent type shown here (cavity is 100mm). Ideal: Acceptable fudge:

Thanks, great advice. It will be a bit more involved for us I think as we are to use a precast slab (Thermabeam) which has particular contact points. I am checking with the manufacturer but I think the vents will have to be lintelled over as shown in the diagram below. The dotted lines are the concrete pillars at the end of the beam and the honeycomb is EPS insulation. I can't see any reasonable way we can hit bond for the outer leaf which is a shame. Complicated a little by Flemish bond, but would be difficult either way.

I'm surprise the calculator was the lowest as it's made by the manufacturer of periscope vents so you'd expect to to come up with the largest number

There is some inconsistency in the number of vents I'm being told I need: My construction drawings have 24 periscope vents, but I don't think much thought was put in to those as at least 3 of them are under doors and another where a drainage lintel sits. NHBC guidance says "Voids should be ventilated by openings providing not less than 1500mm2 per metre run of external wall or 500mm2 per m2 of floor area, whichever gives the greater opening area. Ventilators should be spaced at not more than 2m centres and within 450mm of each end of any wall. Air bricks should be ducted through cavities and be unobstructed." The house perimeter is 60m so that makes for about 34 if every 2m. And this calculator says I need 12 (140m2 and 60m) (Their vents are 7,750 mm2 per unit so 1500mm2/m * 60m / 7750mm2 = 11.6 --> 12) Can I just have 12 vents, 6 on each of two opposing elevations, then?

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.

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).

Unless your parties are particularly wild the mortar won't have to withstand much 'weather' so I'd expect you've plenty of depth there

Looks really nice

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.

100m down a gentle slope I hope ?

Could recessed not be difficult with brick slips as less brick for the mortar to key on?

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

Good point, I'll ask what the spec is for localised loads as it obviously does allow cars to be parked in there

Or with a magnetic 'downforce' system so the cars can run upside down on the ceiling, that would be cool.

In theory a 2t car would need 2m2 per side so thats about 1.5m square on each side. The limiting factor is likely to be the GreenGuard GG300 insulation under the concrete

I've been wondering if you can get a ~10mm thick neoprene mat with built in electric heating elements. Then you can lay it where you need it and it saves tools and elbows from hits on the floor

A quick Google turned up a bloke from the US saying 3000PSI @ 4 inches. 3000PSI > 21,000 kN/m2 so that doesn't make sense [Edit: that's the tensile strength of the concrete not the floor loading]

I've had the floor designed for a 5kN/m2 loading, 150mm mesh reinforced. I don't know if that's enough for a 2 post lift but probably not. Underneath that it's MOT Type 1 for about a metre down to the stiff clay

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

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

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

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.

Cool, thanks With cavity walls being so thick these days you could almost put a step within the wall but that might make for a difficult to clean corner and an odd looking door. You'd have to add something to protect the floor finish too.

So you think 150mm is probably the limit of what feels OK to actually use. What feels weird to one person may be natural to another though and if there are regs that I should be aware of I don't want to get caught out.