Water getting in through the roof: why it happens and what to do

Almost every roof repair I have seen fail, failed for the same reason: it was carried out where the stain was. The stain is the end of the water’s journey, not the beginning.

And there is a bias in the market that makes the problem worse. Whoever quotes for a roof usually sells waterproofing, so the solution proposed is waterproofing. In most of the cases I have seen, the waterproofing was not the problem.

The order in which a roof fails

1 · The upstand: where the water gets in behind

The most fragile point of any flat roof is not in the horizontal plane: it is where that plane ends. The waterproofing has to climb the wall or the parapet far enough, be mechanically fixed at the top and be finished so that water running down the wall cannot get in behind it.

The three ways of failing are always the same. The material climbs too little and the water goes over it once ponding starts. It climbs but is not fixed, and lifts in the sun. Or it is fixed and sealed with a product that ages before the rest, and nobody looks at it again.

2 · Fall: the defect you cannot see until it rains

A flat roof is not flat. It has to fall towards the outlets, and that fall is built with a screed laid to falls. It is a cheap item, one that gets trimmed when the budget is tight, and the trimming does not show on handover day.

There is also a phenomenon that surprises owners and not structural engineers: the slab deflects over time. A slab that was fine can develop a reverse fall years later, through creep in the concrete. The design fall was correct, and there is a puddle today all the same.

Why the puddle matters even where the membrane is sound: standing water finds any defect that running water does not, ultraviolet radiation and the wet-dry cycle degrade the material far faster in the ponded area, and the weight of the standing water loads the slab exactly where it was already deflecting.

3 · Outlets: the maintenance item nobody carries out

It is the easiest problem to solve and the most frequent. A roof outlet fills with leaves, with soil blown in and with grit from the finish breaking down. Nobody looks at it because it is up on the roof.

What turns a blockage into major damage is the absence of an overflow: an emergency outlet through the parapet, higher than the normal one and lower than the top edge. Without it, a blocked outlet turns the roof into a tank, and then two things happen at once. The water rises above the height of the upstand and gets in over the top of the waterproofing, which is the point no membrane protects. And the load of water on the slab grows with every centimetre.

4 · Joints: what moves has to be able to move

A movement joint — an expansion joint — exists because the building changes size with temperature. On the roof the problem is sharper, because it is the surface that swings most: at midday the material can be very much hotter than the air, and in the early hours very much colder.

The practical consequence is the one almost always ignored: a joint is not sealed, it is detailed. Filling it with a rigid material guarantees that it will break, almost always right beside the filling. It is the same logic that makes it useless to fill a live crack with rigid mortar, and it is explained in cracks in walls.

How to recognise it from below: water from a joint produces a stain that is long and straight, and it can cross several rooms following a line. A round stain is hardly ever a joint.

5 · Penetrations: the damage that arrives after the work

This is the one that makes you angriest, because the roof was fine. Somebody goes up to install a tank, an aerial, a solar water heater, a balustrade or the anchor of a safety line, and drills. Every fixing is a hole through the waterproofing.

The clue in time is unmistakable and should always be asked for: if the drip appeared in the months following some minor work on the terrace, do not look anywhere else. And if the building is still under warranty, a later penetration made by a third party is exactly the kind of fact that voids it.

And last of all, the membrane

When the material really is the problem, it is usually for one of three reasons, and only one of them has to do with its quality.

1. 1It reached the end of its service life. Every system has one and none is permanent. A membrane that cracks when bent, a finish that powders underfoot or a surface that has lost its protection against the sun are giving notice.
2. 2It was laid badly. On a dirty or damp surface, with no primer, or with insufficient laps. It is the commonest cause of a new system failing in its first year, and it is recognised because it fails at the edges and the laps, not in the middle of the bays.
3. 3It is the wrong system for that use. A roof that is walked on, a terrace with a planter or one carrying plant does not take the same treatment as a roof nobody ever goes up to.

Where the stain shows is not where the water gets in

It is the rule that prevents the most useless repairs. Between the point of entry and the point where it drips, the water runs across the top face of the slab, finds a crack or a hole left for services, and comes down wherever it can. That journey can be several metres long and is hardly ever vertical.

How the point of entry is narrowed down, in order of cost:

1. 1Ask when it appears. If it appears with any rain at all, it is direct entry. If only with long, windy rain, suspect the upstand or the façade. If it appears without rain, it is not the roof and you have to look for a leak or for condensation.
2. 2Look at the shape of the stain. Tight against the wall, upstand. Long and straight, joint. Round and isolated, membrane or penetration. Diffuse and spread out, ponding.
3. 3Go up and look after the rain, not before. The puddle that stays is the evidence. With no recent rain, the area is wetted with a hose sector by sector and you wait.
4. 4A flood test, sector by sector. The outlets are plugged, a bounded area is flooded and it is watched from below. It is slow, it is the one that really proves the point, and it has to be done with judgement: a slab loaded with water is a loaded slab.
5. 5Thermography, when the building is large or access is poor. It detects differences of temperature caused by trapped moisture. It does not see water: it sees the thermal effect of water, and it has to be read properly.

Why there is no standard to cite here

You will have noticed that so far I have cited no technical standard. It is not an oversight and I think it deserves an explanation, because it is useful information in itself.

I looked for which Colombian technical standard really applies to roof waterproofing. I could not read the title of a single one word for word. The catalogue of the body that issues them cannot be consulted publicly and directly, and the numbers circulating on the internet come from third-party lists that cannot be checked against the source.

What can be asserted, and is what governs in practice: what rules on a roof is the technical specification of the manufacturer of the system and the written warranty of whoever lays it. That is where the thickness, the primer, the surface preparation, the lap, the height of the upstand and the conditions for application are set out. And that is where you should press, because it is what can be read, compared and claimed against.

Three things worth asking for in writing before you sign anything: the technical data sheet for the complete system — not for the individual product —, the warranty with its term and its exclusions, and a photographic record of the surface preparation. The third is the one nobody asks for and the one that decides whether the system will last.

If the building is new: what the warranty covers and what it does not

Here there is a finding that changes the conversation with the builder, and it comes from reading the three texts in full instead of repeating what is said about them: *none of the three sources of warranty uses the word filtración.* Water ingress is not named in any of them.

Read that slowly, because the consequence is uncomfortable. A drip is not, on its own, the building perishing or threatening ruin. It may become that if the water corrodes the reinforcement of the slab over years, or if it compromises stability, but that has to be proved, not assumed. And if it is treated as a finish, the term is not ten years: it is one.

One detail of the Código Civil that is categorical and that hardly anybody invokes: the receipt the owner gives on completion of the works means only that they approve them as outwardly conforming to the plan and to the rules of the art, and it does not release the builder from that liability. Having signed the handover closes nothing.

How the Certificación Técnica de Ocupación works, from when the ten years run and which projects it reaches, is explained in cracks in walls. I do not repeat it here.

In propiedad horizontal: who pays for the drip

The roof is essential common property. Article 3 of Ley 675 de 2001 — the statute governing propiedad horizontal, the Colombian regime under which a building is split into privately owned units and property held in common — says so in those words: essential common property includes, among other things, "the façades and the roofs or slabs serving as covering at any level". Even though the damage shows inside your flat, the origin belongs to the community of owners.

But there is a second regime that is hardly ever applied properly, and it covers precisely where most of the drips are: the terrace.

Article 22 of the same statute creates common property for exclusive use, and expressly names "terraces, roofs, internal courtyards and setbacks" as assignable exclusively to the owner who by position can enjoy them. They remain common property. What changes is who uses them, and with that, who pays for what.

What the general meeting authorises, by what majorities, and what can be done without asking anybody, is in altering a flat in propiedad horizontal.

What not to do

• —Waterproofing over the top without a diagnosis. It is the most sold repair and the one that most often fails. If the problem was the fall, the outlet or the upstand, the new layer changes nothing and covers the evidence as well.
• —Repairing only where it drips. The point of entry is almost never above the stain.
• —Sealing a movement joint with a rigid material. It is going to break. The joint has to go on moving.
• —Applying on a damp surface. It is the number one cause of blisters, and the material is condemned from the first day.
• —Changing the system without drying the wet insulation. The trapped water is still there and still coming out.
• —Fixing anything to the roof without detailing the penetration. Every hole is a future entry point if it is not properly finished.
• —Accepting a verbal warranty. With no written term and no written exclusions, there is no warranty.

In short

Before asking for a waterproofing quotation, go up after a rain and look at three things: whether standing water remains, whether the outlets are clear and whether there is an overflow, and how the junction with the walls and parapets is finished. Those three answers explain most of the water ingress I have seen.

And look at the shape of the stain from below before going up. Tight against the wall is the upstand. Long and straight is a joint. With no rain it is not the roof, and then the problem is another one: that is what damp in walls: telling the origin apart before you repair deals with.