What is wrong with your building? A guide to diagnosing common defects

Damp and moisture

Damp is the most frequently misdiagnosed defect in buildings. There are three main mechanisms plus one hidden variant, they look similar, and they need completely different remedies.

Interstitial condensation is the hidden variant: condensation forming inside the construction rather than on its face. The usual cause is insulating a solid wall internally without addressing ventilation, which leaves the masonry colder and moves the dew point into the wall build-up. It is invisible until decay is advanced, so if damp appeared after energy efficiency works, say so early.

The full treatment, including what to try before spending anything, is in the damp guide.

Cracks and structural movement

Most cracks in most buildings are not structural. Plaster shrinks, materials expand and contract with temperature and moisture, and new buildings settle. The question is never "is there a crack" but is it progressive, and what shape is it.

Reading a crack

• Width. Crack severity is conventionally classified on a scale from hairline through to very wide, with each band implying a different level of concern and repair. [Exact millimetre thresholds to be confirmed against BRE Digest 251 before publication.]
• Direction. Diagonal cracks that step through mortar joints suggest differential movement of the structure. Vertical cracks at corners and above openings are frequently thermal or shrinkage.
• Taper. A crack noticeably wider at the top suggests one part of the building is dropping relative to another. Wider at the bottom can indicate the opposite, ground swelling upward.
• Company it keeps. Sticking doors and windows, sloping floors, and cracks that continue through both internal and external faces raise the level of concern considerably.
• Season. Cracks that open in summer and close in winter are often clay shrinkage or thermal movement rather than permanent failure.

Subsidence and heave

Subsidence is downward movement caused by the ground losing its ability to support the foundation: clay soil shrinking in dry weather, tree roots drawing moisture, or a leaking drain washing out fines. Heave is the opposite, ground swelling and pushing upward, classically after a large tree is removed and the clay it was drying out rehydrates.

Get advice promptly if cracks exceed a few millimetres, appear suddenly, run through both faces of a wall, or come with sticking doors.

Roofs, gutters and water ingress

Water getting in from above or outside is one of the most common and most fixable categories, and often one of the cheapest. It is also the most frequent hidden cause of problems diagnosed as something else, because water travels before it appears.

• Gutters and downpipes. Blocked, leaking or overflowing. Look for vertical staining down the wall and check them during heavy rain rather than on a dry day
• Flashings and abutments, where the roof meets a chimney, parapet or adjoining wall. A very common failure point
• Slipped, cracked or missing tiles and slates, and displaced ridge or hip tiles
• Flat roofs. Ponding water, splits and blisters in the covering, failed upstands, blocked outlets
• Pointing and render. Cracked render is worse than none: it admits water and then holds it against the masonry. Tap it. Hollow means it has lost adhesion
• Sealant around windows and penetrations, including extract vents and service pipes
• Vegetation. Plants growing from a wall or gutter mean there is a permanent water and soil supply, and roots open joints

A stain on a top floor ceiling, in the corner of a room, or on a chimney breast is a roof or flashing problem until proven otherwise. Water tracks along timbers and can emerge a considerable distance from where it entered.

Timber decay

Timber decay always has a moisture source. Fungal decay needs timber to stay above a threshold moisture content sustained over time [figure to be confirmed against BRE guidance], which dry timber in a ventilated building does not reach. This matters because treating the timber without removing the water is guaranteed to fail.

• Wet rot is the common one. It stays confined to the wet area, softens and darkens the timber, and stops when the timber dries. Fix the leak, replace what has gone, and it does not come back
• Dry rot is more serious. It can transport moisture and spread through masonry into timber that was previously dry, so it is not confined to the original wet spot. Signs include cotton-wool-like growth, rust-coloured spore dust, and a distinctive mushroom smell in enclosed spaces like cellars and under floors
• Woodworm shows as small round flight holes with fine dust. The key question is whether it is active or historic, and a lot of treatment is sold for infestations that died out decades ago

Bouncy or sloping floors, and soft skirting at floor level, are worth investigating early. They are usually cheap to fix while the timber is only locally affected and expensive once a structural member has gone.

Get advice promptly if you find a mushroom smell, cotton-wool growth, or timber you can push a screwdriver into.

Concrete and reinforcement

Reinforced concrete relies on the concrete's alkalinity to keep the embedded steel from corroding. Two processes destroy that protection.

• Carbonation. Atmospheric carbon dioxide reacts with the concrete over decades and reduces its alkalinity, removing the chemical protection around the steel
• Chloride attack. Salt penetrates the concrete and attacks the steel directly. This is the dominant mechanism in coastal buildings, and airborne marine salt reaches structures well back from the shore

Once the steel corrodes it expands, and because rust occupies more volume than the steel it replaces, it breaks the surrounding concrete away. That is spalling: concrete flaking or dropping off, usually with rust-coloured staining, cracking that runs in straight lines following the reinforcement beneath, and sometimes exposed bars.

Cold, draughts and heat loss

Cold surfaces are not only a comfort problem. They are the precondition for condensation, so thermal defects and damp defects are usually the same defect described two ways.

• Thermal bridges are paths of higher heat loss through the fabric: lintels, window reveals, the wall to floor slab junction, structural elements that pass from inside to outside. They create locally cold surfaces where condensation and mould concentrate
• Uninsulated or partially insulated elements, including cavities that were never filled or where the fill has slumped
• Air leakage, which is frequently a larger component of heat loss than people expect and is invisible without testing

Two measurements settle most of this. Thermal imaging locates cold bridges and missing insulation precisely rather than by inference. An air tightness test measures leakage as a number instead of a guess, which also converts the weakest input in any condensation assessment into a measured one.

A single room that never gets warm, in an otherwise adequately heated building, is worth investigating rather than living with. It is usually a specific and fixable fabric problem, and it is also the room where mould will appear first.

Ventilation and indoor air quality

Ventilation is the mechanism by which a building removes the moisture and pollutants generated inside it. When it is inadequate, the symptoms show up as damp, mould and stuffiness, so it is usually diagnosed as something else.

• Background ventilation: trickle vents, airbricks, passive vents. Frequently painted shut, blocked, or removed during window replacement
• Extract ventilation in kitchens and bathrooms. Test it: hold a sheet of tissue paper to the grille. If it does not hold, the fan is not moving air, whatever noise it is making
• Purge ventilation: opening windows. Effective and free, and the first thing people stop doing in winter

Sealed-up chimneys are a specific and common trap. Capping a flue without providing a vent creates an unventilated cavity that collects moisture, and the damp then appears on the chimney breast and gets diagnosed as penetrating damp.

How urgent is it?

This is general prioritisation, not advice on your specific building. If something is falling or a structure is visibly moving, treat it as urgent regardless of what any guide says.

Common misdiagnoses

1. An injected damp proof course specified for condensation. The most frequent error in the whole field. A chemical DPC addresses capillary rise and has no effect on moisture condensing out of room air.
2. A crack filled while the movement is still active. Cosmetic repair before establishing whether movement has stopped. The crack returns, and the repair has destroyed the evidence of how fast it was moving.
3. Timber treatment without fixing the water. Fungal decay needs sustained moisture. Remove the source and it stops. Treat the timber and leave the leak, and it continues.
4. Tanking a wall with penetrating damp. This conceals the ingress rather than stopping it. Water keeps entering the masonry and reappears above the tanking or next door.
5. Spalling concrete patched cosmetically. Filling the hole without treating the corroding reinforcement traps the problem. The steel keeps expanding and the patch fails.
6. Insulation added without ventilation. Improves the energy rating and creates a moisture problem. The most common cause of new damp in recently improved buildings.

What you can check yourself, at no cost

1. Walk the outside of the building during heavy rain. Most ingress is visible while it is happening and invisible an hour later
2. Photograph and date any crack with a ruler or coin for scale, and repeat monthly. Whether it moves matters more than how wide it is
3. Test every extract fan with a sheet of tissue paper. Noise is not airflow
4. Check external ground levels against the damp proof course. A new patio or raised bed piled above it bridges it completely
5. Clear gutters and downpipes, and watch them in rain
6. Open trickle vents, unblock airbricks, and move furniture off external walls
7. Stop drying laundry indoors, or confine it to one room with the window open and the door shut
8. Heat low and continuous rather than in short bursts, and do not leave individual rooms unheated with the door closed
9. Write down what changed before the problem started, and when. Building work, a removed tree, a new patio, a capped chimney, replaced windows

Keep a record of what you changed and when. If the problem improves, that is diagnostic in itself and useful evidence later.

When you need a survey

• Anything in the "act now" row above
• You have tried the free measures for a month and the problem persists
• You are buying, and a survey or valuation has flagged something
• You have been quoted for remedial work and want the diagnosis verified before committing
• The problem appeared after building work and responsibility is in question
• You need evidence that will hold in a dispute, claim or negotiation

A survey adds what a guide and photographs cannot: moisture readings at depth as well as at the surface, thermal imaging, air tightness measurement, opening up where needed, and a professional opinion that carries liability.