The Brew Cabin

First of all congratulations for an excellent web site full of very usefull information.

Over the last two weeks I have landscaped my garden with Indian Sandstone (from StoneFlair) called Desert Sand covering almost 125 square meters. The contractor layed the stones on a 200mm bed of scalpings and a 300mm bed of 6:1 sand/cement mix of mortar. The stone was layed directly on the mortar and left overnight to set and the pointing (1:1:2 mix of white cement, Sharp Sand, Soft Sand) was done after a few days.

My concern is that some of the stones are moving (rocking) and cracks are seen between the edge of the stone and the mortar. Also when you tap some of the stones there is hallow sound to indicate there is air between the stone and the mortar. Is this common? If not I need to know immediatly as I have not paid him yet. How should it be fixed?

Is that figure correct? 300mm of mortar - or did you mean 30mm? 300mm is bloody ridiculous!

Also, the mortar pointing mix is very brittle, at 1:1:2; isn't it dazzlingly bright?

If flags are rocking, then, as they've been laid on a mortar bed, they will have to be taken up, the mortar broken out and fresh mortar placed in the bed before re-laying the flag.

If some of the flags sound 'hollow', it may indicate the dreaded 'spot bedding' rather than a full mortar bed or the preferred full dry-mix bed. Spot bedding is a discredited bedding method, but is still being promoted by some manufacturers who want to make it seem easy to lay flags, and by eejit tv presenters who really ought to stick to grinning inanely in front of the camera and leave paving and groundworks to those who actually know what they're doing.

Your contractor should tell you whether they have used a full bed or spot bedding. If they've used spot bedding, ask for their reasoning and why they have not used a full bed.

The loose and/or rocking flags will have to be relaid. One of the problems with mortar or spot bedding is that any movement or rocking only gets worse with time - it never corrects itself. You should keep your wallet closed until the loose flags are fixed. :)

Sorry, I did meam 30mm for base layer and 20mm for the mortar. Thank you for the reply. Will spot bedding cause the flags to crack when a heavy load is dropped on it? I fear that they will.

The mortar is not very bright as it was mixed with red Fobtone cement colour to get the derised light pink to match the sandstone. I am happy with the colour but will the 'brittle' mix cause it to come loose with time?

The spot-bedding may well result in cracked flags, but it's the thickness (or lack of thickness) of the Indian Sandstone flags that's the main problem if you're going to be dropping weights on them. Most are only 30-35mm thick, and that's just enough for them to take average loads, but nothing exceptional, and not vehicular traffic.

Laying on a full bed, either of mortar or semi-dry, helps spread any load applied to the paving, whereas spot bedding means the flag is only properly supported in certain parts and is left to 'bridge' the gaps in those parts where there is no bedding. With a relatively thin flag such as these, a full bed does impart some extra strength, but spot bedding exacerbates their inherent weaknesses.

The brittleness of the mortar is not a major issue, but it will crack and come loose if there is any movement in the flags. It's overly strong for pavement pointing, but it's really not a big problem - I'm more concerned about the spot bedding than the pointing.

Have you got anuy pictures of this Desert Sand? I saw some at Shrewsbury Flower Show on Saturday and it looked very nice, but I'd like to see pictures of it laid in larger areas.

I raised the spot bedding issue with the builder and his reply was that not all the flags are flat at the botton Some are concave causing an air vacuum under the stone. This is what causes the 'hollow' sound and is common with this stone. Out of about 600 flags, about 40 are making the 'hollow' sound. What are your views

He will remove and repoint the flags that are rocking, since the rocking is not much and only about 1 to 2 mm the re-pointing will sort that out.

I have not taken any pictures yet. When I do I will send them to you.

Flags with a less-than-perfect bottom face are not uncommon, particularly with wet-cast products, but there is no way on God's Earth that such an imperfection could possibly cause a vacuum under the flag!! It's what we in the trade refer to as 'talking bollox" ;)

There's a hollow under the flags and there shouldn't be. If the flags are concave, then it is the responsibility of the flagger to adject the bedding to ensure the concavirty is catered for and does not result in a hollow, empty void beneath the flag.

Vacuum! I can't wait to tell my dad that one!! :biggrin:


Looking forward to seeing the piccies - if you want to post them to this thread, see the HELP file, or you can send them to me via info AT pavingexpert.com