Exterior Waterproofing near you

Approved Document C's approach to an external wall is to ask what it has to resist and then what it is made of. Any solid wall meets the requirement if it will hold moisture arising from rain and snow until it can be released in a dry period without penetrating to the inside of the building or causing damage to it, with the necessary thickness depending on the type of brick and block and on the severity of wind-driven rain, measured by the method in BS 8104:1992. The document includes a map of UK zones for exposure to driving rain and a table of maximum recommended exposure zones for insulated masonry walls.

Its technical solution is specific. A solid external wall in conditions of very severe exposure should be protected by external impervious cladding. In conditions of severe exposure it may instead be built with brickwork or stonework at least 328 millimetres thick, dense aggregate concrete blockwork at least 250 millimetres thick, or lightweight aggregate or aerated autoclaved concrete blockwork at least 215 millimetres thick, with the exposed face rendered or given no less protection: rendering in two coats totalling at least 20 millimetres with a scraped or textured finish, mortar strength compatible with the strength of the bricks or blocks, and joints raked out to a depth of at least 10 millimetres where the wall is to be rendered.

The document also asks for the things that actually keep water out at the edges: protection where the top of a wall would otherwise be unprotected, with a damp-proof course as well unless the protection and its joints form a complete barrier; and damp-proof courses, cavity trays and closers provided and designed so that water drains outwards where downward flow is interrupted by an obstruction such as some types of lintel, under openings unless a sill and its joints form a complete barrier, and at abutments between walls and roofs. In driving rain exposure zone 4, it asks for checked rebates in all window and door reveals, with the frame set back behind the outer leaf which overlaps it, or an insulated finned cavity closer.

Almost every wall that needs waterproofing has a defect rather than a deficiency, and finding it is the job. The usual list is short: a rainwater fault above soaking one patch of wall repeatedly; a failed or bridged damp-proof course; ground, paving, decking or a new border raised above the damp-proof course; a missing cavity tray or a blocked cavity above an opening; mortar joints open on an exposed elevation; a cracked or blown render letting water behind an otherwise impermeable finish; a cill without a fall or a drip; or a hard cement render or repointing on an old solid wall that has stopped it drying outwards.

The pattern of the damp tells you which. Water that appears only after prolonged rain from one direction and dries back afterwards is penetrating damp on an exposed elevation. A vertical stripe internally, always in the same place, points at a rainwater fault or an open joint outside. Damp confined to the bottom of the wall, worse in winter, is usually a ground level or damp-proof course question. Damp that is general, worse in cold weather, and worst in corners and behind furniture is condensation and is not a waterproofing problem at all.

None of this is answered by a moisture meter reading at the skirting board and a quotation. Ask any firm proposing a treatment what it thinks the mechanism is, what evidence it has for that, and what it expects to happen to the moisture that is already in the wall. A proposal that cannot answer the third question is proposing to seal water in.

A solid masonry wall handles rain by absorbing some of it and releasing it again when the weather dries. That only works if the surface is more permeable than the water pressure driving into it, and the release route exists. Apply a dense, impermeable coating — a hard cement render, a masonry paint that forms a continuous film, a sealer — and the wall is no longer able to dry outwards. Anything that gets in behind it, through a crack, at the head of the wall, from a leaking gutter, or from within the building, is now trapped.

Two things then happen. The moisture moves inwards instead, showing as damp on internal finishes that were dry before the treatment. And the moisture that remains in the masonry carries dissolved salts to the back of the coating, where crystallisation pushes the coating off in sheets and takes the face of the brick or stone with it in frost. Approved Document C's line about coatings not being the weather-resisting part of a system is the polite version of this; the impolite version is a wall that was damp in one corner and is now damp across a whole elevation.

The narrow case where a breathable water repellent is defensible is a wall of sound masonry on a very exposed elevation where the pointing and details are already in good order, and where the product is genuinely vapour-open rather than film-forming. Even then it is a maintenance measure with a life, not a permanent solution, and it should be trialled on a sample area first. It is never a substitute for repointing, for a cavity tray, or for fixing a gutter.

A pre-twentieth-century solid wall was designed to get wet and dry again, and every impermeable treatment applied to it works against that design. On such a building the measures that improve weather resistance are the traditional ones: sound rainwater goods, open and properly pointed joints in an appropriate mortar, protection at the head of the wall, and correct ground levels. Historic England's repointing guidance, covered in detail elsewhere on this site, sets out why a mortar less permeable than the masonry concentrates moisture movement in the bricks or stones instead.

Where the building is listed or in a conservation area, the choice is also a consent question rather than a technical preference, because rendering, cladding or coating an elevation changes its appearance. That is a matter for the local planning authority and, on a listed building, for listed building consent, both of which other guides on this site explain. A contractor offering to waterproof a period elevation without mentioning either has not looked at the building.

The devolved position matters here too. Approved Document C applies in England; Wales publishes its own approved documents, Scotland works to building standards technical handbooks and a building warrant from the council's building standards department, and Northern Ireland to its own technical booklets, with Booklet C covering site preparation and resistance to contaminants and moisture. Exposure to driving rain is also not evenly distributed across the UK, so the right specification in Sussex is not the right specification on an Atlantic coast.

Above-ground waterproofing is priced on diagnosis, area and access, and the honest quotations usually separate the three. What moves the figure is how much investigation is needed before anything is specified, how many elevations are affected and how large they are, whether the work is repointing, render repair, a cavity tray, ground level work or a coating, the exposure of the site and therefore the specification required, the height and how the elevation is reached, and whether heritage constraints or consent requirements shape the answer. Removing a failed impermeable coating from an older wall is its own significant item and is frequently the largest one.

LokalMatch's part is the introduction. Describe the wall and the damp once, with your postcode — where it shows, when it shows, what the wall is built of and what has already been applied to it — and firms covering your area will contact you directly. It costs homeowners nothing to post a request; the businesses pay for the requests they receive. LokalMatch does not carry out the work, does not set prices and does not vet, rank or recommend anyone, so insisting on a diagnosis before a product, and comparing what each firm says the cause is, is the part of the job that stays with you.