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This method relies on each flag having a bed prepared for it individually. A bed is prepared, the flag is laid and then the next bed prepared and so on.
This method is more commonly used with larger flags where handling and manoeuvrability is a major concern; with natural stone flags where there can be significant difference in flag thickness between adjacent units and for many of the mould cast riven or decorative flags which tend to have a variable thickness ranging ±15mm on individual units – this being a result of the mould not being perfectly level when the wet-cast concrete is allowed to set.
This method requires a screed bed to be prepared before placing the flags. A relatively large area can be screeded in advance and the flags placed one after the other directly onto the bed.
Whilst obviously more efficient than the individual bedding method, this method is only suitable for the smaller flags – generally those less than 600x600mm – which are small and light enough to be man-handled onto a screeded bed, although it can also be used with some calibrated flagstones.
Flags of regular and consistent thickness make screed bedding possible.
Where screeded beds are perfectly flat (ie: no change of plane), flags of almost any size could be laid, but where there are changes in fall or multiple planes, only the smaller flags can normally be used as larger units would rock when placed onto a non-flat bed.
It is essential that the flags be of a constant thickness, hence the small element paving flags, wet-press manufactured patio flags and the smaller calibrated stone flags are the most suitable candidates.
British Standard 7533: Part 4, which covers the installation of both concrete and natural stone flags or slabs, requires that these are laid on a “full bedding layer”. This applies to ALL classes of pavements, including patios and driveways – not just the big projects undertaken in the town centre or on a retail park.
The ‘dollops of mortar’ method, known as “spot bedding”, “dab bedding” or “dot and dab” is not recommended. On public and commercial works, it is rarely, if ever, permitted.
Regardless of what you may have seen on TV DIY and gardening programmes, spot bedding is not an acceptable bedding method because:
• it leaves voids beneath the flags, making them more likely to fracture when loaded.
• the voids can allow surface water to accumulate, leading to subsidence or instability.
• the voids provide a ready-made home for invertebrates, particularly ants, which love to mine an unbound bedding material and/or sub-grade.
• the solidified mortar spots settle differently causing the flag to ‘rock’ when trafficked.
• correcting any rocking flags requires the old mortar to be broken out and disposed of.
• it is often more expensive than solid bedding.
• it often results in damp patches appearing on porous flagstones, especially sandstones.
• it does not comply with the relevant Code of Practice which requires bedding to provide “uniform support”.
A variation on the unacceptable spot bedding is ring bedding. As with spot bedding, it often reveals itself by means of distinctive discolouration or staining on the surface of the flagstones a few weeks after they have been laid.
Ring bedding is defined as a “ring” of mortar being placed around the perimeter of the flagstone with, sometimes, a central dollop which the idiots involved believe improves support!