Where Commercial Facades Leak: Joints, Sills and Penetrations

Facade water leakage is an interface problem

When a commercial facade leaks, the first suspect is usually the glass, or the rain itself: "the storm was unusual." Both are almost always wrong. A glass unit or a cladding panel is rarely the path water takes. Water gets in where two things meet: glass and frame, frame and frame, facade and slab, facade and roof, facade and anything passing through it.

For a building owner the cost is rarely the repair itself. It is the damaged ceilings, carpets and equipment inside; the tenant complaint; the fact that the leak shows up in the monsoon, when access to the outside of the building is hardest; and the weeks spent resealing the wrong joints because the stain was assumed to be the source.

Facade water leakage is solved by knowing where to look and testing in the right order.

Where facades actually leak

The pattern on site is consistent: the leak is at a junction that was detailed late, built by a different trade, or sealed after the fact.

Face-sealed or drained: know which facade you have

How a facade is meant to keep water out decides what a repair should do.

A face-sealed facade relies on a single outer line of sealant and gaskets. If that line is perfect, it is dry. It has no second line of defence, so every gap in the seal is a leak, and the seal must be maintained for the life of the building.

A drained or pressure-equalised facade accepts that some water will pass the outer joint, collects it in a cavity or channel, and drains it back out through weep holes. By letting air into the cavity it can also remove the wind pressure that pushes water inward; the principle is explained under pressure equalisation.

The practical consequence is that the same repair can be right on one and harmful on the other. Sealing over open joints or weep holes on a drained system traps water inside and creates the leak it was meant to cure. Before any remedial sealing, someone should confirm from the shop drawings or by opening up a joint how the system is designed to work.

Why the stain is not the source

Water inside a facade travels. It runs along a transom, drops down a mullion, tracks along a bracket and appears on the slab several metres from where it entered. A stain on a ceiling tile tells you where water ended up, not where it came in.

Diagnosis therefore follows a sequence:

1. Record the pattern. Which elevation, which storms, which floors, after how much rain. Leaks only in wind-driven rain point to joints and pressure; leaks in any rain point to a gross gap or a drainage failure.
2. Inspect from outside. Sealant condition, gaskets, flashings, weeps and anything fixed through the facade since handover.
3. Test from the bottom up. Wet the lowest suspect interface first, then work upward, so water from a higher test does not mislead a lower one.
4. Open up where needed. A few interior or exterior covers removed in the right place save weeks of guessing.

Testing: in the laboratory and on the building

Water performance is tested at two stages, and they answer different questions.

Before production, on a mock-up: static water penetration to ASTM E331 and dynamic water penetration to AAMA 501.1, which uses a wind generator to simulate wind-driven rain. This proves the design. Its role in the programme is covered under facade mock-up testing.

On the installed facade: ASTM E1105 applies water with a pressure difference across the installed facade using a chamber, uniform or cyclic, and is the field test most often specified for acceptance. AAMA 501.2 is a hose test without applied pressure, used as a quality check during installation and as a diagnostic tool for finding where water enters.

Test pressures and acceptance criteria derive from the design wind pressure for the specific building under IS 875 (Part 3). They are set by the facade consultant and should not be taken from another project.

Sealants are a maintenance item

Sealant joints are designed, not just filled. Joint width, depth, backer rod, primer and the movement the joint must take all decide whether the sealant holds. ASTM C1193 is the guide most often used for the use of joint sealants, and the difference between a weatherseal and structural silicone is explained under silicone weatherseal.

Even a correct joint ages under sun, heat and movement. A building owner should expect periodic inspection and, eventually, replacement. Where many joints are failing at once, or the original system was never weathertight, the decision may move from repair to facade retrofit and recladding.

Leaks that start after handover

A facade that passed its tests can still start leaking years later, and the cause is often something added to it rather than something wrong with it. Signage brackets, CCTV cameras, light fittings, antenna mounts, AC pipe sleeves and blind fixings are drilled through mullions, cladding or flashings by trades who never saw the facade drawings. Each fixing is a new penetration with no drainage or sealing detail.

Interior work can cause the same problem from the other side. A fit-out contractor who fixes a partition head into a mullion, packs insulation into a drained cavity, or seals over a sill drainage slot can turn a working facade into a leaking one.

The control is administrative, not technical: a rule that nothing is fixed to or through the facade without the facade contractor's or consultant's detail, and a record of what has been added since handover. When a new leak appears, that record is the first thing to read.

Common mistakes

• Resealing the joint nearest the stain. The water often entered somewhere else.
• Sealing weep holes and drainage slots because they look like gaps.
• Applying new sealant over old without removing it and preparing the surface; it does not bond.
• Testing from the top down, so water from one test contaminates the next.
• Fixing signage, cameras or brackets through the facade after handover, with no flashing or sealing detail.
• Accepting a facade without a field water test on the elevations most exposed to storms.
• Assuming the slab edge firestop is a water seal. It has a different job.

What to ask

• Is the facade face-sealed, drained or pressure-equalised, and do we have the shop drawings that show it?
• Was a mock-up tested to ASTM E331 or AAMA 501.1, and what were the results?
• Was field testing to ASTM E1105 or AAMA 501.2 carried out, where and when?
• Who is responsible for sealant inspection, and at what interval?
• What has been fixed through the facade since handover?
• Are the weep holes clear, and when were they last checked?

Standards referenced

Laboratory water penetration testing under ASTM E331 (static) and AAMA 501.1 (dynamic); field testing under ASTM E1105 and the field check in AAMA 501.2; use of joint sealants under ASTM C1193; and design wind pressures under IS 875 (Part 3). Test pressures, acceptance criteria, joint design and the remedy for a particular facade must be established by the project's facade consultant.