How much water a roof delivers
The scale of this is routinely underestimated. A modest semi-detached house with a roof plan of around 60 square metres will shed roughly 600 litres of water in 10mm of rainfall, which is an unremarkable British afternoon. All of that is meant to be collected by the gutters, carried down two or three downpipes and taken away underground. If a gutter joint is split, a length has dropped below its fall, an outlet is blocked with moss and leaves, or a downpipe has lost its shoe, a substantial part of that volume lands on or beside the wall instead. Repeated week after week, it saturates masonry far beyond anything the wall was designed to cope with.
What it looks like inside
Internally, the signs follow the water. Damp patches appear on the wall behind the defect, often at first-floor or ceiling level where the gutter is, and sometimes lower down where water has run behind the render or down the inside face of a cavity. Decoration blisters and lifts, plaster darkens after storms and dries in settled weather, and skirtings or floors near the affected wall may be wet. Because the wall stays damp for long periods, mould appears on the internal surface and any timber in contact with the masonry, such as a joist end or a bearer, is at risk of wet rot. The tell-tale characteristic is the timing: it worsens during and after rain.
What it looks like outside
Look at the building during a downpour, which is the single most informative inspection available and costs nothing. Water spilling over the front edge of a gutter indicates a blockage or a dropped length; water running down from a joint indicates a failed seal; a dark vertical stripe on masonry below a joint shows where it has been happening for years. Other signs are visible in dry weather: green algae or moss growth on a wall below a gutter, staining and salt deposits on brickwork, spalled or eroded brick faces and eroded mortar, a hollow or damp-sounding render patch, and a downpipe discharging onto the ground beside the wall instead of into a gully.
Secondary damage that costs more than the repair
The gutter repair itself is usually inexpensive. What follows if it is left is not. Persistently saturated masonry loses much of its insulating value, so the wall runs colder and condensation and mould appear internally as well. Frost acting on wet brickwork spalls the faces and erodes pointing. Embedded timber, especially joist ends and wall plates bearing into a wet solid wall, decays by wet rot and, in unventilated conditions, dry rot becomes possible. Render debonds. In severe cases water tracking down beside a wall softens the ground under a shallow foundation. Almost all of that is avoided by maintaining rainwater goods.
Checking your own building safely
Most of the useful checks can be made from the ground with a pair of binoculars or a phone camera on zoom. Look along each gutter line for sagging, gaps at joints, plant growth and debris. Check that each downpipe is complete and that its base discharges into a gully rather than onto the ground or against the wall. Pour a bucket of water into a gully and watch whether it clears. Look for staining, moss and eroded pointing on the wall below each joint. Do not go onto a roof or up a ladder without the right equipment and someone present; roof-level work on a two-storey house is a job for somebody equipped for it.
What a proper repair involves
Clearing the gutters is only part of it. The line needs to be checked for fall towards the outlets, joints resealed or replaced, brackets refixed where the gutter has dropped, and any corroded or split sections renewed. Downpipes need to be complete and discharging into a working drain, with gullies cleared and, where necessary, a soakaway investigated. Where masonry has been saturated for a long period, it needs time to dry before any internal replastering, and the decision on whether contaminated plaster needs removing should follow measurement rather than assumption. If timber has been affected, the extent of decay needs establishing before finishes are reinstated.
When to get an inspection
If internal damp persists after the rainwater goods have been repaired, if the affected wall has been wet for a long period, if timber is involved, or if you are unsure whether the damp is now historic contamination rather than active water, an inspection settles it. We take readings to map the extent and depth of the moisture, assess the timber, and advise on drying and on when reinstatement can safely begin. We would much rather tell you that a £150 gutter repair has already solved the problem than sell you something larger.
Maintenance intervals worth keeping to
Rainwater goods are the cheapest insurance a building has, and they only need a routine. Clear gutters, hoppers and outlets at least once a year, and twice where there are trees nearby, ideally in late autumn once the leaves are down. Flush each downpipe and check that gullies take the water away. Look along the gutter line in a downpour every winter. Check that shoes at the base of downpipes are present and directed into the drain. Repaint or inspect cast-iron goods on their usual cycle and replace perished seals on plastic systems rather than patching them. Keep a note of when this was last done, because on a property that changes hands every decade the gutters are frequently the one element nobody has ever formally maintained.
Limitations of general guidance
This article is general guidance. It cannot confirm the cause of dampness in a specific property. Recommendations should follow an inspection of the building, its external environment and its internal conditions.
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