On 20 Aug, 16:54, Tim Watts wrote:
sm_jamieson wrote:
Well, I'm doing the screeding by laying "rails", then filling in the
bays between. Mix is 1:3 cement: sharp sand, mixed with 1:3 SBR:water
(half-strength SBR mix) to semi-dry (stays clumped when you squeeze it
together).
Bonded with 1:1 water:SBR with cement to a creamy consistency.
Phew, laying screed is not easy !
I was doing a strip along one wall to get me started. 2 metres long by
200mm wide along the wall. I had a guide to level to (a sort of frame
made of 25mm square steel tube). When doing the body of the room I'll
fill inside the frame to do the "rails", but by the wall I was filling
alongside the guide.
I filled to above level, compacted it, then sliced if off level using
a plasterers trowel. Trouble was, when slicing it off, it left behind
a rough surface with some pitting, due to the courseness of the sand.
I had to go over the area trying to fill some of the pitted areas.
Now, all this will be tiled so the pitting that remained did not
matter, but I was getting very frustrated.
Maybe I actually had it too dry, since it would not polish off like
concrete or mortar would.
After a while I hit upon a technique of slicing the surface whilst
sweeping the trowel in circles, which seemed to be good at filling in
the surface, helped by a little water from a sprayer.
Result, it is pretty flat though, find for tiling.
But obviously doing a larger area you have to smooth as you go.
So, what is the technique when scraping level a semi-dry sharp sand
mix, that creates a smooth surface as you go ?
I think this may be the hardest thing I have done so far (except
trying plastering !). Bricklaying is easy !
Cheers,
Simon.
Perhaps a touch more water to the mix?
I did a similar thing when I levelled my kitchen. I didn't want the
levelling scunge and the epoxy DPM under the fire (for obvious reasons) so I
framed off the fireplace out 300mm with taped wood battens and levelled,
DPM'd and topped upto the wood. I did exactly as you did after removing the
wood, infilling a 3/4" deep patch with SBR screed. It was fiddly but not
impossibly so.
I used a straight piece of wood to scrape level then gave it a quick once
over with a metal float as you did. Mine polished up quite well - virtually
perfect (again, wasn't important, tiles over the top). But I tend to default
to wet mixes with everything like that as I can never work with dry mixes.
Usually dry is recommended with screed to reduce shrinkage. I guess to
SBR
helps with that too.
I'll try the next mix just a touch wetter.
I must admit, having screeded a whole small room with 2" SBR with an
assistant, I find larger areas a PITA. I put 2x2 battens round the wall and
used a tamping bar with ears on to guage the level. Trouble was, as the room
was weird shaped, we had to use 3 lengths of tamping bar and when we changed
bar, the screed tended to push back to the previous section and "hump". Had
to go round scraping bits off and eventually gave up and threw 5mm of
Stopgap 300 over the lot (which was easy in comparison).
I had more luck hand finishing a 3m2 concrete slab which went down very
nicely, despite having to stand in it in wellies due to lack of space.
I think in your case, I'd worry less about the surface finish as long as it
is basically flat for tiling.
There are a couple of areas, about 1m2 each, that will have carpet on.
But that rest of
the 25m^2 is to be tiled. I thought I'd leave the non-tiled bits for
now !
It's not going to fall off, that much is assured 
Couple of tips when you infill between strips:
1) Masking tape and a bit of scrap DPM down both sides over the old
(previously just laid) screed to stop the new mix going everywhere
or
2) Bucket of water and wet sponge to run over the old screed edges - cleans
the smearing off (which might be weakly bonded and thus not a good substrate
for the tiles) and gives a nice join.
Thanks, I'll report back.
Anyway, I'm off to London for a weekend break and phantom of the
opera !
I'll see what the screed is like when I take the polythene off on
Monday.
Simon.