Mortar mix for seating slabs

[Scion]

A friend had a low brick garden wall built with slabs on top for seating. A couple of slabs came loose after very little use and it is evident that none of them are fixed properly - a sharp tug will make them come off.

Builder is blaming friend, for having the temerity to sit on the seating area. (Yes, he was told it was for seating.)

It doesn't help that the slabs are wider than the wall; the overhang allows leverage forces to pull the slab up.

I'm happy to have a bash at fixing, my question is, what mix of cement/ sand/whatever should I use to obtain maximum "tensile strength"? Any prep on the underside of the slabs? They are smooth-ish - think slate texture - but fairly thick.

[Andrew]

The problem is not just the mortar mix (which was probably too weak) but the differential thermal expansion and contraction of wall and slabs. Also 'slate texture' slabs won't have the suction required to create a proper cement bond, which could be the major issue which you won't cure even with a stronger mortar mix. I presume the mortar is still well adhered to the wall but the slabs have parted company with the mortar ?. You might have to consider a chemical adhesive which allows some flexibility as the slabs expand and contract.

[Scion]

I hadn't considered thermal expansion but wasn't aware it was an issue in brick/stone.

Hence the 'prep' question. Wondering if roughening the underside would help.

I haven't seen it first hand, I'll ask.

I'll have a Google. As I assumed the mortar was too weak and a stronger mix would cure it I hadn't considered other options. A flexible adhesive might be fine - the overhang would hide it so it doesn't matter if it doesn't match the rest of the brickwork.

[Chris Hogg]

Clean off the old mortar from the slabs and the wall top. Re-fix the slabs with a structural adhesive such as one of the Stixall or Sikaflex range.

[williamwright]

You won't do this I know, but after repeated failures a friend of mine did this with long screws and blue plugs. A complete fix.

Bill

[Scion]

We did consider screwing (stop sniggering at the back!) but all the slabs are cut to size and there are no spares therefore no second chances. If it was mine I wouldn't mind so much but I dislike doing stuff for other people that may end up less than perfect.

[Robin]

Might help to know the height and width of the wall and the width of the slabs. If there's a really big overhang I wondered if it might be worth thinking of (so as to speak) widening the wall to support the slabs. Possibly galvanised steel angle screwed to the wall if they'd be low enough to be hidden?

[Andrew]

Also, if the slabs were laid so that they were tightly butted up to each other then thermal expansion will break the cement bond. This is why the end brick of the top course of a wall frequently has an L-shaped bit of galvanised metal embedded in the mortar and turned up at the end to provide lateral support to the end brick.

If they were laid 'tight' then re-lay but leave a 5 mm gap between them and get some exterior mastic, suitably coloured and fill the gaps with this.

[Andrew]

Cormaic has somethig to say on the question of coping stones on walls

[The Natural Philosopher]

For TENSILE strength use epoxy mortar!

[Brian Gaff (Sofa]

a few bloody great bolts might be of more use. Cement is not good when being pulled apart as it is on a slab on a wall with lop sided pressures. Brian

[John Miller]

In the past, I've solved similar problems by drilling into the underside for a couple of plugs and then inserted screws which stick out and provide a key for the mortar to hang on to. They were standard concrete slabs, so might not be advisable with the stuff you're working with. John M

[John Miller]

In the past I've solved this problem by drilling into the underside for a couple of plugs, then inserted screws which stick out and provide a key for the mortar to hang on to. Those were standard concrete coping stones though so might not be applicable to the stuff you're working with. John M

[newshound]

I was going to suggest adding SBR and Paving Expert even gives you a recipe!

"A lower-cost alternative is to make a slurry using readily available ingredients. The most popular recipe uses nothing more than liquid SBR (3-5 quid per litre) with ordinary Portland cement (5-6 quid per 25kg). A small quantity of the SBR (100ml or so) is placed into a bucket and then dry cement powder is slowly added and stirred in until the desired consistency is achieved."

But please read the original: