BCD Special Report on
Historic Churches
19
th annual edition
19
permanent. However, the screed provides
considerable thermal mass, which means
the water flow temperature can be relatively
low and during the heating season the
heating does not have to be on continually.
Where lime concrete or ‘limecrete’ is used,
the system can have the advantage of being
breathable and flexible and with pre-heating
of the materials it can be installed at any time
of year. However, the drying time is lengthy,
extending the period the building cannot be
used and increasing the contractor’s cost.
Both types of screed also involve
putting a large quantity of wet material into
a building that may be historically very dry.
In historic buildings such a sudden increase
in humidity can harm ancient fabric.
Thin screed
Underfloor heating can be also
be incorporated in a thin (15–25mm) anhydrite
screed and stainless steel matrix, laid on the
same sort of substrate used for a conventional
screed. Because an anhydrite (calcium sulphate)
binder is used instead of cement, the overall
construction thickness is reduced, requiring less
water to be introduced and reducing its drying
time. Being thinner, the slab also heats up
more quickly, although many pipes are needed
as they are smaller than in other systems.
With either screed form it is
usually necessary to allow for carefully
positioned movement joints and these
may be visible in the floor surface.
Insulated tray (dry systems)
Some systems
use a pre-formed insulated tray with recesses
to take the hot water pipes. This system has
been widely used with timber flooring, which
enables the screed to be omitted. However,
one manufacturer has introduced a system of
tongue and groove glued terracotta tiles which
take the place of the screed and enables stone
or clay tiles to be laid on top. This can form part
of a completely dry system as described below.
If timber boards are used, it is essential that
they are engineered to be suitable for underfloor
heating and that a probe is incorporated into the
construction that will cut the heat source when
the underside of the boards reaches 27 degrees,
otherwise the boards will cup and distort.
Dry system installation
The layered, dry system which has recently
come into use is as follows, starting
from the bottom of the excavation:
1.
Old pew platforms, terracotta aisle tiles and
substrate are removed. Any memorial slabs
are carefully recorded and set aside for later
reinstatement. The substrate is probed with
iron rods to locate any voids (alternatively,
if there is time, a ground-penetrating radar
survey can be conducted and the results
interpreted). The lime concrete ‘crust’
commonly found below pew platforms is
then excavated to around 350mm below
finished floor level, which is often the
depth of the void found beneath the pew
platforms. Where sensitive material is
found, excavation must be carried out with
the utmost care, by hand.
2.
Geotextile membrane is then laid
on the substrate. The membrane is a
woven artificial fabric which separates
different types of aggregate material
while allowing moisture to pass through.
150
mm of recycled foamed glass is then
laid and gently compacted. A polythene
damp-proof membrane is laid on top,
followed by 30mm of crushed slate,
which must be carefully levelled.
3.
Next, 20mm tongue and groove gypsum
boards are laid on the slate, with
audiovisual cabling ductwork laid on top
of them and the interstices filled with a
high-density rigid extruded polystyrene
insulation.
4. 30
mm expanded polystyrene moulded
heating pipe tray with aluminium diffusers
and hot water pipes are then installed and
overlaid with a separating membrane.
5.
Finally, a proprietary system of glued
tongue and groove terracotta tiles, 20mm
thick, are laid and then overlaid with the
chosen floor finish. If desired, an audio
induction loop can be laid in the bedding of
the floor finish.
Advantages of the dry system
The dry system has many advantages but
its success relies on a proper understanding
of the site and careful detailing. In a
medieval church being adapted in line with
typical current patterns of use there will
be certain performance criteria which the
dry system can usually meet without risk
to the floor finish. The key performance
criteria for dry systems are set out below.
Loading
The new floor should be able
to support mobile and stationary loads.
Open spaces should enable high-level
lighting and redecoration to be carried out
from a cherry picker. Generally, a mass of
about 500kg per wheel, moving on relatively
soft tyres on 25mm temporary plywood
sheeting can be safely supported.
Electrical services
A new power and
audiovisual system is often required as part
of the re-ordering. Ductwork and access
Excavation to substrate level
Foamed glass on geotextile membrane
Insulated heating trays being laid
Tongue and groove terracotta tiles being installed
panels can readily be included in the dry
system but detailing is crucial to allow for
proper ventilation where required plus
considerable capacity for future-proofing
and maintenance. Access panels should
allow sufficient depth for the long plugs and
terminals needed for audiovisual equipment.
The new floor should also be able to
incorporate a hearing loop if required.
A type of loop known as a ‘phased array’ has
been found to work well even though the
dry system incorporates aluminium cored
pipework and radiant heating plates.
Drainage and ventilation
The advantage
given by the absence of wet trades should
not be negated by lack of control of any
groundwater either below the floor or in the
churchyard outside, which is often at a higher
level than the floor. The new floor will not
be fully breathable and so the junction with
walls and stone columns must allow for the
dissipation of moisture (see diagram overleaf).
All external drainage systems should also be
checked, cleared and, where necessary repaired
or renewed. Where the ground level is higher
than the interior floor level, consideration
should also be given to the introduction of
French drains – a system of drained trenches
which are constructed at the base of the exterior
wall to a depth below the interior floor level, and
backfilled with a free-draining aggregate. The
aim is to reduce the pressure of ground water
by ensuring that the perimeter is well drained.
Protecting and recording historic fabric
The creation of a new floor will, inevitably
mean disturbance of historic fabric but, on
completion of the works, this material will at
least not be encased in concrete. Depending
upon the known history of the church, the
implementation of a dry system will mean either
a full archaeological investigation or a watching
brief following trial pits and a desk study.
The dry system means that the depth
of excavation can be kept to an absolute
minimum but even with the most detailed
research beforehand there are almost always
