Can I sell a house with shale or red ash under the floors?

Yes. If testing puts the sulphate level in the lowest class and the hardcore is dry, many lenders accept the property and an ordinary sale is possible. Higher classes, damp hardcore or visible floor damage push mortgage buyers away — then the realistic routes are replacing the floors first, or selling to a cash buyer who prices the floors in from the start.

What is sulphate attack from shale hardcore?

Between the 1940s and about 1972, industrial waste — red ash or shale, black ash, slag — was commonly used as the fill under solid concrete ground floors, especially in mining and industrial areas. If moisture reaches it, sulphates in the fill react with the cement in the slab and the reaction products expand, pushing the floor upwards. The classic sign is a floor that domes up in the middle.

Will a mortgage lender accept a house with shale floors?

It depends on a laboratory test of the fill. The lowest sulphate class is generally acceptable to surveyors and lenders; the middle class is the borderline case, usually needing dry hardcore, no damage and a membrane; the higher classes are generally not acceptable, especially with no membrane under the slab. Some lenders ask for remedial work before they will lend at all.

Shale · red ash · sulphate floors

The floor is rising because of what's under it. The house can still be sold.

A doming floor, doors that won't shut, and a surveyor saying "shale" or "red ash" — it sounds like the end of the sale, and to a mortgage buyer it often is. But this problem has a test, a class, a fix and a price. Here's all four, honestly.

1940s–1972when shale, red ash and other industrial waste was commonly laid under solid concrete floors — nobody kept records of how many homes (Kiwa · AMS Surveys)

4 classesthe laboratory test result lenders read: the lowest is generally acceptable, the middle is the battleground, the higher two generally aren't (AMS Surveys)

£5,000+excavation is the only real fix — digging out the floors and the fill beneath them — and it isn't usually covered by insurance (Crookshank · Kiwa)

What's happening under the slab

Tap each number. The whole problem lives in a layer you've never seen, laid before you were born.

1The layer nobody recorded

From the 1940s to about 1972, the fill under solid concrete ground floors was often whatever industry had spare: red ash and shale, black ash, slag — especially in mining and industrial areas. It wasn't identified as a problem material until the 1970s, the Building Regulations only restricted it from 1967, and nobody kept records of which houses got it. Yours wasn't badly built; it was built normally, for its time and place.

2The slow chemistry

The trouble needs one ingredient: moisture. When damp reaches the fill, sulphates dissolve and react with the cement in the slab above, forming expansive compounds — and the floor is slowly jacked upwards from below. It can take decades to show, because the early expansion just fills the gaps in the material first. That's why a house can sit quietly for fifty years and then start moving.

3What you see upstairs

The classic sign is a floor that domes up in the middle. Then doors start catching, internal walls and doorways distort as the slab pushes against them, skirtings lift, and cracking can appear in brickwork below the damp-proof course — the expanding slab can push outwards as well as up. If any of that sounds like your hallway, the next step is a test, not a panic.

4The test that decides everything

A specialist takes samples of the fill from under the slab, and a laboratory grades the sulphate level into four classes. The lowest class is generally acceptable to surveyors and lenders. The middle class is the battleground — usually needing dry hardcore, no visible damage and a membrane to pass. The higher classes are generally not acceptable, especially with no membrane, and some lenders want remedial work before lending at all. The report comes with a budget cost for replacing the floors — which is the number every route below turns on.

Sources: Kiwa CMT, sulphate attack in UK homes · AMS Surveys, sulphate attack on concrete floors · Crookshank Surveyors, red ash and sulphate attacks. General information, not structural advice — the test, not a webpage, answers what's under your floor. Last reviewed August 2026.

Your three honest routes

All three start with the same move: get the test. It costs a fraction of what guessing costs.

Test — and maybe sell normally

If the laboratory puts your fill in the lowest class and it's dry, many lenders accept the house as it stands — you may not need us or anyone like us, and we'll say so. Even a middle-class result with no damage and dry hardcore can pass with the right lender. Test before you decide anything.

The gamble is the result: a high class or damp fill turns a routine sale into the situation this page is about — and once the report exists, it exists for every buyer who asks.

Dig out, replace, sell at full value

Excavation is the only real fix: the affected floors and the fill beneath them come out, a modern membrane and slab go in. Done and documented, the house sells as an ordinary house — the problem is genuinely gone, not managed.

Upwards of £5,000 and rising with every affected room, not usually covered by insurance, and it's major works: the ground floor of your home becomes a building site while you're living in it or paying to live elsewhere.

Sell to us, floors as they are

We buy without a lender, so there's no class threshold to pass and no retention to fund. Tested or untested, doming or quiet — tell us what you know and the offer is made knowing it, so our survey can't shrink it. You pick the completion date; the dig-out becomes our problem, not yours.

The price is below market value — 65–82% depending on the date you choose, judged against what the house is realistically worth with its floors, and shown before you give us anything.

The certain route: no lender to fail the class test, a figure made after the bad news, and a date in writing with £500 a day on us if we miss it.

Why a buyer's lender walks — and why we don't

A mortgage valuer who sees a doming slab in a 1950s house in a mining area writes three words — "sulphate attack suspected" — and from that moment your buyer needs a specialist test, a lender willing to read it kindly, and thousands in hand if it reads badly. Most quietly disappear instead. We price on the floor you actually have: the test result if you've got one, the honest symptoms if you haven't, and the budget cost of replacement either way. A known problem with a known fix is exactly the kind of thing a cash buyer can price — it's uncertainty that kills sales, not shale.

And the usual promise holds here too: disclose everything — the doming, the old test, the neighbour who had their floors out — and it's locked by the price promise. Disclosed facts can't be used to move the figure later.

The questions shale-floor owners ask

The floors have been fine for fifty years. Why is everyone worried now?

Because the damage takes decades to show — the early expansion fills the gaps in the fill before it starts lifting the slab — and because surveyors in the affected regions now test for it routinely on houses of the right age. Fifty quiet years genuinely is evidence in your favour: dry fill that's never moved often tests in the acceptable classes. But it's the test that proves it, not the years.

How do I even know if my house has shale under it?

Age and place are the clues: solid concrete ground floors laid between the 1940s and about 1972, in a mining or industrial area — the practice was common everywhere industry had waste to spare, from the North East coalfields to Stoke and the North West. No records were kept, so the only certainty is a sample through the slab. If you're selling to us, you don't need to test first — tell us the age and the area and we'll take it from there.

My buyer's survey mentioned a "retention" for the floors. What now?

The lender is holding back money until the floors are tested or fixed — which means your buyer must fund the gap, and most can't or won't. You can pay for the test yourself to try to save the sale (a low class might), replace the floors, or take the certain route and sell to a buyer who doesn't have a lender. What rarely works is waiting for the buyer to solve it.

Is it dangerous to live with?

It's a slow, mechanical problem, not a sudden one — floors lift over years, not overnight. But the expansion can push walls as well as floors, and cracking below the damp-proof course deserves a professional look. If anything about the movement feels rapid or the cracking is widening, get a surveyor's opinion for your own peace of mind, whatever you decide about selling.

Get a figure made after the bad news

There's no obligation and nothing to sign. We'll give you a figure in writing, with our reasoning, and you can take it to anyone you like before you decide.

Or call us: 0191 249 3969 · 8am–8pm, seven days