# Where Commercial Facades Leak: Joints, Sills and Penetrations

> Commercial facades almost never leak through the glass or the panel. Water gets in at interfaces: stack and sill joints, head and sill flashings, the slab edge, penetrations, failed or badly installed sealant, cladding joints, displaced gaskets and blocked drainage. Finding a leak means testing those interfaces systematically, because the stain inside is rarely where the water entered.

## 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

| Location | Why it leaks | What to check |
| --- | --- | --- |
| Stack joints (horizontal joints between unitised panels) | Gasket displaced or not engaged, splice missing, joint not drained | Gasket continuity, splice sleeves at mullions, drainage path |
| Sill and head flashings | Flashing laps reversed, ends not turned up, no end dams | Laps shed water outward, end dams present and sealed |
| Slab edge | Gap between facade and floor not closed, perimeter seal failed | Perimeter seal, slab edge closure, whether the firestop is also doing a water job it was not designed for |
| Penetrations | Brackets, signage fixings, pipes or louvres through the line of seal | Each penetration individually sealed and flashed |
| Sealant joints | Poor adhesion, wrong joint size, no backer rod, sealant painted over | Adhesion, joint geometry, cracking or hardening |
| ACP and metal cladding joints | Panel joints face-sealed without a drained backing, routed edges open | Whether the system is face-sealed or drained, and if the backing wall is weathertight |
| Glazing gaskets | Shrinkage at corners, gaps at gasket joins | Corners and joins, gaskets pulled back from the glass |
| Weep holes and drainage | Blocked by debris, insects, paint or sealant | Weeps clear and draining to the outside |
| Facade-to-roof junction | Parapet capping, coping laps and the top of the system | Coping joints, how the top of the facade is closed |

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](/glossary/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](/glossary/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](/glossary/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](/services/facade-glazing/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.

## Frequently asked questions

### Why does my glass facade leak only in the monsoon?

Because ordinary rain runs down the face and drains away, while monsoon storms combine heavy water with wind pressure that pushes it into any opening. A joint that stays dry for most of the year can leak under wind-driven rain, and leaks often appear on the elevations facing the prevailing storm. That pattern points to a joint or drainage problem rather than a defective glass unit.

### The water is coming in at the floor. Is the sill the problem?

Possibly, but not necessarily. Water travels along mullions, transoms, brackets and the top of the slab before it shows up, so it often appears below or to one side of the actual entry point. The sill, the stack joint above it and the slab edge are all candidates. Testing each in turn, starting from the lowest, is how the real source is isolated.

### Can we simply seal over all the joints from outside?

Usually that makes things worse. Many curtain walls and rainscreens are designed to let some water into a drained cavity and take it back out through weep holes. Sealing over drainage paths traps that water inside the system, where it finds its way into the building. A face seal also only lasts as long as its adhesion, so it becomes a maintenance item in its own right.

### What is the difference between ASTM E1105 and a hose test?

ASTM E1105 is a field test that sprays water on the installed facade while a pressure difference is applied across it with a chamber, simulating wind-driven rain. 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. The test method and pressures for a project should be set by the facade consultant.

### How often should facade sealants be inspected?

Periodically, and after severe weather, by someone who can reach the joints safely. The interval depends on the exposure, the sealant and the building, and the sealant manufacturer and facade consultant should set it. Inspection looks for loss of adhesion, splitting, hardening and joints that have been painted or patched over. Replacement follows the joint design and the manufacturer's instructions, not a cosmetic top-up.

### Is a leaking facade a design fault or a workmanship fault?

It can be either, or both. A joint detail with no drainage path is a design problem; a sealant applied to a dusty substrate without primer is a workmanship problem; a weep hole blocked by debris is a maintenance problem. Diagnosis has to establish which, because the remedy and the responsibility are different. A facade consultant's investigation and testing is the reliable way to decide.

## Sources

- [ASTM E331](https://www.astm.org/products-services/standards-and-publications.html) — Water penetration of exterior windows, skylights, doors and curtain walls by uniform static air pressure difference
- [AAMA 501.1](https://fgiaonline.org/) — Water penetration of windows, curtain walls and doors using dynamic pressure
- [ASTM E1105](https://www.astm.org/products-services/standards-and-publications.html) — Field determination of water penetration of installed exterior windows, skylights, doors and curtain walls
- [AAMA 501.2](https://fgiaonline.org/) — Quality assurance and diagnostic water leakage field check of installed storefronts, curtain walls and sloped glazing
- [ASTM C1193](https://www.astm.org/products-services/standards-and-publications.html) — Standard guide for use of joint sealants
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings and structures

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Source: https://hagerstone.com/blog/where-facades-leak-joints-sills-penetrations