Diagonal cracks above windows in Toulon: foundation settlement?

You’re looking at the lintel above your living room window, and that stair-step crack runs from the top corner up toward the ceiling. In Toulon, this kind of damage often shows up after a wet winter or a dry summer. Honestly, a diagonal crack crossing the corner of a bay window is not a paint crack. DTU 20.1 (small-element masonry) requires horizontal and vertical ties precisely to absorb these forces; when they’re missing or the ground moves, the crack appears at an angle, around 45°, following the diagonal of tensile stress.

Telling an active crack from a stabilised one

Concretely, before panicking, measure. Set a tell-tale: a thin plaster strip or a simple pencil line with the date, across the crack. If the opening moves more than 0.2 mm over a few months, it’s active. The FD P 18-201 documentation sheet distinguishes micro cracks (under 0.2 mm), fine cracks (0.2 to 2 mm) and open cracks (beyond that). Above 2 mm with movement, it’s no longer simple crazing: you look for a structural cause.

A diagonal crack above a window that slopes down toward the opposite corner points to differential movement. One part of the foundation drops, the other doesn’t. The masonry, being rigid, can’t follow: it breaks diagonally, where tension is highest.

Differential settlement: the first lead to check

In Toulon, the subsoil is rarely uniform. You find old fill, marl, fractured limestone, and sometimes former stream beds that were filled in. Two neighbouring footings can rest on soils with very different bearing capacity. Standard NF P 94-500 governs geotechnical missions: it defines when a G5 study (diagnosis) becomes necessary on an existing cracked building.

Signs pointing that way:

• The crack is wider at the top than at the bottom, or vice versa, with a steady variation.
• A door or window has been hard to close for a few months.
• Tiles lift or split along the line of the damage.
• Outside ground slopes toward the house, concentrating water at the base of the foundation.

Don’t confuse this with simple clay shrink-swell, also common, but which tends to produce cracks on facades, often vertical or Y-shaped. The BRGM publishes shrink-swell exposure maps available by municipality; Toulon and its outskirts appear for some areas, which doesn’t rule out settlement from a different cause.

What to do, in order

First, document. Dated photos, tell-tales in place, opening measurements with a crack gauge. A clean photo report is worth more than a long speech when the time comes to talk to an insurer or a neighbour.

Then, get an outside eye. An independent building expert, with nothing to sell on the works side, will tell you whether the damage is structural or cosmetic. That’s the only way to avoid a €15,000 repair quote slapped on a render that’s just cracking.

Then, if doubt remains, a G5 geotechnical study. It costs, but it settles things: load-bearing soil or not, foundation depth, presence of water. Without it, any foundation repair is a gamble.

Repairing: tackle the cause, not just the render

Filling a diagonal crack without treating the soil means watching it reappear within one or two winters. The classic solutions: underpinning with micropiles or expansive resin injection, depending on soil type and access. The choice depends on the study, not on a contractor’s preference.

On an old house in Toulon, in the steep neighbourhoods, access is often the first obstacle. A drilling truck doesn’t go everywhere. That’s a criterion to factor in from the start, not after signing.

Remedies and guarantees

If the damage comes from a neighbour (earthworks, pool leak, gullying), Article 1244 of the French Civil Code (formerly 1386) makes them liable for damage caused to your building. If you sell, the contractor’s ten-year decennial guarantee runs ten years after acceptance of the works, and the two-year biennial guarantee covers equipment elements. Also check your home insurance: some cover differential subsidence, with strict conditions. Finally, in a dispute, a joint amicable expert assessment is often the required step before any legal action.

Frequently asked questions

Is a diagonal crack above a window always serious?

No. A fine, stable crack with no movement can stay cosmetic for years. What matters is how it evolves over time and how wide it opens. Set a tell-tale and monitor over several months.

How do I know if my crack is still moving?

Set a tell-tale (plaster or a dated mark) and measure the opening with a crack gauge every two to three months. A variation above 0.2 mm indicates an active crack. Also note whether doors close less easily.

Do I need a geotechnical study before repairing?

If the damage is structural and progressing, yes. Standard NF P 94-500 defines the G5 mission suited to existing buildings. Without it, you’re repairing blind and the risk of recurrence stays high.

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📅 Mis à jour le 23/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).