I've just done the first layer (scratch coat)...the mix was 2 parts cement + 3.5 sharp sand + 3.5 builder sand + waterproofer (Celtec). I've ran out of Celtec and wondering if I can use a different mix for the next coat - I have some LIME and some liquid waterproofer - what would you recommend? Shall I go buy some more Celtec or can I use a different mix with Lime and/or liquid waterproofer for the next layers?
On the plastering and rendering course I did some years ago, we were told you only include waterproofing in one of the two coats, assuming you are applying the finish coat to the scratch coat whilst it's still wet, as you should be (24-48h later). Otherwise the double waterproofing can interfere with the drying out after it's set. IIRC, convention is waterproof scratch coat when rendering in the summer, or waterproof the finish coat when rendering in the winter (because wall dries out in a different way in summer and winter). I'm not familiar with Celtec, so check its instructions to see if they say something specific for it.
BTW, lime is not a waterproofer. In cement renders, its a plasticiser. I could well imagine lime might interfere with the waterproofer (it might use up proportionally more of the waterproofer because it's more porous, making the waterproofer less effective, but see if the waterproofer says anything about that). You can buy plasticiser, or combined waterproofer/plasticiser, and many cements include plasticiser admixtures anyway nowadays (it will say so on the bag if it does).
Our brickie seems to favour platiciser over lime plus it is a winteriser too and allows work in cold temperatures. I think the waterproofer is added separately.
I'm interested in adding external insulation (say 75mm celotex attached with direct screw fittings, multi monti generic, and some wire mesh) and then rendering over that with screed fibres to add more strength.
The reason being dense concrete blocks make for really fast construction with no cavity and are cheaper than bricks but would this meet modern building regs?
AJH
They recently insulated a nine story(ish) block of flats by fixing insulation slabs to the outside and rendering it. AFAICS they were mineral wool slabs and I never drove past when there was anything to support the render attached but I would think some sort of mesh would be required. It appeared to be a well tried system as there were big plastic disks tied through to the wall holding the slabs on and they probably support the render too.
Remind me to look in 25 years and I will let you know how durable it is.
Thanks for your reply... The message is clearer now: no plasticiser on second coat - this is also recommended in other sites I've just visited. I think I'm still going to use lime as an additive for the next coat.
Much what was done here for out re-build. Mind you ISTR BR only called for 2.3W/m2 then, so we only got 25mm of Celotex.
Mesh was galvanised expanded metal lath and, yes you can see the cracks where they join.
Put 75mm on the inside with the plaster board ready attached. Met the BR for our extension with external feather edge cladding but I guess render would have been OK.
regards
Er, lime is a plasticiser, or did you mean no waterproofer?
Also, if you've left it more than 48 hours since scratch coat, I don't know how well the two coats will bond. Probably worth looking that up, as you might need to use a bonding agent (and if so, it will have to be a waterproof one if your finish coat doesn't have a waterproofer).
You can leave the scratch coat a good deal longer if you like. So long as its reasonably well keyed you should have no difficulty with the finish coat. IIRC they used to leave it a week or more before doing the top coat.
I am seriously considering something similar for our place. From what I have seen so far there are some nylon hammer in fixings with big round mesh type heads. The idea being I believe that you slap up the insulation with expanded metal lath over it, drill through the lot and into the wall, then hammer the fixing in. Being nylon it does not form a cold bridge.
What I am not sure about, is what type of render you can put on the outside of it - whether ordinary sand and cement is ok, or if you need something special and light weight. I can see some fibre re-enforcing could be good.
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Ideal is 24 hours, as the cement in the scratch coat still has enough setting to do to actually bond with the finish coat, and at 24-48h, the scratch coat should be ideally damp enough so you don't need to kill the suction (which also kills the bond strength) although the waterproofer may reduce the need for this.
After 48 hours, you may need to rely on something else to do the bonding, such as keying and/or a bonding agent, as the cement bond between the two coats rapidly becomes much weaker the longer the time between the two coats.
bonding agent like PVA or SBR? Anyhow, it's been now about a week since I've done my first coat...would it be feasible to use an angle grinder to improve the keying? I've done the scratching but maybe I'll get a better and deeper key...
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In my (somewhat limited) experience, keying is much overrated. On a gable end wall I did, where some was properly keyed and, for reasons I can't remember, some wasn't, it made no difference to how well the next coat went on.
"It made no difference"...do you mean the lot fell down in the same way? No disrespect here, but in my limited knowledge the main point of keying (or scratching) is to improve adhesion between 2 coats - in simpler words: to keep the second coat hanging in place as long as possible (correct me if I'm wrong).
AJH>>I'm interested in adding external insulation (say 75mm celotex attached AJH>>with direct screw fittings, multi monti generic, and some wire mesh) and AJH>>then rendering over that with screed fibres to add more strength.
What sort of U value does one of these zero energy houses aim for in the wall? I work on a block of flats that has 10" of glass fibre filled cavity and 100mm of celotex should be near that. From the way the boiler runs in cold weather each flat seems to require 2.5kW. and I guess a small flat is around 250m^3 so with 2 air changes an hour you could lose 3.5kW without heat recovery for a delta T 20C.
All the more reason to add screed fibres?
I've already done this on a store room that has been converted to an office but I used extraliner 12mm plasterboard with 50mm celotex, quite expensive at GBP30/sheet. I raised the floor with 75mm celotex and 75mm of screed with 150' of polyplumb spiral embedded in it.
AFAIK...AIUI...IME...etc etc... The bond between coats operates at a microscopic level, so that mortar sticks to the face of a brick even though it has no visible grooves or scratches. Keying helps reduce the initial slump when the mortar is trowelled on to a large, flat area, but doesn't affect the long term adhesion. IOW it's less arm work for the plasterer. When I was young and inexperienced, I built a breeze block garden wall and got a plasterer in to render it. He told me to pva the blocks, so I did 3 or 4 coats on to be on the safe side, with the result that the wall had a nice satin sheen :-). When he arrived to do the render, he raised his eyes to the heavens and said he couldn't do the job because the render would slide down the wall. However, once suction took over, the stuff went on well, and stayed on for at least 2 decades until the house was sold. On this basis I have every confidence that you could successfully render a sheet of glass. You can certainly render flimsy plastic pots
For what? The wall, or the pot?
the wall of course... For the first coat I did use some PVA diluted initially but it dried out too quickly and the rendering kept falling - I managed in the end but I guess I've wasted 50% of the mixing on the floor...so I decided to use just water and in my case it made the job much easier.
Dunno, it was over 30 years ago. Nowadays I find 1 pva to 4 water about right for most jobs.
Yes, but not PVA (as it's water soluable) unless you use waterproofer in the finish coat too (which is probably OK now the scratch coat is well dry).
To be honest, I think you need to consult a professional at this point or you might end up with something which fails and is difficult to put right.
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