Why buildings in France suffer from subsidence and cracking

Why buildings in France suffer from subsidence and cracking

Buildings in France suffer from the same problems as buildings in the UK. However there are several factors that make matters worse in France.

• The incidence of swelling and shrinking clays in France is far more widespread in France and covers a much larger area of built up land

• Building standards and their enforcement has only fairly recently been brought up to UK levels i.e. very modern French buildings will be built to a very high standard but buildings of 20 years old and more may not be

• There are a lot more old barns and derelict dwellings in very attractive rural areas that can probably be renovated whereas permission for new build in these areas is likely to be refused

• Old stone houses and barns built in lime mortar can accept massive deformation before significant cracks occur - there are many walls around that have had all the cracks lovingly restored but are on the point of collapse

There are several different reasons why buildings crack and show other signs of structural distress such as settlement of floors and sagging roof-lines.

These include:

• Swelling and shrinking clays

• Trees

• Underground water flow

• Leaking drains and surface water run-off

• Thermal expansion and contraction and differential thermal movement between different materials

• Inadequate design and installation of the roof structures

Clay sub-soils

Cracking in walls in France is most often caused by swelling and shrinking clays. Historically the French have not worried too much about the odd crack but the recent droughts and the increase in weather variability has changed all this. There is no doubt that the number of affected dwellings in France is at least as high as in the UK where a large industry has developed in the repairs for property owners who have claimed successfully for insurance money to carry out the work. Will your french house insurance pay for any remedial works? - the answer to this is maybe - click on subsidence insurance in France to get more information.

Trees

Trees are only problematic when swelling and shrinking clays soils are present. Oak trees seem to cause the most damage but other species such as poplars can also be a problem. There are well established rules that relate tree heights to the safe distance from a property - but these only apply when susceptible clays soil is present.

Underground water flow

Underground water flow is normally only a problem when a property has been built near a steep slope or an old quarry or on filled ground where the original land contours promote steep underground gradients in the ground water table.

Leaking drains

Leaking drains are very common in France and over the course of several years can significantly soften the soil beneath property foundations. This particularly applies to roof drains as they are often in very bad condition with leaking / broken downpipes that direct water straight down onto the adjacent wall foundations. Many rural properties and barns are built on and indeed into hillsides without any drainage to direct surface water run-off away from the building itself. In such circumstances it will invariably be better to provide land drains even if all the evidence and not least the sworn evidence by the vendor that the barn has always been as dry as a bone! Do not believe it! - if a barn is built into a slope without an adequate land drainage system it will be very damp from time to time.

Thermal expansion and contraction

Thermal expansion and contraction cracking occurs when new cement based materials (such as concrete block rendered walls) are built without the necessary expansion and contraction joints. This is rarely a problem in the rural properties in France that are rarely large enough to require such joints. But differential thermal movement can cause the ends of roof trusses and other types of beam to slip off their supporting wall - this is a common cause of floor / roof failure in old French buildings.

Old roof structures

The delightful old roof structures in centuries old oak that we all love to see were installed by carpenters who designed and installed them as they had been done for centuries before by their ancestors. Most of these roofs are robust and very strong but they are suceptible to several types of defect - these include:

• Bottom booms (the main horizontal beam of the roof truss) of inadequate depth / thickness

• Bottom booms made from a poor quality piece of oak that has split between the knots or that was not very straight in the first place

• Excessive distortion of the bottom boom caused by shrinkage

• Excessive joint movement in the roof truss because of broken / rotten dowels

• Wet rot caused by roof leaks

The main affect of these defects is that the trusses can move on the walls that support them, push the walls out and even slip off the supporting wall.In most old roofs you will be able to see some evidence of repair - many of these are very makeshift and are inadequate if total property renovation is planned. We have also seen many instances where the main boom of a truss has been cut to increase head heights for doorways and the roof is held up by internal partitioning. Removal or change to the partition layout in these circumstances can lead to total collapse.

Where is subsidence and cracking likely to occur in France

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