Green Building Forum - Pebbledash or Roughcast over EWI

Pebbledash or Roughcast over EWI

WillInAberdeen — Tue, 17 Oct 2023 10:50:32 +0100

A neighbour is thinking about EWI. Like all the buildings nearby, he has a pebbledash/Roughcast/harling render (I'm not sure which) with stone chips of ~10mm embedded in the surface of cement render, visually in good condition. He would like the same finish on top of the EWI to match the surrounding buildings.

1) has anyone had EWI applied onto an existing roughcast surface, or does it have to be chipped off first? How is it glued on without voids?

2) can a roughcast surface be applied onto EWI? Most of the systems seem to use a thin smooth top coat with much smaller aggregate in it, 3mm?

Pebbledash or Roughcast over EWI

owlman — Tue, 17 Oct 2023 11:12:12 +0100

You can get a rough finish with sprayed on monocouche see attached:-

I'm guessing, but for a good job you may need to smooth render the pebbledash to true it up first.

ttps://www.uk.weber/blog/how-guides/how-achieve-roughcast-texture-finish

Pebbledash or Roughcast over EWI

tony — Tue, 17 Oct 2023 11:38:52 +0100

No need to chip off existing render, ensure air canâ€™t flow round and in and out of any voids.

We have affixes expanded metal lathing over the insulation using insulation fixings then three coat render with finish applied to the topcoat. Might need engineering detail for it

Nylon mesh and high tech render might work out cheaper

Pebbledash or Roughcast over EWI

Peter_in_Hungary — Tue, 17 Oct 2023 13:09:43 +0100

When I am putting EWI over an uneven surface I put a bead of adhesive top and bottom of each board with 3 or 4 dollops in the middle. This will stop any air flow at the back of the EWI

Some contractors recommend rendering the wall to a smooth flat finish but IMO that is unnecessary and 'job creation'.

Whether to use mechanical fixing or not depends upon the wall and the location. E.g. I have a basalt stone wall that it is impossible to get mechanical fixing in. The wall is fairly exposed and faces north. The EWI has been on about 12 years without mechanical fixings and no problems.

On top of the EPS there should be a 5mm coat of adhesive with glass mesh bedded in. This is important to stop cracks appearing in the final finish.

The thin film render is the final coat to add a decorative colour and better weatherproofing. This is about 3mm thick and the thickness is governed by the size of the aggregate. Over here you can get 1.5 mm or 2 mm aggregate depending upon what thickness you want. (I think thicker is seen as a better quality job but obviously more expensive as more render is used).

Over here the colour you want is mixed in the builders merchant with machine measured colouring agents so there is no waiting for your particular colour to be ordered. I have never had colour shading issues with this system.

Pebbledash or Roughcast over EWI

bhommels — Tue, 17 Oct 2023 14:24:13 +0100

Wetherby do a spar dash receiver finish: https://www.wbs-ltd.co.uk/systems-solutions/system-finishes/dry-dash-aggregate/
The downloadable brochure contains a list of aggregate colour options. I have used a WBS EWI system and it was very good quality and their support was very good. Other than that I have no interest in WPS, just to be clear.

Pebbledash or Roughcast over EWI

Peter_in_Hungary — Tue, 17 Oct 2023 17:07:27 +0100

Posted By: bhommelsWetherby do a spar dash receiver finish: https://www.wbs-ltd.co.uk/systems-solutions/system-finishes/dry-dash-aggregate/

The site says it is waterproof - it doesn't say it is breathable (that I could see)

Pebbledash or Roughcast over EWI

djh — Tue, 17 Oct 2023 18:27:07 +0100

Posted By: WillInAberdeenpebbledash/Roughcast/harling render

pebbledash is where dry chippings/pebbles are thrown against a wet render. The pebbles are sometimes not terribly well embedded and consequently have a habit of falling out over time. I lived in a house with pebbledash when I was a child (and helped some of the pebbles to fall off ). I have no experience but I'd be cautious about putting EWI over it without making sure any lose chippings are dislodged first.

Harling starts by applying a smooth render. Then the chippings are mixed with render and thrown on to the wall. Traditionally the render is lime. So the chippings are completely enclosed in render and the bond is render to render. So all should be secure.

Roughcast is the same thing as harling, AFAICT.

Pebbledash or Roughcast over EWI

jamesingram — Tue, 17 Oct 2023 22:11:59 +0100

Ive installed EWI of rough cast.
We chipped off any larger protruding bits and hard brushed it to get rid of any loose bits.
We then fixed boards using expanding foam and 2 mechanical fixings.
Foam fixed boards and stopped air movement , bead of foam round edge and two line creating thirds
Then did the standard mesh coat render over top .
10 years and its still looking good. house is just round the corner from me.

My neighbours council, they EWI terraces along my road with pebbledash , looks good , use two different undercoat colours and a horizantal bead at storey height which broke it up a bit.
its wearing much better than the finer grits finish on my place and the flats near by.

Pebbledash or Roughcast over EWI

fostertom — Wed, 18 Oct 2023 09:46:03 +0100

Both the Weber and the Wetherby refs v valuable - thanks. Have been aware that one day I'd need a solution to that but not so far.

However, neither quite make it, when what's needed is wet dash, as well described by djh above - which uses round shingle, not jagged, and ends up smoothly knobbly if that makes sense. tony describes wet cast by resorting to a full 3-coat render. More thought still needed.

Pebbledash or Roughcast over EWI

WillInAberdeen — Wed, 18 Oct 2023 10:06:57 +0100

'Harling' is used round here as a catch-all term to mean all the above techniques (guess every region has different building language!). Most usually, sharp dry 8-10mm chippings of local stone are thrown (harled = hurled) against wet cement render. Then firmed in with a trowel, pushed in deep enough so the colour of the chippings is still visible but they don't fall off (even when wiggled!).

The colour of the building comes from the local stone chippings, which are the right colour to match local stone buildings, and often have some quartz in them which 'sparkles' a little in the sun. After a few years the raw stone chippings get mellowed by lichen (?) growing between them. That's the look that is wanted here.

Thanks everyone for the links!

Still looking for an EWI 'system' that allows this IE a thick enough render (10mm?) to receive the chippings. Is it as simple as just using the usual cement render, onto mechanically fixed mesh over the insulation boards?

Edit: looked again at Bhommel's link https://www.wbs-ltd.co.uk/systems-solutions/system-finishes/dry-dash-aggregate/ which looks ideal, mentions use over EWI. Thanks!

Pebbledash or Roughcast over EWI

Peter_in_Hungary — Wed, 18 Oct 2023 11:48:51 +0100

Posted By: WillInAberdeenIs it as simple as just using the usual cement render, onto mechanically fixed mesh over the insulation boards?

The mechanical fixings go on to the boards before the cement render is applied.

AFAIK the mechanical fixing is to help prevent the wind suction pulling off the EWI. Over here mechanical fixings are not recommended (required) on thermal blocks e.g.Ytong, I presume because the plastic fixing don't hold in those type of blocks . Originally mech. fixings were mandated everywhere but now only where possible and needed.

Of course if a contractor is doing the work they may demand mech. fixings as a call back prevention exercise. AKA a##e covering.

Pebbledash or Roughcast over EWI

WillInAberdeen — Wed, 18 Oct 2023 12:13:21 +0100

I mean like Tony mentioned - the mesh (he mentioned metal plaster lath) is mech fixed onto/through the insulation, then the heavy render+chips are applied to the mesh. The mech fixings support the weight of the thick render+chippings.

Pebbledash or Roughcast over EWI

Peter_in_Hungary — Wed, 18 Oct 2023 13:23:52 +0100

Understand - I thought you were talking about the normal glass mesh used to stop cracking at the board joints and around the windows etc.

Pebbledash or Roughcast over EWI

dathi — Sat, 11 Nov 2023 14:50:14 +0000

Posted By: tonyNo need to chip off existing render, ensure air canâ€™t flow round and in and out of any voids.

We have affixes expanded metal lathing over the insulation using insulation fixings then three coat render with finish applied to the topcoat. Might need engineering detail for it

i did the above on my external insulation using K rend and black and white marble chips its done 15 years now and its still fine i just clean it once a year with diluted bleach from a nap sack sprayer to keep it white

Pebbledash or Roughcast over EWI

Peter_in_Hungary — Sat, 11 Nov 2023 21:55:41 +0000

Posted By: dathii just clean it once a year with diluted bleach from a nap sack sprayer to keep it white

What dilution do you use ?

Pebbledash or Roughcast over EWI

bxman — Sun, 12 Nov 2023 09:52:14 +0000

I have an " aggregate dash" finish on my EWI no metal reinforcement simply the reinforcement scrim as shown in the product video

The finish blends in well not found any need for maintenance in approx 10 years.

The chips, I chose still have very sharp edges and still gradually reduce in number but without detracting from the appearance

see 7 min video

https://www.youtube.com/watch?v=4zfo-vcXLJE

I did buy shares in the company (which I still hold) in an unsuccessful attempt to get remedial work done without actually going to court

Company wise a disappointing investment but I still think it to be a first class product but as ever ; it relies on the contractor actually doing the work correctly.

Make no mistake .

My investment in EWI was very well worthwhile and I would and do recommend it to everyone .

It has saved unknown tonnes of Co2 release .

As far as I could see, at that time , the BBA were simply publishers of companies literature,( for a fee. )

and my so called insurance was worthless IMO.

One would like to think times have changed but some how I doubt it

Pebbledash or Roughcast over EWI

revor — Sun, 12 Nov 2023 15:07:57 +0000

I have found "wet and forget" good on my rendered chimneys. Spray on leave, takes while to clean but it also keeps working and stops render getting dirty again. Known to be used to clean the grime off historical buildings.

https://www.wetandforget.co.uk/

Pebbledash or Roughcast over EWI

dathi — Mon, 13 Nov 2023 13:38:43 +0000

Posted By: Peter_in_Hungary
Posted By: dathii just clean it once a year with diluted bleach from a nap sack sprayer to keep it white

What dilution do you use ?

i just use toilet bleach out of aldi/lidl mixed 50/50 with water i just spray it on and leave it i don't wash it off