Voids In Laying Beds
As mentioned several times in this text, with vivid illustrations shown in the Useless Projects
(Failures) article, there are many reasons for laying full mortar beds under paving slabs. The
old ‘dot and dab’ or ‘five spot’ method of placing mortar has long been discredited as being
unsuited to any project requiring longevity.
Voids, or gaps left in the laying bed are unwelcome – even the practice of running the trowel
through the mortar bed to ‘fluff it up’ by creating ridges with the purpose of making laying
easier is fraught with problems, as any moisture left under the slab – or has migrated upwards
through the laying bed – will remain trapped in the hardened materials and cause delamination
between the slab and laying bed under frosty conditions with the effects of freeze/thaw.
Voids do not allow for full contact between the (slurried) paving slab. Water trapped in the
gaps will show as reflective staining on the surface of most natural stone paving, especially
sandstones and sawn granites, where they will manifest by appearing as blotches on the slabs.
They can allow ants to colonise the voids, bringing soil and other detritus in with them, further
potentially disrupting the bond between slab and laying bed.
Unless the laying bed is 100% full, jointing compounds or grouts may settle lower than the
ideal depth of 20mm (thickness of the majority of pointed slabs) by entering the void, and
causing slumping in the grout line, which will become filled with dirt and grime, acting as a
channel instead of a full grout finish.
