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Posted: Tue Feb 07, 2006 7:25 pm
by Brooksy
I have just finished a patio job, and as ever it's taken me ages due to uplifting and relaying rocking slabs!
I've been in this business now about 2-3 years and have used this site loads (thanks for that), yet I still can't lay flags on a semi-dry mix without most of them rocking (checked lightly by hand) after I have tapped them down. What am I doing wrong?
I have tried using quite dry mixes (too dry probably) all the way to what I would call a fairly damp dry mix (clumps together easily), but it doesn't seem to make any difference.
I am finding that if I have to tap the slabs any more than quite lightly, then the chances are it's going to rock afterwards, and I have to uplift and relay (or force more mix under the corners).
I'm only talking about very slight rocking here (you can barely see it), but it's the sort of thing you can feel through your feet by walking on it once the cement has gone off.
I have worked with people who aren't concerned about laying slightly rocking slabs, and I'm sure they get away with it a lot of the time, but I'm just trying to avoid being called back to jobs.
I have at times resorted to using sloppy wet mixes to lay slabs on but this is even more time consuming, and generally leaves the slabs needing to be washed afterwards. They never rock when laid this way though - is this why Tommy Walsh always seems to lay slabs on wet mortar?
Any help would be appreciated
thanks.
Posted: Tue Feb 07, 2006 11:38 pm
by Tony McC
Without seeing you "in action", it's hard to say where you're going wrong, but the most likely cause (that is, the reason most often encountered when training our apprentices over the years) is overfilling the centre of the bed. This results in one or more edges being 'looser' than the rest of the bed following consolidation.
When you lay out your bedding mix, trample it down with yer boots. Scrape it off to form a level-ish bed that's 5-8mm low, then add an extra 10-12mm of loose bedding mix over this. Use a spirit level or short screed board to level this off so that you are reasonably sure you've got an even bed, and then place your flag. Consolidate as shown on the main website making sure the maul is used in the places indicated rather than trying to smack down the centre or the edges.
Once the flag is down to level, pack the free edges from the side to ensure there's no wobble. This isn't shown on the Laying Flags page, but it is explained. I keep meaning to get a photie done, but I've a memory like a... a .... a wotsit, a thingy, one o'them yokes with holes in it, in the kitchen, next to the grater.
Also, with cement-bound bedding, it's important to keep off the flags as much as is poss for the first 12 hours or so, and if you do have to walk on them, make sure you plonk yer size 10s in the centre of each flag and not near the edges: better still, put down a board or decking sheet to help spread the load. This is a handy tip if you're mortar pointing.
Like yu, many flaggers find that a wet mix is more forgiving, but as you've noticed, it's also bloody messy and with some flags, it can cause problems with picture-framing and efflorescence. As a contractor, once a job was finished and paid for, the last thing I wanted was the client on the phone wanting me back there because of ring-stains on the flag, or an eff-bloom that forced you to wear sunglasses when looking at it. Swings and roundabouts, innit!
You will eventually find a mix consistency that does it for you. it might not be semi-dry: that's what wored best for me - you might find you need a moist mix. It's a good idea to play around with a moist or wet-moist mix for a few months while you perfect your bed-making technique, and then move on to gradually stiffer and drier mixes.
Th'owd flaggers, like my old fellah, can create a bed with three flicks of a spade, and no matter whether it's sand, cinders, concrete or semi-dry mix, it'll be right first time - that ability comes with years and years of practice, and it's once aspect of the trade where you can never stop improving. It's a knack - some learn the basics in 6 days; some need 6 months; and a few of the useless twonks that passed through our books would never have it if they worked for 60 years.
Slow down; work on getting the bed right, and everything else will follow. It's better to spend an extra couple of extra minutes making sure the flags are stock solid when first laid than have to come back in a couple of weeks and spend half a day trying to wet-grout half-a-dozen of them.
Posted: Thu Feb 09, 2006 12:21 am
by Brooksy
Yeah, I suspected it might just be down to getting a bit more practise.
I tend to use a float to try and get the bed sorted, which does sometimes leave it a bit uneven (maybe higher in the middle as you say).
Anyway thanks for your help - I'll try out your tips on my next slabs job.
Posted: Thu Feb 09, 2006 11:22 pm
by MatthewWakeman
an engineer is watching a slab layer at work gently tapping down his slabs to a line checking and tapping checking and tapping, the slab layer looks up " how you doing mate " the engineeer replies " i'm an aerospace engineering and i work to a thousanth of an inch and i was just admiring your eye for accuracy working to the line" the slab layer responds a thousanth of an inch thats no good here mate this has got to be cock on"
Nice
Posted: Fri Feb 17, 2006 12:33 am
by Tony McC
I worked on a project in a northern city centre at the back end of the 1980s. The RE was terribly precious about setting all levels and would never allow anyone other than himself use the auto-level. It was an honour to be allowed to hold the staff!
One section of paving, running along a series of very, very swanky shops was to be paved with brand new, cut-for-the-job, sawn-six-sides yorkstone flags at 90 quid per square metre.
The kerb line was in and verified; the RE came out of his Ivory Tower and set levels for the back edge at 3 metre intervals.
We'd laid roughly 180m² of the total 240m² when up pops the RE again to check the work. He notices the straightedge isn't quite tying-in with the shop thresholds - there was roughly a bugs' dick and a rizla in it. It would be no trouble at all to make up the diff with a 25mm wide accommodation fillet, but no! That won't do at all, says Mr High and Mighty.
So: he gets himself into a right panic, and has me spending two hours walking up and down with a straightedge, trying to work out what's gone wrong and how it can be all my fault before he gets out the auto-level. Up and down I trudge with the staff, taking spot levels for him at what seems like 300mm intervals, and then he reveals the awful truth: he's miscalculated the crossfall. Instead of having 75mm of crossfall, he must have had a branestawm when doing the sums and has only allowed 72mm. Shock! Horror! Instead of a crossfall of 1:40, we have a mere 1:40½. How will the water ever get away??
So, no word of a lie, he has us take up all 180m² and re-lay the lot to the corrected level, and he agrees to pay for the lifting, plus 18 hours daywork to clean off the salvaged flags, and then pay us the contract rate to re-lay the lot at the new level.
Most expensive bit of flagging in the city - and did anyone ever notice?? Did they f....!! :p