… which are not French, but definitely are drains – and sometimes useful ones
at that. Douglas Kent, SPAB Technical Secretary, explains when they might be used
to fight damp and water penetration
Q. What are French drains?
A. Named after Henry French, a 19th-century judge and farmer, French drains are
a form of land drainage. A French drain comprises a perforated pipe laid in a
shingle-filled trench topped, if desired, with turf. The pipe has a fall of approximately
1:80 and is typically 100-150mm in diameter accommodated within a trench 150-200mm
wide by 200-300mm deep. Modern methods employ a geotextile filter membrane to
line the trench and the pipe is sometimes wrapped in the same material. The membrane
excludes sediment from the drain, allowing a coarser (20-50mm) fill. uPVC pipes
can be used but clay ones are superior. Good installations incorporate frequent
rodding points to facilitate the clearance of any blockages. Water is conveyed
to a soakaway, watercourse or mains drainage system.
Q. Why might a French drain be considered?
A. Inexpensive and easy to install, French drains can, in certain circumstances,
help control damp in old buildings by re-directing surface and subsoil water away
from foundations. Laid near or occasionally inside a building, they may, for example,
be an effective means of intercepting surface flow from uphill or dispersing liquid
moisture in a clay soil that is not free-draining. However, they are not the answer
in every case. Open drains can be a better solution if evaporation of moisture
from the bottom of walls is needed where ground levels have risen around a building.
In other situations, the use of “hi-tech” alternatives, such as drainage composites,
might be appropriate.
Always, therefore, accurately identify your moisture source before considering
any remedial action. With severe or extensive dampness problems, consult an independent
professional rather than a remedial treatment contractor with a vested commercial
interest encouraging over-specification. The SPAB may be able to advise on names
of independent professionals.
Q. What should I check before digging a French drain?
A. Do not overlook the archaeological implications of disturbing the vulnerable
area close to the walls of an old building. In many cases consent will be required
prior to starting excavation, together, of course, with any permission necessary
to discharge the water collected. Remember also to check in advance for the likely
existence of underground services in the area in which you propose to dig. In
order to ascertain the depth of a building’s foundations and the nature of the
ground, it is frequently prudent to undertake a small trial excavation.
Q. Can French drains cause structural damage?
A. Yes, if care is not taken. Old buildings tend to rest on shallow foundations
and there is a risk of undermining these. There is also a danger of the ground
drying out excessively and causing settlement in buildings. Wherever possible,
therefore, a French drain should be positioned a metre or so away from walls,
rather than right up against them. Alternatively, the drain might be placed to
align with the drip line beneath eaves that have no gutters. If a French drain
is installed close to a wall, the trench must not encroach within a 45 downward
angle from the bottom of the foundations, to maintain adequate lateral support
from the ground.
When backfilling a trench, each 150mm or so of aggregate should be compacted
before more is added. This is to minimise settlement of the reinstatement and
consequent movement of the ground on which the building is founded.
Q. How should I maintain a French drain?
A. French drains require regular maintenance, like all drainage, and should be
inspected annually for signs of obstruction. Neglected drainage may be worse than
no drainage at all. There is a serious risk of a French drain becoming blocked
and forming a moat around a building, thereby exacerbating the dampness problems
that it is intended to help address.
Further Reading
The Control of Damp in Old Buildings, SPAB Technical Pamphlet 8
Damp Buildings Old and New, by Ippolito and Giovanni Massari, ICCROM, 1993
