I am going to be fitting the floor joists later this week and was always conscious that it's one area prone to significant issue when it comes to the air tightness test.  I have read about the 'Tony Tray' method and that certainly seems like a more reliable option than the joist end caps which still require a diligent brick layer and given we have different width joists, will probably be a faff to source.  It's a brick/block/full fill build up and was thinking that using DPC will suffice?

Is there any reason why you need to lap the DPC down the face of the internal blockwork.  So long as you have a full mortar course here, then air leakage will be negligible and you could trim flush with the wall?

I also see the term 'breathable' mentioned,  does that mean that I should not use DPC and should use another product?


 

1 hour ago, flanagaj said:

I also see the term 'breathable' mentioned,

 

Where? Not in this dwg, unless I am missing it.

35 minutes ago, Redbeard said:

 

Where? Not in this dwg, unless I am missing it.

Ok, not in that one, but it is here 

http://www.tonyshouse.readinguk.org/tonytray.pdf

With the amount of good flexible liquid coatings available I think I would forget the pain of the Tony tray and just paint around the joists when it is all up and the roof is on. 
you could spray airtight paint around them and that would get every nook and cranny. 

17 minutes ago, Russell griffiths said:

With the amount of good flexible liquid coatings available I think I would forget the pain of the Tony tray and just paint around the joists when it is all up and the roof is on. 
you could spray airtight paint around them and that would get every nook and cranny. 

After further research, that is what I have also concluded.  Given the likely shrinkage, it makes sense to do the final sealing one it's water tight and nicely dried out inside. 

I think the tray is a good idea i have some leftover air tight membrane so will be trying it. I'd be wary or using dpc its not very flexible I don't think you'd get a great seal around it.  

Edited October 11Oct 11 by Oz07

16 hours ago, Oz07 said:

I think the tray is a good idea i have some leftover air tight membrane so will be trying it. I'd be wary or using dpc its not very flexible I don't think you'd get a great seal around it.  

I can see how the tray is a great idea in theory, but when you then add in wall ties and deeper 305mm posi joists, it's no longer simple.

Any role for Proclima Tescon Vana tape re this ?

I know goddamn nothing, but our architect agreed to its use where ceiling joists went into blockwork - we are not doing airtight/passiv or anything though.

On 12/10/2026 at 11:25, Spinny said:

Any role for Proclima Tescon Vana tape re this ?

I know goddamn nothing, but our architect agreed to its use where ceiling joists went into blockwork - we are not doing airtight/passiv or anything though.

Yes, if OP has not done anything else, but for a large-scale project with good access the 'Tony Tray' seems better. With a/t tape you are probably (based on my own experience) looking at a scratch-coat of plaster/render round each joist end, wait till it dries, prime it with a/t tape primer, wait for it to 'dry' tacky, apply the tape tight into every corner and undulation... And what about the end ones adjacent to an internal or external wall? Individual taping OK if there is no other option, but at the outset there is.

I have never used Passiv Purple or similar. If that really fills 'dints' (like the face of a block) really reliably then that may be an alternative to tape.

Sorry, I can't offer ay advice, but did you investigate the idea of using ledger boards & hanging the posi joists from those? If so, what made you choose the sticking them through holes in the wall method? Just asking, because I'm not at this stage yet, & based on what I've learned, I strongly favour the ledger board method.

Legers are soooo much better / easier, I’d never make all those holes if I was making a house.

6 hours ago, Redbeard said:

I have never used Passiv Purple or similar. If that really fills 'dints' (like the face of a block) really reliably then that may be an alternative to tape.

You parge all around them first, and then use the PP or other liquid AT membrane, then tape from wood to PP for movement.

18 minutes ago, Tony L said:

the idea of using ledger boards & hanging the posi joists from those

My build was a timber frame, so I used ledger boards and the metal web joists were hung on them. Very straightforward and good for airtightness.

I couldn't be arsed with ledger boards on blockwork. I'd rather try tony tray. Ledgers might sound great in practice but fixing and putting them up sounds a pita. What if the walls are out by 10mm on the width.

On 24/10/2026 at 17:21, Oz07 said:

Ledgers might sound great in practice but fixing and putting them up sounds a pita

Really? I don't have experience of either method, but leaving all those holes looks a lot more difficult to me, especially when you consider the depth of the holes will need to be more than the height of a block. & with the ledgers, you're not restricted to setting them only at the height of the top of one of the courses of blocks - you can position them where you want to to give floor to ceiling heights that suit both ground floor & first floor.

On 24/10/2026 at 17:21, Oz07 said:

What if the walls are out by 10mm on the width.

Given the lack of competence of the builder I sacked, if the next guy manages to get my walls to within 10mm of being parallel, I will be celebrating. Aren't the posi joists designed to be trimmed at the ends? Ordering with 20mm or so additional length may be a good odea. If a posi joist ends up being 10mm too short then I suppose a big plywood shim would be an easy fix.

Edited yesterday at 19:061 day by Tony L

28 minutes ago, Tony L said:

Really? I don't have experience of either method, but leaving all those holes looks a lot more difficult to me, especially when you consider the depth of the holes will need to be more than the height of a block. & with the ledgers, you're not restricted to setting them only at the height of the top of one of the courses of blocks - you can position them where you want to to give floor to ceiling heights that suit both ground floor & first floor.

Given the lack of competence of the builder I sacked, if the next guy manages to get my walls to within 10mm of being parallel, I will be celebrating. Aren't the posi joists designed to be trimmed at the ends? Ordering with 20mm or so additional length may be a good odea. If a posi joist ends up being 10mm too short then I suppose a big plywood shim would be an easy fix.

🤣🤣🤣 you do realise you put the joists on as you build the wall up not leave pockets out for them. Then you use the joists as a platform to keep building up.

Yeh it would just be a right pita to cut and fit pozis to size between ledger plates. They are about 70mm thick for a start. Then your relying and having to install a lot of fixings whereas you just rely on gravity with built in method and the odd wall restraint strap.

21 hours ago, Oz07 said:

🤣🤣🤣 you do realise you put the joists on as you build the wall

Yes.

I wanted to hear the case for holes in the walls vs ledger boards. Thanks for explaining.

Not done this (and think @Oz07 has) but one of the advantages of ledger boards was that I thought it was easier to ensure a level floor. More room for error/fiddling bedding in joists on a masonry wall than putting up the ledgerboards.

I know nothing about this. I thought you were not allowed holes for joists through to the cavity any more?

Having googled "ledger boards" how does one "lose" the thickness of the ledger board on an interior wall?

You need Posis designed to hang on the top chord instead of resting on the bottom chord. That way the ledger's "inside" the Posi joist.

19 minutes ago, LaChab said:

You need Posis designed to hang on the top chord instead of resting on the bottom chord. That way the ledger's "inside" the Posi joist.

Ahh, i see. Yes, makes sense. Thanks.

On 24/10/2026 at 17:21, Oz07 said:

I couldn't be arsed with ledger boards on blockwork. I'd rather try tony tray. Ledgers might sound great in practice but fixing and putting them up sounds a pita. What if the walls are out by 10mm on the width.

Use a tape measure? Order posis that have stock ends which can be cut on site. Simpler than drinking beer and falling down.