SBR screeding - first attempt

[sm_jamieson]

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.

[Tim Watts]

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.

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.

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.

Cheers

Tim

[sm_jamieson]

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.

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 !

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.

[sm_jamieson]

Well, phantom was awsome. Proper singers. And the fairly small orchestra were perfect. You know when a singer and the orchestra are so perfectly in tune that they totally blend on some held note ... We watched xfactor back at the hotel. Oh how we laughed (even more that normal).

Anyway, the test patch of screed seems pretty reasonable. With the half strength SBR if has not totally turned to rubber (!) and I could just scratch down a couple of 1-2mm high spots that were causing the spirit level to rock. The patch looks very grey. I could probably reduce the 1:3 cement sand ratio a bit. Simon.