Clay tile tolerances
Clay tiles generally need the largest manufacturing tolerances, due to
the shrinkage of the clay during drying, which can be between 10 and 12
percent, the stacking arrangement and the firing temperature in the kiln.
Some kilns and clays produce tiles that are very consistent, but
variations must be expected, such as twist and camber.
Concrete tile tolerances
Concrete tiles can be manufactured to fairly fine tolerances due to
negligible shrinkage during curing and the stability of the material.
However, concrete in the wet stage can be deformed due to recovery of what
is a liquid during the extrusion and cutting process. The result can be an
uneven top surface or bowed sides, especially the left-hand interlock.
Resin slate tolerances
Single lap resin slate, made in a press, has a smaller manufacturing
tolerance than either clay or concrete. It is pressed into a mould and
heat cured, making the finished product a very close replica of the mould.
Resin slates not made in a press can often be more variable than clay
tiles due to the wet nature of the process.
Having made and delivered the tiles to site every roof is slightly
different in size and accuracy from eaves to ridge, hip to valley, verge
to verge. The building process needs components that can be fitted
together regardless of the problems left by the previous and adjacent
construction trades and components, and can cope with the movement of the
structure during its 50 to 100 year life span. With roof tiles this is
achieved with differing tile modules, variable head lap, interlock shunt,
half tile modules with some designs, and as a last resort, cutting the
tiles. Wherever possible the tiles should be laid as close to mid shunt as
Single lap tiles
Interlock shunt with single lap tiles can be anything from 0 to 10mm
depending upon the tile design and material. Flat concrete interlocking
tiles generally have 6mm of shunt (±3mm). A tile designed to have a
coverage width of 300mm can be laid at anything between 297mm and 303mm.
By laying 50 tiles (15m) along a batten it is possible to gain or lose
half a tile by shunting in or out. The upper and lower halves of the
interlock should not come into contact with each other when laid
correctly. If the upper interlock has been shunted out too far, or the
interlock is not straight (lock splay) the ribs are likely to come into
contact with each other and result in corner or interlock breakage.