Technical Q&A 24 : Clay Plain Tiles
Q. What are Plain Tiles?
A. Plain tiles are
the widely-seen flat form of roof tile, which, in practice is usually gently cambered.
Being “double-lap”,
the tail overlaps two courses below. Unlike the uniform, mass-produced plain
tiles from Victorian times onwards, earlier hand-made ones were hung not by
protruding nibs but wooden pegs and possessed greater individuality. Sizes of
these ‘peg tiles’ range from just 229x152mm in Kent,
to 305x203mm in York.
Most are a shade of red, although soft, variegated colours are associated with
Cambridgeshire.
Q. How do Plain-tiled Roofs
Deteriorate?
A. Fixings and their supporting
timbers usually deteriorate before tiles. Over time, the corrosion of nails
(“nail sickness”) may cause tiles to slip or crack, and battens or pegs may
decay due to insect or fungal attack.
Other causes of deterioration
include: frost action, exacerbated where moss retains moisture; mechanical
damage (such as wind-lift); and the decay of mortar “torching” underneath
tiles.
Q. Should I Repair or Re-tile my
Roof?
A. This depends on the extent of
deterioration. Consideration should normally be given to re-tiling when repairs
are no longer cost effective. This is typically after one-fifth of the tiles
have been reinstated. The condition of gutters and ancillary items such as
flashings will also influence the course of action.
Repair or re-tiling should be
entrusted to a reputable roofer, and the SPAB may be able to advise on suitable
contractors in your area.
Q. What might Maintaining and
Repairing a Plain-tiled Roof Entail?
A. Slipped, broken or missing tiles
will, from time to time, require reinstating by slotting them or suitable
replacements up over the battens or laths. They can be torched with mortar,
spot bedded or nailed, as appropriate, to secure them. Mortar fillets at
abutments will need repointing periodically and any excess moss should be
carefully brushed off tiles.
Foam or bituminous-type remedial
treatments are inadvisable. They prevent proper inspection, hinder the re-use
of tiles and, by reducing ventilation, increase the risk of timber decay.
Q. How Should I Introduce New
Plain Tiles?
A. New tiles should usually match
the type, colour, texture, size and thickness of existing ones. Avoid using
second-hand plain tiles from other buildings. Substitution with concrete tiles
is undesirable as, generally, is replacement of hand-made plain tiles with
machine-pressed ones (or vice versa).
When
completely re-covering a roof, the maximum number of old tiles should be
re-used. Depending on the circumstances, new tiles can be mixed with old ones
across the roof or reserved for less prominent slopes.
Q. Should I use Sawn Battens and
Modern Roofing Underlay with Plain Tiles?
A. Where the
appearance of the roof underside is important, like-for-like replacement is
normally desirable, using, if appropriate, riven laths, oak pegs and torching.
In other situations, the use of sawn
battens, copper or aluminium or nails, or non-ferrous pegs, and possibly
roofing underlay, could be justified, but additional ventilation may be
required. Here, selective courses and verges with peg-tiling are nailed, as
with nib tiles, for wind-resistance. Nails for fixing battens or laths may be
of stainless steel, although screw-fixing is recommended above fragile
ceilings.
Q. How Should Plain Tile Details
be Treated?
A. The emphasis should be on
matching existing details. The detailing of hips, valleys, ridges, dormers,
eaves and abutments should therefore be recorded.
Details
such as tile-and-a-half rather than cut-tile verges may be inappropriate for
pre-Victorian buildings and the lime mortar fillets found on many old roof
abutments usually deserve retention rather than replacement with lead
flashings. Bedding mortar for hand-made tiles should normally be lime-based
(without cement).
Q. How do I Improve Ventilation
with Plain Tile Roofs?
A. Ventilation can be increased
sensitively, but might require innovation. The purpose is to prevent
condensation, thereby minimising the risk of timber decay.
Typical
solutions are proprietary “in-line” vents for ridges, timber pieces between
horizontal underlay laps within roof slopes, and, where standard eaves vents
are not used, ventilation through the soffit or open eaves.
Further Reading
Roof Slating and Tiling, by F Bennett and A Pinion, Donhead, 2000 (reprint of
1948 edition)
© Douglas Kent 2006