New extension DPC Tray Flat / Sagging - What to do?

[boxrick]

I've recently sacked my builder. Whilst trying to fix some issues he left behind I remember someone commenting on my DPC Tray at the bottom of my walls.

 

They are either sagging or flat.

 

Id like some thoughts about how serious this is and what I should do? 

 

[Mr Punter]

The main thing is that you need a DPC.  It can be separate for inner and outer leaf or linked like a tray / skirt with a fall to the outside.  Because the walls are already built there is not much you can do if the DPC bridges the cavity and is flat.  No water will rise past your Marmox blocks unless you are doing a basement. 

[Spinny]

I am no builder, but as I understand it the principle is that any water which might enter the wall cavity would move down the wall cavity under gravity. It would then encounter the 'cavity tray' with a slope towards the outer leaf, so that it could then find its way out via weep holes in the outer leaf.

 

I think the reality is that this is a second order mechanism because you should not have water in the cavity in the first place. Obviously moisture could enter through the outer leaf, through the top of the wall if the roof/parapet/gutter leak, or I guess through some condensation process. But under normal circumstances this shouldn't occur. However when building things with long lives the non-normal needs to be considered.

 

Others will be able to advise more, but I wonder have the works been seen by a building control person as I think DPC level is a normal inspection point ?

 

I'd also like to say you have my sincere sympathies for your situation because I have been through the same process of having to terminate a builder's contract and I know how difficult and stressful things become and know you will find some good and very experienced people here.

[Mike]

2 hours ago, Spinny said:

I think the reality is that this is a second order mechanism because you should not have water in the cavity in the first place

If it's not rendered, then with hard rain in an exposed location it can be a continuous flow, especially if the brickies haven't fully filled all the joints.

 

3 hours ago, boxrick said:

how serious this is and what I should do?

Unless I had a rendered wall, I'd definitely be wanting to correct it.

 

You'd need to cut out short sections of brick, cut the insulation and insert 'retrofit cavity trays' designed for the purpose, reinstating the insulation above and below, then the add back new bricks, making sure that the joints are well filled, but adding preformed weep holes. Then leave it several days before do the next section. You can have different sections on the go at once, as long as they're well away from each other to maintain the structural integrity of the wall. I've done similar (as a manager) retrofitting trays above lintels that the original builders 'forgot'.

 

Edited Monday at 13:17 by Mike

[Mr Punter]

48 minutes ago, Mike said:

If it's not rendered, then with hard rain in an exposed location it can be a continuous flow, especially if the brickies haven't fully filled all the joints.

 

Unless I had a rendered wall, I'd definitely be wanting to correct it.

 

You'd need to cut out short sections of brick, cut the insulation and insert 'retrofit cavity trays' designed for the purpose, reinstating the insulation above and below, then the add back new bricks, making sure that the joints are well filled, but adding preformed weep holes. Then leave it several days before do the next section. You can have different sections on the go at once, as long as they're well away from each other to maintain the structural integrity of the wall. I've done similar (as a manager) retrofitting trays above lintels that the original builders 'forgot'.

 

 

This is a base level DPC.  As long as it prevent water rising up the wall it has done the job.  Chopping out bricks in sections just sounds silly.  This is not halfway up the building where it will possibly allow water into the wall below, it is at ground level, where it will soak harmlessly into the ground.

[Mike]

24 minutes ago, Mr Punter said:

it is at ground level, where it will soak harmlessly into the ground.

Or non-harmlessly though to the inner leaf. One or the other.

[Nickfromwales]

20 hours ago, Mike said:

If it's not rendered, then with hard rain in an exposed location it can be a continuous flow, especially if the brickies haven't fully filled all the joints.

 

Unless I had a rendered wall, I'd definitely be wanting to correct it.

 

You'd need to cut out short sections of brick, cut the insulation and insert 'retrofit cavity trays' designed for the purpose, reinstating the insulation above and below, then the add back new bricks, making sure that the joints are well filled, but adding preformed weep holes. Then leave it several days before do the next section. You can have different sections on the go at once, as long as they're well away from each other to maintain the structural integrity of the wall. I've done similar (as a manager) retrofitting trays above lintels that the original builders 'forgot'.

 

IIRC, there’s 120mm of EPS EWI and a thin coat render going all sides around on this job. Does that change the ‘importance’ of how these things have been done or if they’re now virtually redundant, also with full fill glass and blown bonded beads in the new / original cavities?

 

I’m struggling to see how any moisture would ever get in here, if the EWI is done well and the render is mastic sealed to a decent depth of soffit too? 

[boxrick]

120EWI all the way around with a combination of either render or cladding finish. 

[Mr Punter]

Did you consider using Rockwool instead?  No issues with fire risk.

[boxrick]

13 minutes ago, Mr Punter said:

Did you consider using Rockwool instead?  No issues with fire risk.

In respect of EWI?

 

That's likely the material I'll have to go with for EWI anyway above DPC.

[Nickfromwales]

If there’s only fire risk at one elevation, then the EPS EWI should be okay for everywhere else. 
 

At nearly £25 per single 600x1200mm batt, 120mm rockwool isn’t kind on the pocket!

 

Still, the question is about the EWI negating any concerns about your cavity tray / DPC issue.

 

So, with a fully rendered EWI, is there any real world chance of water / moisture getting behind the EWI, through the external leaf of masonry, and into the fully filled cavities, and causing issues there?
 

I’d assume that the item in question isn’t fully continuous or completely intact either, so if it stops and starts or is perforated / damaged, that would allow any puddling water to eventually escape; as a last possible line of defence. 

[Nickfromwales]

Assuming weep vents would need to go through the external masonry and through the EWI and be open at the rendered facade if so necessary too? 

[Mike]

5 hours ago, Nickfromwales said:

IIRC, there’s 120mm of EPS EWI and a thin coat render going all sides around on this job. Does that change the ‘importance’ of how these things have been done

If there's EWI, then indeed it's very unlikely to be a problem.