Cracks on a house:
serious or not?

Faced with a crack, four observations are enough for a first sort. None requires professional tools.

1. How wide?

The building trade uses simple benchmarks. Under 0.2 mm, it is a hairline crack, or crazing when the cracks cross in all directions: a barely visible line, where a fingernail does not go in. Between 0.2 and 2 mm, it is a crack: on a facade, it already lets rain through and deserves to be monitored. Beyond 2 mm, it is a wide crack (lézarde): it needs an opinion on site. To measure, a graduated ruler is enough; a crack gauge, a small transparent card printed with lines of increasing widths, is more precise.

2. What shape, what direction?

Spider-web cracking on a render: this is surface shrinkage. A straight line along a plasterboard joint, or at the junction of two materials: two substrates that do not move together, with no consequence for solidity. By contrast, a stepped crack that follows the joints of block or brick masonry, a diagonal crack starting from the corner of a window or door, or a horizontal crack under the roof or at floor level reveal a stress in the wall, most often a support that is moving.

3. Does it go through the wall?

Look for the same crack on the other side: inside if you spotted it on the facade, and vice versa. If it can be found on both faces, it goes right through: it is the masonry that is cut, not just its covering. On a load-bearing wall, that alone is a reason to ask for an opinion.

4. Is it still moving?

This is the question that decides what comes next. A stabilised crack, known as passive, can be repaired. An active crack, which opens, lengthens or varies with the seasons, will reopen under any render for as long as its cause is at work. Only monitoring over several months can settle it.

The accompanying signs matter as much as the crack: doors and windows that stick, tiles that split, a partition coming away from the ceiling, a garage, terrace or front step pulling away from the house. The Ministry of Ecological Transition lists them among the typical damage from clay soil movements.

This table gives guidance; it does not replace a visit. On a single house, two causes often combine.

Before filling anything, set up a way to know whether the crack is moving. Three methods, from the simplest to the most precise.

Dated photos. A close-up photo, ruler placed against the crack, taken every month from the same spot and in the same light. Draw a small pencil line at each end, with the date: if the crack goes past the line, it is lengthening.

The plaster tell-tale. A plaster patty a few centimetres across, placed across the crack and dated. If it splits, the wall is moving. It is simple and telling, but it only answers yes or no.

The crack gauge. Two graduated plates stuck on either side of the crack, which slide over each other when the wall moves: you read the opening to a tenth of a millimetre and note it at each reading.

Monitor the crack for several months, ideally a full cycle of seasons: a crack that opens in summer and closes in winter points to a soil that dries then rehydrates. Also note what happens around it: drought, heavy storms, work at the neighbour's, a leak, a tree felled or planted. This logbook will serve the engineering consultancy, the expert and the insurer.

Shrinkage of materials

A render, a concrete, a mortar lose water as they dry and shrink. If drying is too fast — full sun, wind, a substrate that is too dry — or the render too heavy in cement, the surface crazes. This is the origin of crazing and of many fine cracks on recent buildings: annoying, rarely serious.

Expansion

Materials lengthen in heat and shorten in cold. A facade facing due south or due west, a long terrace slab, a wall mixing concrete and brick move differently from one season to the next. Cracks form at the junctions: under a roof slab, between a wall and a post, at the corner of an attached extension.

Differential settlement

Every house sinks a little into the ground after it is built. The problem comes when it does not sink in the same way everywhere: uneven ground, poorly compacted fill under one part, an extension laid on foundations different from those of the house, a leaking buried pipe washing out the soil. The part that goes down pulls on the other, and the cracks appear stepped or diagonal, often at the weak points that are the corners of openings.

Drought and clay soils

A clay soil swells when it is saturated with water and shrinks when it dries. These slow movements, repeated from one summer to the next, mainly damage individual houses, whose foundations are shallower than those of apartment buildings. According to the Ministry of Ecological Transition, medium and high exposure zones cover 55% of mainland France, and this phenomenon accounts for 42% of the damage compensated under natural disaster cover; a claim costs on average €16,500 on an individual house. Vegetation makes the phenomenon worse: roots pump water from the soil right under the foundations. To find out your house's exposure, the Géorisques map (georisques.gouv.fr) allows a search by address.

The roof frame pushing

A traditional roof frame holds thanks to its tie beams, the horizontal members that link the feet of the trusses and stop the roof spreading the walls. When a tie beam has been cut to convert the loft, or when a timber has rotted at its foot, the roof pushes the walls outwards: horizontal cracks under the roof, vertical cracks at the corners of the top level, a wall that loses its plumb. This case is dealt with first at the roof frame; our guide to roof refurbishment describes the condition of the timbers to check.

Neighbouring works

Excavation on the boundary, a party demolition, compaction or the passage of machinery can decompress the soil or make fragile masonry vibrate. If a site starts next to your home, photograph your walls before it begins; you can also have their condition recorded by a bailiff (commissaire de justice). If cracks appear, write to the neighbour and to your insurer, with photos as evidence.

Poorly drained water

A gutter that discharges at the foot of a wall, a broken downpipe, a buried leak soften the soil under part of the foundation and prepare the way for settlement. In the other direction, a crack open on the facade lets rain in: that is then a leak, which we deal with in our article on water infiltration in a wall.

A stone house more than a century old almost always has cracks. Many are old and stabilised: the building found its balance long ago, and its thick walls built in lime absorb small movements without damage. What should raise concern is change: a crack that appears, opens or lengthens.

Three situations recur in old houses in Savoie. A cement render applied to stone, too rigid, which splits and holds water in the wall: the masonry deteriorates behind it, a case detailed in our article on the old crumbling wall. An opening made or widened in a load-bearing wall without a sufficient lintel: diagonal cracks above the opening, see our guide on opening up a load-bearing wall. A converted loft from which tie beams have been removed: the roof frame pushes, as described above.

On these houses, the repair stays compatible with the wall: lime mortar and render, never cement. Our guide to renovating an old house places these repairs in the order of works.

Filling straight away. A filler applied to a moving crack splits again, and it wipes out in passing the trace that would have allowed it to be measured. Monitor first.

Repainting or re-rendering over it. Redoing the facade of a wall that is still moving means paying for scaffolding twice.

Filling the crack of an old wall with cement. Harder than stone and lime mortar, cement creates a rigid point: the wall cracks right next to it.

Letting insurance deadlines slip. After a natural disaster order, you have 30 days to declare; for a recent house, the 10-year structural guarantee (garantie décennale) expires ten years after handover.

Signing for a heavy solution without a written cause. Injections, micropiles, underpinning: none of them is chosen without a diagnosis.

A fine crack that does not move is dealt with by a mason or a painter. You need to go further when the crack exceeds 2 mm or widens, when it goes through a load-bearing wall, when several diagonal or stepped cracks appear at the same time, when doors and windows distort, when a slab or tiling splits, when an extension pulls away from the house, or when the cracks follow dry summers on clay ground.

Two specialists can then step in. A structural engineering consultancy checks walls, floors and foundations, and sizes the repair: staples, tie rods, reinforcement. A geotechnical engineering consultancy carries out the ground survey (boreholes, nature of the ground, depth of the existing foundations); for a house already affected, this is called a geotechnical diagnosis, the G5 mission. The Ministry of Ecological Transition cites these two consultancies, together with the insurance expert, among the professionals qualified to diagnose a house exposed to clay.

We do not carry out these studies. After a visit, we tell you whether they are necessary, and our work follows their conclusions. If a claim is declared, the insurer's expert assessment is added: keep your monitoring logbook.

One rule applies to each solution below: only a stabilised crack, or a crack whose cause has been removed, is repaired durably. Prices taken from our 2024-2026 quotes in Savoie and Haute-Savoie, excluding VAT.

Interior hairline cracks: fill and repaint

The crack is opened with a scraper so the filler grips, dusted, filled, sanded, then a tape or glass fabric is applied if it tends to return, before painting. On our quotes, painting a wall in good condition, filling, sanding and primer included, comes to €25 to €37 excl. VAT/m²; our guide to the price of painting per m² details the lines, and the one on redoing walls and ceilings the heavier cases.

On ceilings, cracks often have their own causes: a poorly reinforced plasterboard joint, a floor that sags above, old plaster coming away from its substrate. Our article on cracks in a ceiling is devoted to them.

Facade cracks: filling and facade renovation

A fine render crack is dealt with at the time of painting or facade renovation: it is opened up, filled with a mortar suited to the substrate, reinforced with a mesh if needed, then covered over. General crazing requires a new render. On our quotes: facade painting with cleaning, €80 excl. VAT/m² (one 2025 project); redone facade render, €85 to €100 excl. VAT/m² (three projects); lime render on an old wall, €140 excl. VAT/m² (two 2025 projects); scaffolding, €3,000 to €7,900 excl. VAT as a fixed sum for a house. The detail is in our guide to the price of facade renovation, and the method in the one on facade renovation.

Stabilised through crack: stitching

When a crack cuts the wall but no longer moves, the masonry is stitched back together: grooves cut across the crack receive steel staples or bars sealed with mortar, then the crack is refilled in depth, with lime mortar on an old wall. Our recent quotes contain no standalone stitching line: the price depends on the number of staples, the thickness of the wall and access, and is calculated after a visit.

Walls moving apart: roof frame, tie rods, ring beam

If the roof is pushing or two walls are separating at the corner, the force must first be taken up: restore or replace the tie beam, fit metal tie rods linking opposite walls, or create a ring beam (chaînage). Sizing is the job of the structural engineering consultancy; the price depends on the roof frame and is calculated after a visit.

Foundation that has moved: underpinning

When the ground no longer carries part of the house, the support is taken deeper. Classic underpinning is done in sections: one digs under the existing foundation in short lengths, alternating, and pours concrete down to good ground, never exposing the whole wall at once. On our quotes, underpinning comes to €1,000 excl. VAT per linear metre: this is the line of a 2025 quote, to consolidate about sixty centimetres of a barn's foundation against an adjoining extension, i.e. €1,100 incl. VAT with 10% VAT. A single project: take this figure as an order of magnitude. Our guide to the price of foundations places it among the other items.

Other techniques exist when good ground is deep: micropiles drilled down to a bearing layer, expanding resin injections under the foundations. They are the work of specialist firms, do not appear in our quotes, and are decided only after a ground survey.

Removing the cause around the house

On a clay soil, part of the solution lies around the house. The Ministry of Ecological Transition classes prevention work in three families: water management, vegetation management and waterproofing of the ground around the house. It specifies the timing: drains, rainwater downpipes and leak repairs at the end of summer, at the end of the dry period; root barriers, at least 2 metres deep, at the end of the wet period, before spring. It also flags water traps at the foot of facades, down to the condensate from a heat pump. For roof water, see our guide to the price of a zinc gutter.

House under ten years old: the 10-year structural guarantee (garantie décennale)

For a new house, an extension or structural work accepted less than ten years ago, the builder's 10-year structural guarantee (garantie décennale) covers defects that threaten the soundness of the building or make it unfit for its purpose, even when they stem from a defect in the ground. The period starts the day after the acceptance report is signed, as service-public.gouv.fr points out. If you have taken out construction damage insurance (assurance dommages-ouvrage), report the claim to it: it funds the repairs covered by the 10-year guarantee. Also write to the builder, with photos and measurements to back up your letter; if the builder no longer exists, contact the insurer named on its 10-year insurance certificate. A fine crack that threatens neither soundness nor use is not covered by this guarantee.

Drought: the natural disaster scheme

Cracks caused by ground movement following a drought are covered by your home insurance under the natural disaster scheme, provided that an inter-ministerial order recognises this state of natural disaster for your commune and is published in the Journal officiel. The commune submits the request to the prefect, generally within 24 months of the event: report your damage at the town hall. Once the order is published, you have 30 days to report the claim to your insurer. It pays an advance within two months, then the full compensation within three months. The excess is €1,520 for this type of damage, against €380 for a home affected by another natural disaster (service-public.gouv.fr). Since the ordinance of 8 February 2023, compensation focuses on damage likely to affect the soundness of the building or hinder its normal use.

The Clay Prevention Fund, launched on a trial basis to help owners protect their homes, does not cover the two Savoie départements: the list of selected départements, consulted on 5 November 2026, includes eleven, among them Allier and Puy-de-Dôme for the Auvergne-Rhône-Alpes region.

Other cases

Outside the 10-year guarantee and the natural disaster scheme, everything depends on your home insurance contract and on the cause: water leak, damage caused by a third party, neighbouring works. Ask your insurer in writing whether the claim is covered, enclosing the identified cause and your dated photos. For neighbouring works, the condition report taken before the site started will make the difference.

At Atelier NovAlp, a cracked house starts with a visit from the architect and one of our masons. They record each crack — width, shape, faces affected —, examine the roof structure, water drainage, floors and surroundings, fit crack monitors if the crack needs to be tracked, then write down the probable cause. If a ground survey or a structural engineering firm is needed, we say so, and we wait for its conclusions before pricing the repair.

The work is carried out by our salaried craftsmen, each in their own trade: our masons for stitching, crack repairs, underpinning, drains and renders; our carpenters when the roof is pushing the walls; our roofers for gutters and downpipes; our plasterboard fitters and our painters for interior finishes. To find out who does what, see our teams.

1. The cause, in writing, and what backs it up: measurements, crack monitors, ground or structural survey where relevant.

2. The state of the crack: stabilised or active, and for how long it has been monitored.

3. The method, item by item, in quantities: metres of crack treated, number of staples, linear metres of underpinning, square metres of render.

4. Materials compatible with the wall: lime on an old wall, reinforcing mesh in a façade render.

5. Separate items: scaffolding, propping, rubble removal, finishes.

6. VAT: generally 10% for work in a home completed more than two years ago, unless the work completely renews most of the foundations. Our guide to VAT on renovation work sets out the rules.

The visit and quote are free: request a visit.