Render onto s/s mesh

Hello, The pebbledash above my bay window (30s semi) had cracked extensively, revealing a galvanised mesh that was now beyond rust.
So, I have knocked all the exterior render off the bay and now have stainless steel expamet mesh ready to start the repair.
I understand that I should use stainless nails and tie wire with s/s mesh, not galvanised.
That I should overlap sections of mesh by four inches. That the mesh is backed by 1/4" thick wooden battens to allow the render to penetrate the mesh and hang on.
That these battens are also to hold the bituminised felt sheet* behind the mesh in place against the wooden studding behind it (I guess I could use 1200 ga polythene sheet too).
*Thinking of 'undertile felt' from B&Q - similar thickness to the original, though the B&Q stuff is lightly sanded.
Things I am less certain about - my house is from 1938 and has lime plaster inside and lime mortar between the bricks, it's quite white in appearance and the mesh had this in it too - but there appear to be three coats of render, the inner one fairly white, the next a little darker, the third with all the pebbles stuck in it. I am thing the base coat is not pure lime mortar though? More likely 1:1:5 lime:cement:sharp sand? What about coarse sand - is that better?
I believe the layers should get weaker as you go out, so the next layer might be 1:1:6?
Not sure about the pebbledash layer - needs to be able to take pebbles though having tried this once they just seem to bounce off ;0)
Also not sure about 'bonding' to the mesh and/or the brickwork at the edges with pva or 'sbr' slurry?
Is the initial 'pricking' coat to the mesh done in a special way or is it just a thin coat to get the 'tongues' of render stuck through the back?
Also not sure about putting waterproofer in the render - after all, lime mortar is breathable, so is cement/lime mortar to a certain extent. I don't want to trap water where a lime render could have 'breathed' it out so to speak.
Any comments/advice/tips??
Andy
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Andrew Phillips wrote:

If it's a square bay, I'd strongly advise you to clad it in upvc rather than rendering, if it's rounded, IE curved, then render is the way forward.

yes
You've lost me already, 1/4 of an inch batens? - these are lath, like what's used indoors - they won't last long outdooors, nor would I fancy trying to affix them to mesh.

Breathable membrane is what you require

Red building sand, like what's used for mortar is what everyone renders with

I'd be temted to use 3:1 sand cement on first coat, then 4:1 for top coat and pebbledashing. You don't need three coats.

They stick better when wet.

not required on the mesh at all, but yes everywhere else

Just applied as normal, although it will need to be a fairly stiff mix, too wet and it will just sag through the mesh

You can put W/P in the top coat but not the first (the 2nd coat won't stick to it when it's dry.)
Also, forget about the lime
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Also if you'd like it to look fecking ugly afterwards and entirely inappropriate for a '30s house.
You can round the UPVC "Shameless accident compensation cheque" effect off nicely by finding a '30s semi with a semicircular arch porch, then fitting a tiny UPVC door into the middle of it. Fill the gap with more UPVC - you can get this in raised & fielded panels for a classy effect.
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On Monday, 13 August 2012 23:51:07 UTC+1, Phil L wrote:

Sadly, I can't stand the sight of uPVC. Instant character bypass for a house ;0) In any case, it's a single return bay and cladding that would look odd. If I were to go that way I'd do it in slate shingles, but I'd probably need planning permission to change the look of the house at the front like that plus it'd still look odd as a single return bay done in slate.

Tick.
The laths are as good as they day they were nailed in position 75 years ago - seriously, if I don't damage them taking them off, I'll use them again. They go behind the mesh - they (a) give a 1/4" gap for 'pricking out' of the mortar to occur through the mesh before the felt starts and (b) hold the bituminous felt dpm to the wooden studs behind.

Surely breathable membrane is not of much use if pressed hard up against the render coming through the mesh - it also looks too flimsy to have something touching it - since there is no airflow in the stud what is the point of breathable membrane? I would in effect be rendering onto breathable membrane, as the render will reach back through the mesh to it. If warm humid air could enter the stud from inside somehow then it might condense on the back of the cold render, but it can't so I don't see why brethable is necessary. The bituminous felt isn't breathable and the studwork behind is 100% sound.

Good idea

Excellent
Stiff mix, check.

Good point - needs suction I suppose as well as scratches. >

You want me to mix a lime rendered house with a cement render? As I understand it, cement is very hard and can crack with older houses, plus if any moisture gets in through a crack it can't get out again - except inside. My brother had this problem with his house which had been inappropriately cement-rendered.
Thing is, the original house render is fine after 75 years so I'm loathe to change it or mix and match - the only thing that failed was the galvanised mesh...
On Monday, 13 August 2012 23:51:07 UTC+1, Phil L wrote:

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On Tuesday, 14 August 2012 00:21:05 UTC+1, (unknown) wrote:

Regarding the studding behind the render etc - if it is ventilated I haven't seen how yet, but I only took a quick peek behind the bituminous felt dpm sheet. Since it is not ventilated from inside or outside, the only way it could be is via cavity wall air but again I'll have to have another look. The felt dpm by the way had a horizontal overlap join in it near the top - so does actually pass air, though as I say the air has nowhere to go as far as I've yet seen.

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On 13/08/2012 23:34, Andrew Phillips wrote:

I am guessing you are describing similar to what I did here:
http://www.internode.co.uk/loft/rendering.htm

Or building paper is a another common backing...

Yup, something with some Hessian reinforcement is easier to fix.

If you use cement in it, then its not really lime render. The lime in these situations acts as a plasticiser, but does not bestow the traditional attributes of "proper" lime mortar.

Not sure its that critical. You don't want the render stronger than the substrate though.

Never tried it, so can't comment... there are some videos on youtube though.

Use corner beads at the corners (fixed first), and a render stop at any edges. Keeps it all tidy an in place.

I just apply normal thickness backing coat, which pretty much hides the mesh. Scratch it up, and then stick the finish coat on that.

Don't think I would. Impermeable coatings can cause as many problems as they solve.
--
Cheers,

John.

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On Tuesday, August 14, 2012 1:14:55 AM UTC+1, John Rumm wrote:

Hello John,
Yes, That's exactly what I'm doing!

I've heard of it, will look it up - sounds...fragile.

Mine is held onto the studding by laths and clout nails and onto the brickwork at the edges by clout nails into wooden rawlplugs and with penny sized copper washers.

I understand that, although I do believe it is somewhat breathable compared to solely cement mortar - the cement fills up the inter-grain spaces pretty much completely I believe.
My problem is that I can't tell if the original builder used lime mortar or cement/lime mortar. The sand where I am is silver and when you add lime it's snowy white. That's the colour the base coat is, snowy white. The next coat is a shade darker, but that could just be a weaker mix so more sane. I guess it's hard to tell what mortar is made of...

The substrate is mesh and those common bricks that are too ugly to use without a coating.

The render stop at the bottom - the bell-casting - must have been a piece of wood, removed after the render set. The render also sits partly on a beam above the window below. The beam has the outside edge sloping so as to offer the render some support where it touches it. There is a little rot in the centre where the render sat on it but the beam is otherwise sound. I will cut the rot out and splice in a small facing strip profiled to blend in, with nails and Cascamite boat glue, plus treat the wood.
There is no corner bead that I could see - the mesh just went around the corner. There are extra laths in the corner to support the mesh at it turns 90 degrees.

Yes, I have seen this with my own eyes in other places. It is a concern, this render mix/waterproofer business, as the success of this type of construction might depend on the use of breathable lime - I don't know enough about it either way to make my mind up. Like I said, everything in the house contains lime, the brickwork mortar, the render, the interior lime plaster. The only unknown is if it's got any cement in it at all...

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On 14/08/2012 02:00, snipped-for-privacy@gmail.com wrote:

Ultimately the strength comes from the rendered metal lath - all you really need is something to stop it falling through as you render.

That ought to do it. Sticking in normal wire nails half way and clenching them over was another common way (gives a length of grip along the nail rather than just the head)

Since you are replacing it all, you don't necessarily have to use the same.

Yup, quite common. Also common to staple a bit of sash cord along the wood before putting in place. That then casts a "drip" into the bottom of the render so the water falls away from the wall rather than run down it.

Nothing to stop you adding them though - its easier to render up to beads.

I suppose if you add an impermeable coat, you can always add trickle ventilation somewhere if you need it.
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Cheers,

John.

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I've only rendered onto expanded stainless steel lath once. The problem I had was forcing the render through the lath at all. It tends to just push the lath against the wall behind, but not go through and stick to the wall itself, and springs away when you stop applying the pressure. I was only doing a very small area, but I was very close to taking the lath off, rendering a very thin layer direct to the wall, and then putting the lath back in the top of that layer before continuing.
BTW, handling a sheet of expanded stainless steel lath is not very dissimilar from handling a coil of razor wire - very difficult to avoid cutting yourself to shreds. I might even suggest wearing goggles.
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Andrew Gabriel
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On 14/08/2012 15:18, Andrew Gabriel wrote:

Sorry - meant to say the lath and the render there!

Might have needed a wetter mix... generally I have found that it flows through easy enough. You get better integration of the lath if there is a bit of space behind it every now and then.

I used the galvanised stuff last time. Slightly less nasty, but still quite hazardous. Stiff gloves and a small disc in an 115mm AG to cut.
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John.

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Andrew Gabriel wrote:

When the guys did here, (wood frame) they ply-ed the whole exterior, shoved a semi-permeable membrane over that and then nailed *vertical battens* to that and the metal lath over that, with a drip bead at the bottom. That meant the render keyed very well and there was an air gap between the render and the wood, which allows it to dry out if there is any penetration, and also allows the walls to breath a little.

Good idea. Gloves certainly.
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On Tuesday, 14 August 2012 15:18:01 UTC+1, Andrew Gabriel wrote:

I have already found out what 'orrible stuff s/s mesh is - it was hard enough trying to get it to roll up to get it into the car! I read that it is best to angle the holding nails for it so as to tension the sheet. Best to start in the middle and work out apparently. I expect I'll preform it if I need to take it around the corner to the return leg of the bay.
Regarding lime mortar, I read somewhere that lime fizzes when exposed to brick acid, whereas ordinary portland cement just bubbles gently. This might be an interesting test to try on what render I've already hacked off - though I have battery (sulphuric) acid available, not 'brick acid' whatever that is. I presume the lime will react and dissolve/turn to gas whilst the sand will be relatively unaffected. I'll give it a try with a few bits of mortar of different types of known provenance.
I'm veering back towards straight lime:sand render now, of the 1:2.5 variety for the base coat, but need to research it more as I know lime washes away in rain eventually ( I lime washed a wall once and after a couple of years it started to wear thin) so what's to stop a pebble-dash bearing final coat from eroding? Plus, people advise you should make up lime putty/coarse stuff weeks before you use it which is a pain, though I think I have at least two weeks before I'm ready to start rendering....
It still seems to me I haven't quite figured out what the original builders used as a render mix for all the layers, though lime was part of it.>

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On 14/08/2012 19:48, snipped-for-privacy@gmail.com wrote:

Brick acid is HCL, not H2S04
(can fizz on cement as well)
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John.

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On Aug 14, 7:48pm, snipped-for-privacy@gmail.com wrote:

I can't see that distinguishing lime from cement & lime. Simple strength might - cement is far harder. But really, why would anyone pay extra for white cement? If its white, its lime.

HCl, but any acid reacts with lime.

Set lime doesnt wash away. Chalk & glue paint does, I guess that's what you had. Also lime does initially if it sees much rain before its set, so check the forecast for the 2 days ahead.

one piece of advice I take no notice of. Builder's lime works.



NT
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Best way I find to get it through the EML is to go over it with the trowel at a fairly strong angle, so just the edge inch or so pushes the mix through
NT
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On Tuesday, 14 August 2012 01:14:55 UTC+1, John Rumm wrote:

cement, and sets like cement - but the result isn't as stiff, and is breathable.

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Metalwork to enable easy clean edges makes work quicker, but it also builds in a failure point, since its galv, and its perfect straightness doesn't always suit old houses. I'd be inclined to leave it out and form your edges by hand.
I would defintely not use plain cement render for the base layer when you can use an insulating render such as papercrete, or standard render with expanded stone included.
Vapour barrier should go on the warm side of any insulation, otherwise condensation and rot is likely.
Pure white render is pretty well bound to be lime. Lime mortar is normally 3:1 to 2.5:1, and isnt generally varied the way cement mixes are. Lime is much softer, takes ages to set, and needs to be trowelled during setting as it cracks. Unlike cement, trowelling when it part set doesn't harm it.
Finally mixing a very little plastic fibre into the 1st coat can reduce the risk of cracking to a degree. You can buy it at daft prices, or just cut synthetic carpet and rag into diagonal strips and feed it through a shredder.
NT
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On Monday, August 13, 2012 at 11:34:13 PM UTC+1, Andrew Phillips wrote:

Need some info on rendering a round bay window.All the rendering had blown and cracked just on the round bay,so i removed all the loose,then found a l ike a corrugated plastic sheet with bitumen on it,and felt behind it.Their isnt many supports ie timber behind the sheet,so could this be reason why i t had blown ?If i added more timber supports and made it more secure,would this cure it ?
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     snipped-for-privacy@sky.com writes:

It doesn't sound like a product suitable for rendering on. Is this on the inside or the outside?
If you are pulling it off anyway, now is the time to make sure it's well insulated behind it.
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