When should you be worried about wall cracks in French property?

The growing number of people with cracks in their properties from clay subsidence or ground heave recently sparked debate in the French media of how to tell if they are dangerous.

Anyone who has looked at old stone houses in France will have seen examples of cracked walls that have stood for hundreds of years and are still solid.

At the other end of the scale there have been horrendous collapses of old apartment blocks in cities such as Marseille and Toulouse, where residents said they had complained about cracks appearing without anything being done.

And after earthquakes, surveyors condemn some buildings because of seemingly innocuous looking cracks, arguing that the structural integrity of the building is damaged.

Read more: Hundreds evacuated after building collapses in centre of Toulouse

Different wall cracks 

The general consensus in France is that cracks are dangerous if they are wider than two millimetres and go all the way through a wall.

They have their own descriptive names: microfissure for a small crack in plaster or rendering which can be easily filled in; fissure horizontale for cracks running horizontally along a wall, sometimes at the level of a floor; fissure en moustache for cracks which run from the corner of a door or window; fissure en escalier, for cracks which zig-zag up, following joints between building blocks, dressed stone or bricks; and fissure transversante for any of the above where the crack goes all the way through a wall.

Moving cracks (those that are getting bigger) are more of a worry than stable ones. Measure its height and the width and keep notes.

Some people photograph the cracks regularly – a ruler gives you a reliable scale for comparison.

The general advice is to always get a professional opinion on cracks that worry you. 

In practice, however, French bureaucracy and complicated professional rules mean that builders, surveyors, engineers and architects are seldom willing to commit to a written report outside specific cases. 

Building surveyors as a separate, independent profession do not exist in France in the same way as in the UK, for example.

Meanwhile builders are often too busy to look at a house which may or may not result in work.

In addition, younger builders, trained in France to build with concrete blocks, may not have enough experience with old stone houses to give good advice, nor want to get involved in renovation work.

Read more: Shrink-swell heat damage probably a factor in fatal building collapse

Legal cover

Some household insurance companies have legal cover which extends to getting opinions on structural matters – it might be an idea to explore this avenue.

However, problems traced to a lack of maintenance, such as allowing water to seep into walls, are not covered by most policies.

Similarly, cracks caused by drought-related subsidence or ground heave on clay soils are only covered by insurance if a natural catastrophe has been declared in your commune, and that is not always straightforward.

Broad sweep analysis suggests serious horizontal cracks are due to too much weight on the structure; cracks running from doors or windows are caused if they are not properly installed, with undersized lintels, for example, or built with badly cut stone; and zig-zag cracks are due to poor foundations or building techniques.

Read more: New tool helps homeowners in France assess drought damage risk

DIY guide to repairing cracks

Readers with old undressed stone houses looking to tackle cracks themselves are generally advised to start from the top and move down.

You will have to open up the sides of the crack about 50cm from the top with a hammer and stone chisel, enough to put in a big “bridging” or tie-in stone, which you mortar in.

Build up to the top from there with smaller stones, avoiding piling them up in a straight line to reduce the chance of a new crack opening up.

Next, go down another 50cm or so and repeat the process.

Some builders working on old stone properties make a big show of “injecting” a slurry of building lime and water mixed in a big fizzy drink bottle from the top of cracks – others dismiss this as merely “cinema”.

The usefulness of the technique probably depends on the thickness of the wall and the binding material it was first built with. If it is the mix of mud, straw or horsehair often used, the liquid lime could presumably return it to a more binding state.