Keeping Water Out of Old Buildings: What the Past Can Teach Us
Submitted by Andy_B on Thursday, 01 October 2026 Page Views: 2
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Water is the oldest enemy of any building. Long before damp proof membranes and modern sealants, builders in Britain were fighting rain, groundwater and frost with the materials they had to hand. Many of the churches, castles, barns and houses they built are still standing centuries later, and that is no accident.Today the same buildings are being repaired, extended and put to new uses, and the question of how to keep water out is as live as ever. Looking at how the past handled it can help us make better choices now.
How medieval builders dealt with rain
The first line of defence was the roof. Steep pitches on thatched and tiled roofs threw rain off quickly before it had a chance to soak in. On churches and grand buildings, sheets of lead covered lower roofs and gutters, and lead has protected some of those roofs for a very long time with regular care.Next came the overhang. Generous eaves kept rain off the tops of walls. Hood moulds above windows and doors, sometimes called drip moulds, caught water running down the face of a wall and steered it to the sides. Gargoyles and water spouts on churches and cathedrals were not just decoration. They threw rainwater well clear of the walls so it would not soak the foundations.
Walls were built of stone or brick laid in lime mortar and often finished with lime render or limewash. These materials let a little water in, but they also let it out again. The mortar was softer than the stone, so moisture and salts moved through the joints rather than the stone itself. When the mortar wore away, it could be repointed, and the stone survived. Timber was protected with tar or pitch where it met the ground or the weather.
Why breathable fabric still matters
The key idea behind all of this is that old buildings manage water rather than trying to block it completely. They shed most of it, and they dry out the rest. That balance can be upset very easily.Hard cement render on a soft stone wall is a classic example. Water gets in through small cracks but cannot get out through the cement, so it builds up behind it. In winter, the trapped water freezes and pushes the face of the stone off. Waterproof coatings on old brick can cause the same problem. Timber beams built into damp walls can rot at their ends, out of sight, for years before anyone notices.
This is why conservation specialists are wary of any product that promises to seal an old wall for good. In most cases, the right answer above ground is to repair gutters and downpipes, lower soil levels against walls, repoint in lime and let the building dry.
Where modern waterproofing earns its place
That said, there are places in historic buildings where managing water in the old way is not enough, and where modern waterproofing solutions have a real part to play. Basements, cellars and crypts are the most common. When these spaces are converted into museum stores, cafés, meeting rooms or homes, they need to be dry enough for people, furniture and collections. Flat roofs on extensions, roof terraces and link buildings between old and new parts also have to keep water out completely.For these spaces, owners and architects look for systems that stop water reaching the inside while still respecting the old fabric around them. The British Standard for below ground waterproofing, BS 8102, describes three broad approaches. One is a barrier applied to the structure, often called tanking. Another relies on the structure itself being watertight. The third is drained protection, where a membrane fixed to the inside of the wall lets any water that gets through run down to a drainage channel and away.
In historic basements, drained protection is often preferred. It works with the water that will always find its way into an old cellar rather than trying to hold it back under pressure, and it can be fixed with little damage to the old walls. If the space is ever returned to its earlier use, the system can usually be taken out again.
What conservation architects look for
Anyone working on a listed building will hear the same few principles again and again. They owe a lot to the Society for the Protection of Ancient Buildings, founded by William Morris in 1877, which argued for careful repair over heavy restoration.The first principle is to understand the problem before treating it. Many damp cellars are wet because of a broken drain or a blocked gutter above, not because the walls have failed. The second is minimal intervention. Do as much as is needed and no more. The third is compatibility. New materials should work with old ones, not against them. The fourth is reversibility. Where possible, a new layer should be removable without harming what lies beneath it.
A good waterproofing specialist working on a heritage project will be happy to follow these principles. Listed building consent will usually be needed for work that affects the character of the building, so it pays to involve the conservation officer early.
Maintenance deserves a mention too. Even the best modern system depends on the building around it. Gutters still need clearing, drainage channels and pumps in a basement need checking, and flat roofs need an eye kept on them after storms. Old buildings have always rewarded regular, modest care, and that has not changed.
Old wisdom, new tools
The builders of our churches, castles and barns never had modern membranes, but they understood that water always finds a way. They shed it, steered it and let walls dry out, and their buildings have lasted as a result.The best modern work follows the same thinking. Above ground, it restores the old ways of shedding water. Below ground and on new flat roofs, it uses modern systems carefully, in a way that respects the fabric around them. Put together, old wisdom and new tools can keep a historic building dry for the next few centuries too.
With many thanks to AngusKirk for the Creative Commons photo at top.
