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Posted: Tue Oct 19, 2004 12:28 pm
by Rigman
I first must say that this site has proven to be an invaluable source of reference for my current DIY project landscaping the back garden - probably one of the most comprehensive web sites I have ever seen.
Forgive me if this subject has been raised before (I did scan through a good few pages of post but could not see what I needed), however I am becoming increasing confused as to what I need for bedding slabs. I have chosen Bradstone 'Old Cathedral' flags for our 2 patios of about 17m^2 each. In the Bradstone catalogue, they suggest that these slabs should be laid on a towelled mortar bed. The illustrations in their brochure show a mortar bed (6:1) being applied directly on to MOT tp1. I have read your various pages, and until I read Bradstones notes, was going to apply a MOT tp1 sub-base, with a sharp sand partially compacted screed (probably with the addition of 10:1 OPC as you have suggested in your page 'How to Lay Flags').
Incidentally, the patio is being constructed on freshly excavated clay (about 1m below original ground level), which is mostly firm, however soft in parts.
I have also seen 5-spot mortar being recommended - although I have taken note of your comments in that respect, so won't be using that method. I have also had a hardscaper recommend putting reinforcing mesh in the sharp sand - that will rot, so wont be doing that either. If you go to an un-named high street DIY store, they recommend a mortar bed on loose bedding.
What is your recommendation?
A Confused Patio Layer!
Posted: Wed Oct 20, 2004 2:15 pm
by Tony McC
For most patios, there is NO NEED to use a sub-base. Sub-bases are primarily load bearing layers, and as the patio would normally carry nothing heavier than a wheel barrow or a lawn mower, then there's no real need for a load-bearing layer: the bedding and the flags themselves should be more than capable of carrying the load.
However, if levels are being elevated, or the ground is bad, or you've dug off too deep, then a sub-base CAN be used, and is preferable to using an overly deep bedding layer. A good rule of thumb is that the bedding layer should never be more than 60mm (50mm is my own preferred limit). Once you find yourself on a project where the bedding would be more than this, then build up using sub-base material rather than increasing the depth of the bedding. This is especially true of unbound bedding (sand or grit).
Hopefully, that's cleared up the sub-base business, so now for the bedding.
Most flaggers use sand as a bedding material. Clean, sharp or grit sand, as would be used for block paving. However, getting a bed of sand to compact evenly and accurately is a work of some skill, so, for diyers, and for the thinner patio flags (owt less than 45mm), a cement-bound bed can be used. The most popular cement-bound material to use is a mortar, made using the grit sand mentioned earlier, but the consistency of the mortar is down to individual preference.
Some folk use a wet mix, mortar with the sort of consistency used when laying bricks. I don't like to use a wet-mix as I find it gives problems with 'picture framing' at the edges of the flags and it's bloody messey. It's damn near impossible to prevent wet mortar being tread onto the surface of the flags.
Some folk use what is known as a 'moist mix', which is a much stiffer mortar than the wet mix, with just enough added water to bind together the sands and the cement. When a handful of moist mix is squeezed in the hand, there should be no excess water dripping out between the fingers (Note - wear gloves if trying this, or you risk cement burns!), but the squeezed handful should remain bound together when released and be reluctant to return to a granular form.
The truly god-like amongst us prefer a semi-dry mix. This is a mix with NO added water. It relies on the natural moisture content of the sand to initiate hydration of the cement. The squeeze-a-handful test described in the preceding paragraph applies to a semi-dry mix, but, the compacted ball of mix can be crumbled back to a granular form, if required, because there isn't sufficient moisture present to bind the particles by cohesion.
The reason I prefer a semi-dry mix is that it's less messy, there is much less, or even no, discernable picture framing, and you have a longer working life for the bedding.
Of the three, a wet-mix is probably the most forgiving, in that it allows you to make more errors with the prepared bed level, and because the mix is so fluid, it can move around when pressed and shift the high spots into low spots. This is not possible with a semi-dry, which behaves as would a standard sand, and so needs a significant degree of accuracy, but is less demanding than would be a clean sand, because even incompletely compacted areas of the bed will harden and offer full support after 24 hours or so. The moist mix could well be a happy medium, I suppose, but I was brought up using sand or semi-dry mixes and "if it ain't broke, don't fix it".
There is no need for a 'bond' between the flags and the bedding. Some so-called diy experts are promoting the idea that flags have to be "stuck down" like floor tiles. This is bollocks. As long as the bed offers full and uniform support, there is nothing to be gained by flags being bonded. Many wet-cast flags have an uneven base that gives an adequate 'key' to prevent any lateral slippage or creep, and if mortar butter and pointing is used, this acts as an anchor between individual flags. There is a case for flags being 'bonded' or anchored in some way at free edges, but there are ways and means of doing this without having to have the flags permanently attached to a block of mortar - see the Laying at Free Edges page.
So, if you've not fallen asleep yet, I hope that covers your question and explains the whys and wherefores of sub-bases and bedding. Choose a method that suits you best, and not necessarily one that suits us "experts", and good luck! 
Posted: Fri Oct 22, 2004 1:16 pm
by Rigman
Tony,
Many thanks for the prompt reply. At all seems much clearer.
Rigman 