Page 1 of 2
Posted: Tue Mar 05, 2013 4:40 pm
by Trevski
Ok I know it's not paving related but the wealth of knowledge on here is amazing.
I've been asked to render a 30m2 piece of external block wall, the wall has been up for some years and probably needs a good wire brushing to give a clean surface to render onto.
The owner has asked if I will be using a galv lathe mesh before putting the scratch coat on.
I figure that the blocks are rough enough to provide a good key and as long as they are wetted down then the suction should be controlled enough. The plan is to put on a 4:1 10mm scratch coat and then a 4:1 10mm finish coat the follwing day. All beading will be in stainless.
Of course I don't want to have it pull away in the first winter so thought I would ask how others would tackle this job
Posted: Tue Mar 05, 2013 5:16 pm
by dig dug dan
I am not a rendering expert, but done lots of it using a subbie
Pva the wall first, then its 4-1 soft/plastering sand mix, using waterproofer in the mix.
beading is essential. Scratch coat is also a good idea on the first coat.
LLL will no doubt have some advice
Posted: Tue Mar 05, 2013 7:54 pm
by lutonlagerlout
EML is good but who is paying for it?
we have done it with EML,but has to be fixed with screws and penny washers
if going straight on the blocks i would key the block work with a grinder 10mm deep every 150 mm diagonally ,both ways
if that makes sense
LLL 
Posted: Tue Mar 05, 2013 8:21 pm
by Trevski
Thanks LLL, I think that will suit his budget and equally as effective.
Other than damping the wall down, do I need to consider an application of PVA?
Posted: Tue Mar 05, 2013 8:35 pm
by lutonlagerlout
pva/sbr will help to prime the wall
mix it with water 5:1 then spray it on with a pump/nozzle gun
LLL
Posted: Tue Mar 05, 2013 8:41 pm
by local patios and driveway
Make sure the scratch coat is nice and damp you dont want the finish coat to dry too fast, i always work the final coat with a big sponge to get a nice smooth textured finish.
Posted: Tue Mar 05, 2013 8:49 pm
by Trevski
Thanks fellas, good advice and I'm far more comfortable taking this on now, I do a bit of plastering quite happily but this will only be the second rendering.
Onward we go.
Posted: Thu Mar 07, 2013 1:34 pm
by hickeymaster
Hi, It might be an idea to scud (spatter dash) the wall first with a slurry mix of 1:2 cement and sand before the scratch coat. This gives the render a good key to the block work. I thought this was standard practice.
hickeymaster
Posted: Fri Mar 08, 2013 2:56 pm
by simeonronacrete
SBR is a much safer way forward than PVA. PVA will reemulsify on contact with water.
Key the block work as suggested before you start.
Rake out mortar joints if possible.
Damp with clean water.
Primer is a 1:1 mix of Ronafix (SBR) with cement.
Brush primer on to damp surface and render onto wet/tacky primer.
Clean, sharp sand - whilst harder to apply - will give more water/frost resistance. SBR in the render will make it totally (depending on cement:sand content, grading, etc). Good mixing, proper mixer, careful curing - all the usual - will minimise shrinkage and drying cracking.
Posted: Fri Mar 08, 2013 3:18 pm
by DNgroundworks
i did a few years plastering, and ive never heard or been taught to add mechanical key to blockwork before? Just damp down and if old blockwork SBR bond the wall and if new blockwork no primer just straight on with the mortar?
Posted: Fri Mar 08, 2013 4:48 pm
by simeonronacrete
We always go for the belt and braces approach - far better to eliminate or at best to reduce the risk of something going wrong than have to go back and do it again.
Some blockwork can be very porous and draw all the moisture from the primer and even the render into the block. Other blocks can be so dense that a stipple coat is first needed to key the surface. Stipple coat in our book of recipes would be 1:1:1 cement: shard sand:Ronafix (SBR) stippled onto the dry surface and left to cure overnight until hard enough and sufficiently bonded to act as the key for the render (and we would still advocate a primer coat of SBR and cement before the render goes an - and put SBR (Ronafix) in the render too) as previously recommended.
There are of course many unmodified renders with soft sand and no bond coats been hanging there for many years with no failure. But there are also brand new renders falling off, cracking, shrinking, debonding, frost-attacked - that we see weekly.
So we err on the side of caution and go for a secure spec.
Other opinions will differ.
Posted: Fri Mar 08, 2013 5:25 pm
by lutonlagerlout
^^ I agree with simeon here
do it once and do it right
nothing ever failed from being over engineered
LLL
Posted: Fri Mar 08, 2013 8:57 pm
by GB_Groundworks
On the big house we built few years ago we had a big plastering contractor to do the k rend they just applied it to the damp block, I guess it's upto manufacturers to lead the way but like the concrete thread a lot of bad habits spread through out the building game.
Posted: Sun Mar 17, 2013 12:21 am
by henpecked
I was always taught 6:1 base and 5:1 top coat. More flexible base for movement, harder top coat for weather resistance.
Posted: Sun Mar 17, 2013 6:51 am
by seanandruby
Maybe power was the wall first, then i'm with dn g'works on this.