Flat roof side extension under gable - no cavity tray

[akist]

I am reading about cavity trays and panicking about :

I have built a flat roof side extension measuring 9.2m x 4m alongside the gable wall on one side of the house. There is no cavity tray. If I understand it correctly from what I am reading now, the water from the gable wall above the flat roof structure will run down the cavity and all the way down to the foundations, but on its way can come out the blockwork into the rooms? Should there be a huge cavity tray running the whole length of the side extension then to take water out of the gable brick wall and potentially onto the flat roof?

 

[freddiemercurystwin]

Is the external skin brick or rendered and what condition is it in?

 

[akist]

Hi, it is all brand new brickwork. The house is under construction. There is no flat roof yet, just the walls. But it is too late to fit a cavity tray. The bricklayers fit cavity trays over windows and doors, but not over the side extension wall. I attach a photo (do not have a better one).

 

[freddiemercurystwin]

All brand new? Whoever's managing it needs sacking.

 

[akist]

Then I would have to fire myself The plans are silent on this topic, I did not miss it, it just was not specified.

The solution now might be to use a breathable brick sealer? eg https://emperorpaint.co.uk/brick-sealers

Thankfully this can be done at any time in the future?

 

[tony1851]

If the plans did not note a cavity tray, then maybe the plan-drawer should be questioned about it? A cavity tray is an important part of the fabric when a single-storey extension is going up against a higher wall.

Did Building Control not spot this omission when plans were submitted?

 

[^woody^]

It's so standard that there is no need to actually mention it.

The assumption is that you don't use shiite bricklayers.

 

[Notch7]

But it is too late to fit a cavity tray

it is not too late.

Get some trays retrofitted - the bricks are cut out, a section at a time and trays inserted.

or go to cavitytrayman and get them to fit their tray - it’s basically a strip of metal with a bit of a bend along its length - they grind out one line of mortar and insert the tray - it doesn’t go right across the cavity, but the water that gets in runs down the inner face of the outer skin.

Either way, it’ll cost you about £1k ish

The solution now might be to use a breathable brick sealer

Sealers have a limited life span - will probably only last a year, although the most expensive ones containing silvan / siloxane do work pretty well


If you choose not to do a tray - I would suggest on the wall in the extension, fit 50 x 25 battens then fit 12mm foil backed plasterboard - that creates a cavity so any water running down the bricks are less likely to show on the plaster - bear in mind any water running down the cavity will hit any steel lintels above openings and will run out at the ends of the lintels

 

[tony1851]

It's so standard that there is no need to actually mention it.

Thanks for the head-up; in that case, next time I draw plans for an extension I'll not bother drawing any concrete strip foundation on the basis that it's 'so standard'.
While I'm at it, can I miss off any wall ties, as they're also 'so standard'??

 

[^woody^]

Thanks for the head-up; in that case, next time I draw plans for an extension I'll not bother drawing any concrete strip foundation on the basis that it's 'so standard'.
While I'm at it, can I miss off any wall ties, as they're also 'so standard'??

Other foundation types are available, so don't do that, they will laugh at you in the office. Likewise for wall types.

Cavity trays however......

 

[Deleted member 281264]

It seems a lot of builders are leaving out cavity trays



has my building regs guy left off a cavity tray from the detail? lol

 

[akist]

Did Building Control not spot this omission when plans were submitted?

No, and neither did the inspector who actually was here a few days ago and literally measured the cavity right over the utility rooms! He could see right down into the wall, as he stood over it.

The assumption is that you don't use shiite bricklayers.

Yes they inserted cavity trays over every window and door, but not on that wall. And I do have a full roll of DPM left they could have easily used if only we knew.

or go to cavitytrayman and get them to fit their tray - it’s basically a strip of metal with a bit of a bend along its length - they grind out one line of mortar and insert the tray - it doesn’t go right across the cavity, but the water that gets in runs down the inner face of the outer skin.

OK thanks, I will research a bit more on this.

 

[johnny2007]

Lots of properties don't have cavity trays and don't suffer any damp.
In the old days cavity trays didn't exist.
In reality, unless the extension is on the exposed side of the house and you get heavy rain, you won't have a problem.
Although, as it's a fresh construction, installing the tray now won't be too difficult and would be a suitable belt and brace approach.