Cavity trays above windows on timber frame

[dogman]

I have been doing a bit of planning on the rendered section of our new build. Its a twin wall MBC build. 

About half the build will be clad with render board and thin coat render.

What i cannot find is how to deal with cavity trays above the windows and doors and also at a roof abutment

 

 

The above is from the Knauf detail and it does not show any form of a tray. Another manufacture has a metal channel above the window that drains to one side.

I think @JSHarris used flexible flashing on a timber clad section

Does anybody have details of how this should be done or how they managed the detail on their own build

 

[Jeremy Harris]

There are no cavity trays, as such, but TRADA have some good guidance on how to deal with the door and window heads where you have cladding.  I pretty much followed the TRADA detail, except with flexible non-lead flashing rather than specially formed aluminium flashing.  The idea is to allow the cavity above the door or window to freely drain, whilst allowing ventilation.

 

With render board I don't think there is the same need to ensure water drainage, as it should be pretty impermeable, which is probably why Knauf has that sealed detail.

 

Edited February 26, 2017 by JSHarris

[ADLIan]

Both

 

knauf render panel requires drained/vent airspace behind. See manuf instructions and BBA certificate. They omit to say how to deal with this detail above window however and diag is misleading. As JSH says TRADA will have more details (I don't have these to hand however).

 

ian

 

[dogman]

@ADLIan i have seen JSH post and see how he dealt with the issue with timber cladding and see what he did will work

 

The issue with cement board and render is it is in effect a sealed system vented at the top and bottom of the wall.

Not sure which of the manufacturers it was but the only detail i have seen is a u section between the timber frame and  cement board set at a slight angle so it will catch any moisture and drain it away from the window head.

 

[ADLIan]

I've not seen JSH detail but noting his attention to detail I'm sure it'll work!

 

My concern is with the diag you show at the top which fails to show the vent required to this airspace - I find this a little misleading from the manuf.

 

ian

[ADLIan]

Perhaps the solid timber here is fire fire reasons - internal fire breaks thru window and could then travel within hidden cavity??

 

ian

[dogman]

this is from wetherby 

TF-Epsitec15-RO-05-Window head.pdf

 

It shows a bell cast fixed to the timber frame to deflect moisture away not as good as the u channel as moisture will sit on the inner face of the board

 

The timber frame is counter battened so air flow should not be an issue if you pay attention to detail. ( and yes i agree the detail from Knauf is misleading)

 

[TerryE]

We have a stone skin, so the issues are different; nonetheless, the point that would concern me abut this detailing is that if  the integrity of the outer skin is compromised for any reason, then the closer / external panel will act in such a way as to hold the moisture against the internal frame.  Not good.  I would have expected some form of DPC and drip ventilation about the window to carry this moisture away from the inner frame and to ensure that it stays dry.

[dogman]

I decided to brave the plasterers forum and ask

They are worse than us for banter but do stay on topic

 

anyhow Prorend have details on how to flash the windows in the prowall details

 

Edited February 27, 2017 by dogman
.

[Jeremy Harris]

8 hours ago, dogman said:

I decided to brave the plasterers forum and ask

They are worse than us for banter but do stay on topic

 

anyhow Prorend have details on how to flash the windows in the prowall details

 

 

Back in the early days of Ebuild it was usually the plasterers who started trouble...............

[Mr Punter]

Lift up the breather paper, tuck up and staple some 225mm DPC to the sheathing  and drape over the horizontal batten.  Even without weep holes, any water will be trapped between the render board and the DPC and will eventually find its way down the sides.

[TerryE]

8 hours ago, Mr Punter said:

Lift up the breather paper, tuck up and staple some 225mm DPC to the sheathing  and drape over the horizontal batten.  Even without weep holes, any water will be trapped between the render board and the DPC and will eventually find its way down the sides.

 

Sounds like a plan, but if you are doing this then perhaps you'd be better letting the DPC turn up behind the render board in a J profile, and that way the water will escape down the sides rather than rotting the render board.

[dogman]

I have managed to speak with several board manufacturers as well as a couple of the render manufacturers.

 

key is to ensure that 

a. the battens do not create dead spots. 

b. That there is a method to move any moisture from the inner skin to the outer skin.

c. weep holes are not needed on any boarded system.

This is one way of doing it using flashing

Edited March 2, 2017 by dogman

[Trw144]

On 01/03/2017 at 15:23, Mr Punter said:

Lift up the breather paper, tuck up and staple some 225mm DPC to the sheathing  and drape over the horizontal batten.  Even without weep holes, any water will be trapped between the render board and the DPC and will eventually find its way down the sides.

 

Exactly what I did, and taped some outdoor air permeability tape over the joint on the membranes for good measure. Did this for both the render board and timber clad detail.

[dogman]

One of the board and render manufacturers has stated that you must not counter batten the frame as it causes the render to crack.

Only use vertical batten and leave gaps for air to move horizontally

 

[ProDave]

Seeing all the complications of a cavity and board+render system, have you considered what I have done, wood fibre direct onto TF with no cavity, and render onto wood fibre board?
 

[dogman]

@ProDaveI have looked at yours in the past and have not discounted it. Have asked MBC about the battening as they specify counter battening. 

[Jeremy Harris]

I went through this with the TRADA detailing, and they too advise vertical counter battens, with a gap at the bottom of any that are over window and door heads to allow cross-ventilation.  Their detail calls for specially formed aluminium flashing, made to slip under the breather membrane, with a tall enough upstand to be trapped under the counter battens.  This then continues down and out, to deflect any penetrating moisture away from the inner structure.  The heads are fixed to short intermediate counter battens fitted between the main counter battens, so that the ventilation path is maintained.

 

I didn't have time to find someone to bend up custom aluminium flashing (but have since heard that the people that made our aluminium dry verge will do it) and so I used non-lead flashing, the stuff that has something like aluminium mesh embedded in some form of plastic, that is lead-coloured.  It was easy to shape this to fit.  Don't be tempted to use lead, as it may stain the glass underneath permanently if any run-off gets blown onto the glazing..

Edited March 3, 2017 by JSHarris

[dogman]

Mbc are happy with just vertical battens to the specification of the board manufacturer.

 

Has anyone used SAS Prowall/Prorend they and Knauf have been very helpful, waiting for Parex to send out colour samples 

 

 

[Stones]

On 3/1/2017 at 15:23, Mr Punter said:

Lift up the breather paper, tuck up and staple some 225mm DPC to the sheathing  and drape over the horizontal batten.  Even without weep holes, any water will be trapped between the render board and the DPC and will eventually find its way down the sides.

 

This is what we did in our timber clad section (the DPC being cut into the ICF block and sealed in place with soudal) simply to ensure that any water that does find its way behind the cladding will run out over the horizontal batten over the windows where it can find its way out, rather than tracking back to the seal between window and ICF blockwork.