Hairline cracks after skimming: what's normal, what's the job, and what to tell the customer

The skim went on flat. A fortnight later the customer sends a photo: fine cracks along the ceiling, or where the wall meets it.

The easy answer is that the house is drying out. Before giving it, know what British Gypsum, which makes Thistle, says about its own plaster. Its manual says Thistle finish plasters "do not suffer from inherent shrinkage cracks". NHBC's note for homeowners is broader: mortar, plaster and concrete all start wet, and can shrink as they dry. So a crack in a new skim is worth reading before it's filled. It can come from what's behind the skim, from how it dried, or from the house moving.

Here's how to tell which, how to put each right, and what to say before the work starts.

Where the cracks come from

British Gypsum's data sheets and answers name the causes, and so does NHBC's guidance for homeowners.

Board joints. Some joints move more than others: around door and window openings, where board edges aren't supported, and on ceilings under floors that flex. For those, British Gypsum says to bed its paper joint tape in the plaster, for better resistance to cracking than its self-adhesive fibre tape gives. It allows the fibre tape on ordinary flat joints. Siniat goes further, and recommends no self-adhesive mesh tape under a skim at all.

A sand and cement undercoat. Cement undercoats shrink as they dry, and British Gypsum says they can crack days or even weeks after they go on. Skim before that shrinkage has finished and the finish is more likely to crack or come away, particularly over a poor key. Its advice is a much better key, and a much longer wait than a gypsum undercoat needs.

Drying too fast. Plaster needs some of its mixing water to set. In hot weather British Gypsum says to keep the windows shut, because warm air drying the face causes crazing, a mesh of fine surface cracks. If crazing starts, its advice is to stop and come back when it's cooler.

Cold. British Gypsum says Thistle plasters should only go onto backgrounds that will stay at 2°C or above until they're dry. DuraFinish needs 5°C, and none of them goes onto a frozen background.

Timber drying out. In a new build or an extension, the timber and other materials shrink as they dry. NHBC calls small cracks in walls a common sign of it. By NHBC's reckoning a new home takes around nine months to a year to dry out. Heat speeds the shrinkage up, which is why it tells owners not to turn the central heating up to its highest setting.

The seasons. Materials expand and shrink with the temperature and the damp through the year, each at its own rate. NHBC says these thermal movement cracks are common where an outside wall meets a plasterboard one, and where boards are joined on a plasterboard ceiling. They also turn up around a concrete or steel lintel over a window or door. NHBC adds that hairline cracks may appear on ceilings underneath the joists, and that thermal cracks can open and close through the year.

What counts as normal

The nearest published yardsticks are the tolerances the new-home warranty providers use.

LABC Warranty's technical manual says some cracking, up to 2mm wide, may occur at wall, floor and ceiling junctions, from shrinkage and materials moving differently. In the same list, board joints shouldn't be readily visible and jointing tape shouldn't show in the finished surface. The finish is judged from at least 2m away in natural daylight, with no artificial light.

NHBC tells homeowners that a crack of 2mm or less is cosmetic and won't affect the structure. It also says most homes crack at some point, however well they were designed or built.

Those are written for new homes, but I'd use them as the test for any skim. A fine crack at a junction in the first year is inside them. Joints you can see from across the room, or tape showing through, aren't.

Cracks that need a closer look

NHBC's note for homeowners lists the cracks to report rather than fill:

• wider than 5mm at any point
• wider at one end than the other
• horizontal or vertical, and constantly more than 2mm wide
• diagonal or stepped, at any width
• visible inside and outside
• noticeably wider or longer than when it was first spotted

None of those is a plastering fault to fill and forget. They can be the building moving, and that's for a surveyor or a structural engineer to look at, not for filler.

Telling them apart

• A fine crack where the walls meet the ceiling, in the first year. Shrinkage and materials moving differently as the house dries. Up to 2mm is inside the warranty tolerances.
• A hairline crack along joints in a plasterboard ceiling. Thermal movement, which NHBC calls common. If the joints show from across the room, look first at the tape, whether the board edges are supported, and whether the floor above flexes.
• A mesh of fine cracks across the surface. Crazing: the face dried too fast, from heat, sun or warm air while it set.
• A crack running from the corner of a door or window. Movement at the opening. British Gypsum wants paper tape there, and NHBC says to report a diagonal crack of any width.
• Cracks or patches coming away over sand and cement. Skimmed before the undercoat finished shrinking, or over a poor key.
• Wide, tapering, stepped or growing. The building, not the skim. Get it looked at.

Putting them right

For fine cracks, NHBC's advice is ordinary filler and a lick of paint during routine redecoration. It warns that a crack may reappear after a year or so from thermal movement, which isn't a worry unless it keeps getting wider. That's a reason to wait: fill in the first weeks of a new extension and you may fill the same crack twice.

British Gypsum sells EasiFill for holes, patches and cracks up to 6mm deep, movement cracks included, sanded to a fine finish before painting. For a junction crack that keeps coming back, its answer is cove, which it sells for covering imperfections and settlement cracks.

Where the cause is the job, filler only hides it for a while. A joint that cracked because of the tape, or a skim coming away from its undercoat, needs taking back and doing again.

Whose problem it is

Work for a homeowner comes under the Consumer Rights Act 2015, and it has to be done with reasonable care and skill. If it isn't, the customer can ask you to do it again. That has to happen within a reasonable time, without significant inconvenience to them, and at your cost for labour and materials. If that's impossible, or you don't do it in a reasonable time, they can ask for money off.

Two other parts of the Act matter here. What you tell the customer about the work counts as part of the deal if they took it into account. So "it won't crack" is a promise. And a term that tries to take away your duty of reasonable care and skill isn't binding on them. A line on the quote can set expectations, but it can't excuse a bad job.

When a builder is your customer rather than a homeowner, that part of the Act doesn't apply, and your contract with the builder does.

Claims don't last forever. In England and Wales a claim on a contract has to be brought within six years. In Scotland the period is five years. For a claim for damages, they run from when the customer knew or could reasonably have known about the loss and who caused it.

Say it before the job

Most of the argument can be had before there's anything to argue about.

Put a line in the quote's notes about fine cracks at junctions and joints in the first months, and what you'll do about them. Tell the customer to keep the heating steady rather than high while the plaster dries. Ask them not to paint until it's dry right through, beyond a coat of breathable paint. And photograph the background before you skim: the tape on the joints, the undercoat, the board fixings. If a crack turns up in March, a dated photo from November shows what was under it.

A version you can send the customer, in your own words if you'd rather:

Fine cracks can appear where the walls meet the ceiling, or along the joints in a plasterboard ceiling. They're common in the first months, while the new plaster and what's behind it dry out. Keep the heating steady rather than turning it up high. Don't paint until the plaster is dry right through, apart from a coat of breathable paint. Let me know if a crack is wider than 5mm, wider at one end than the other, diagonal from a door or window, or getting longer. I'll come and look.

What priming fresh plaster adds to a decorating price is in what it costs to paint a room.

Where grafter.ly fits

A quote in grafter.ly has a notes box, and whatever you write there is on the customer's copy, so the line about cracks goes out with the price. Photos taken on a job stay on it with the date they were taken, and they queue on the phone when there's no signal. If a customer rings months later, the job, the quote they accepted and the photos of what was behind the skim are in one place.

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Sources. British Gypsum, read from Internet Archive copies of its own documents: the White Book chapters on Thistle plaster finishes (captured 30 May 2023) and finishes (14 August 2024); the Thistle MultiFinish data sheet (26 June 2023); its answers on plastering in excessive heat and the minimum temperature (19 April 2024); and its repair range selector (15 May 2025). Siniat's guide to finishing internal walls (September 2024). LABC Warranty's Technical Manual, version 11, section 20. NHBC's Cracking in your home and Guide to your new home, both November 2025. The law: the Consumer Rights Act 2015, sections 48, 49, 50, 55, 56 and 57; the Limitation Act 1980, section 5; and the Prescription and Limitation (Scotland) Act 1973, section 6, section 11 and Schedule 1. All checked 19 October 2026. Contains public sector information licensed under the Open Government Licence v3.0.

This is general information about plaster and the law, not advice on a particular job or dispute.