MortarThePoint

They're leaving some gaps in the blockwork like this. Will be harder in the 140 blockwork mind you

Surely shouldn't need to bother the Structural Engineer for this as the lintels are only 600mm long?

I have asked them for a detail for this a number of times over the last few months and only had words back "Support is required at bearing level above the opening for the vent." I take that to mean there must be a lintel. The sides have pillars to and whilst in theory you could plan them not to clash (as 600 c/c) that could easily go wrong and present a bigger problem that is only discovered late in the game when the slabs are being fitted (all in a day)

Not sure what the correct forum protocol is here (new thread or use old one) but I am facing a similar issue. Is it not possible to have the vent sunk down in a box with a horizontal grate over the top as long as the box can drain? Obviously it would be a problem if the drain pipe blocked and the top was open to rain. We are working out how to get vents in to our porch whilst having level access. The porch has a roof. I'd like to keep the ability to have a level patio at the back too don the line. There are lots of vents along that elevation so... There'd be mesh under the brass vent plate shown screwed over the box.

We're using a slab flooring product that has support pillars moulded in that have to bear on something. Cavity concrete fill comes up 200mm then there is only one more blockwork layer until the floor slabs.

Your probably right, but they need to be linteled anyway

Working out the bond:

Will be good to use if I mess up by bond calculations ? What ducting did you use with that?

Nice ideas. I think I'm trying to get them on bond and putting them in a 350 or 400mm slot to allow adjustment.

Building Regulations Part C says that for "SUSPENDED CONCRETE GROUND FLOORS MOISTURE FROM THE GROUND" But sets no c/c requirement

Well it pays to be looking at the most up to date documents. NHBC had changed their wording since 2011 (I've added the highlight):

I'm intrigued. Are you just testing your pipes are watertight? If drainage they shouldn't see any real pressure unless blocked should they. There are worse days to have had a soaking on

It's not block and beam but a slab based flooring. It would be bearing directly on the lintel of the inner leaf and chances are only one of the two.

Well a bit of an update here. The Structural Engineer has said he isn't happy about the central wall misalignment but has kept me waiting for about a week now to do calculations as to how much offset is OK. The groundworks guys did day 2 of blockwork yesterday and that looked to be fitting nicely by eye. I went out this morning and they have taken it back down. They checked yesterday's work with the total station and realised they had made a mistake with a mark that meant a wall went 40mm off target (OK one end, straight, but 90 degrees 13 minutes to the other wall). They have now identified that the front area of foundation is out of shape and they are talking about digging next to it and pouring more foundation. 450mm width if just butting up or 600mm if doweled. I prefer the sound of doweled. To their credit they have worked all this out themselves and corrected their work, rather than coming to me and asking if it is OK, which gives me some confidence.

Clear span is about 1000mm. On the 215 blockwork they are planning to use two lintels (side by side). I don't know what Type but they look to be 100x70 with a single metal bar about 6mm in diameter near the bottom edge.

But you'd be happy with it just bearing on the blockwork or coursing blocks?

Floor goes down before outer skin is established unfortunately. The BCO said "what does your warranty provider require" and the warranty provider said "what does your BCO require" so that's handy. Obviously NHBC aren't regulations, but they are a standard worth sticking to if possible.

I'd take it even if it leaked like a sieve.

I've a run of subfloor blockwork that is specified as 190mm 10.4N blocks. The drainage pipes pass through this wall and I wanted to make sure the lintels are strong enough and have the correct bearing. They've only fitted the one in the outer leaf in the photo below but are looking to use 2 of the same type of lintel bearing directly on to the Stranlite 10.4N blocks for the inner leaf. I should have measured the span but I would guess it is 1000mm. Looking at the Structual Engineering calculations there looks to be about 6000 kg per metre length of that inner leaf so that seems like a lot of load to put on those lintels and the 150mm bearing at each end. On paper the pressure would be 60kN/(2 * 150mm * 200mm) = 1N/mm2 which is obviously much less than the blocks 10.4N/mm2 spec but perhaps that's missing something.

That's what I'm concluding too. I think 600mm lintels for these with a 400mm opening would make it easy. That leaves a big opening on the inside of the cavity for critters to get in to the cavity though.

That would be good as 450mm is too tight really otherwise. It would still be good to find a 4" x 3" air brick though. I'm tempted to 3D print an adapter but that would take quite a lot of time and plastic Edit: Here is something that could possibly do the trick, stepping a brick wide vent down to half brick wide

It's basically impossible to hit Flemish bond whilst satisfying the NHBC "within 450mm of each end of any wall" requirement if you have cavity walls over 338mm which is the vast majority of mine at sub floor level. 496mm is the first point on bond at which it doesn't clash with the inner leaf of the perpendicular wall. There is only one Stretcher with any part of it within 450mm of the quoin and the edge quoin side of that would cause a clash. I'm presuming the NHBC requirement is any part of the vent within 450mm of end of wall. I don't know what this adapted (no Queen Closers, uses 3/4 batts) form of Flemish bond is called but it is possible to get that to work:

The NHBC stipulation of "within 450mm of each end of any wall" is quite restrictive as if that is referring to the outer leaf (which is where it is ultimately visible from) you have to take wall thickness away from that to see where it ends up on the inner leaf. I have some 390mm thick walls so that puts the vents right at the corner. Can that be right, looks pretty extreme.

As there was made ground on site the Site Investigation said there could be Ground Gas. Testing for Ground Gas looked as expensive as installing a Ground Gas membrane, so we are just doing the membrane instead. That needs a ventilated void below floor.

This image made me think of a possible solution. Whenever feeling the need to hide something it's worth thinking whether it can be turned in to a feature. I could get some metal grates 335mm wide and recess them in to the wall with the vent behind it.