A building can remain completely dry during light rain and then suddenly develop a leak during a heavy storm. This does not necessarily mean that a new defect has appeared overnight.
In many cases, stronger wind and heavier rainfall expose weaknesses in the external building envelope that may not be affected during normal weather. Rainwater can be driven against façades, window joints, parapets and other external details from directions they are not usually exposed to.
This is commonly referred to as wind-driven rain. Understanding when and how a leak appears can provide useful clues when investigating water ingress in commercial, residential and high-rise buildings.
Where the source of a recurring external leak is difficult to identify, see our rope access leak detection and waterproofing service in London .
What Is Wind-Driven Rain?
During calm conditions, rainfall generally falls predominantly downward and is managed by roofs, gutters, flashings and other drainage details designed to direct water away from the building.
Strong wind changes the way rain interacts with a property. Water can be driven against vertical and inclined surfaces including external walls, windows, cladding systems, balconies and parapets.
Instead of affecting mainly the roof and horizontal surfaces, rainfall may reach façade joints and building details that receive much less water during ordinary conditions.
This can be particularly noticeable on tall or exposed buildings where upper elevations may experience greater wind exposure.
If the building envelope already contains a defect, wind-driven rain can expose that weakness and create a route for water ingress.
Why Does a Building Leak Only During Heavy Rain?
Some leaks only appear when several conditions occur at the same time.
A small gap around a window frame, façade joint or flashing may remain dry during light rainfall because relatively little water reaches that specific area.
During strong wind and prolonged rain, however, the same detail may be exposed to a much greater volume of water.
This can reveal existing defects that do not produce visible symptoms during normal weather conditions.
Common Entry Points for Wind-Driven Rain
Potential water entry points can include:
- Failed window sealants
- Cracked masonry
- Deteriorated pointing
- Parapet walls and coping details
- Cladding joints and interfaces
- Damaged flashings
- Movement joints
- Roof-to-wall junctions
- Balcony waterproofing details
- Blocked drainage systems
This is why a leak that occurs during a storm should not automatically be assumed to originate from the main roof covering.
Failed Window Sealants
External window sealants form a weather-resistant joint between the window system and the surrounding façade.
Over time, sealants can crack, become brittle, detach from the substrate or lose their ability to accommodate movement.
Even a relatively small gap may allow rainwater to pass behind the external joint when that elevation is exposed to prolonged wind-driven rain.
Once water enters behind the seal, it may travel around the window frame or through adjacent construction before becoming visible internally.
If damp patches or leaks regularly appear around windows after storms, deteriorated perimeter sealing is one of the areas worth investigating.
Our guide to window sealant failure explains the common warning signs and repair considerations.
Cracked Brickwork and Deteriorated Mortar
Brick and masonry façades are exposed continuously to rain, temperature changes and movement.
Cracked masonry, damaged pointing and open mortar joints can create potential routes for moisture into the external wall construction.
During prolonged or wind-driven rainfall, the façade may remain wet for longer and defects can receive greater direct exposure to rainwater.
Problems may occur around:
- Mortar joints
- Movement cracks
- Window openings
- Damaged bricks
- Previous repairs
- Interfaces between different materials
Read our guide to masonry defects and repairs for more information.
Parapet Walls and Coping Details
Parapet walls are highly exposed building elements because they sit above the main façade and are subjected directly to wind, rain and temperature changes.
Potential defects can include:
- Cracked masonry
- Deteriorated mortar joints
- Damaged coping stones
- Failed joints between coping units
- Defective flashings
- Failed waterproofing details
Where moisture enters at parapet level, it can travel downward through the construction before becoming visible internally.
This means a damp patch can sometimes appear considerably lower than the original entry point.
See our guide to parapet wall damage for common defects and repair considerations.
Roof-Level Details and Flashings
Not every roof-related leak originates from the main roof surface. Problems can occur around joints and interfaces where different building elements meet.
Areas worth inspecting can include:
- Flashings
- Upstands
- Roof penetrations
- Membrane joints
- Parapet interfaces
- Roof edges
- Roof-to-wall junctions
During severe weather, water may reach deteriorated or poorly sealed details that appear dry during lighter rainfall.
Our commercial roof inspection guide covers common roof-level defects and warning signs.
Blocked Gutters and Rainwater Systems
Water ingress during heavy rainfall can also be associated with drainage problems.
If gutters, outlets or downpipes are restricted, water may overflow or collect around roof edges and façade details.
During a storm this can expose areas to more water than they would normally receive.
Possible consequences include:
- Overflowing gutters
- Water running down façades
- Standing water near roof edges
- Increased exposure around flashings
- Moisture reaching façade joints or cladding interfaces
See our guide to blocked gutters on commercial buildings for more information.
Balcony Waterproofing and Drainage
Balconies contain several interfaces where water may collect or enter the building envelope.
Potential defects can include:
- Damaged waterproofing
- Failed sealant joints
- Cracking
- Blocked drainage outlets
- Deteriorated coatings
- Failed balcony edge details
During prolonged rainfall, water may remain on the balcony surface for longer and increase exposure around existing defects.
For additional information, see our guide to balcony repairs and common high-rise balcony defects .
Cladding Joints and Façade Interfaces
Modern cladding systems contain multiple joints, seals, panels and interfaces that form part of the external building envelope.
Potential water entry points can include:
- Panel joints
- Failed seals
- Damaged panels
- Window interfaces
- Movement joints
- Façade penetrations
- Flashings
Some defects can be difficult to identify from ground level. A close-up cladding inspection may help identify visible deterioration and possible water entry points.
Why the Visible Leak May Be Far Away From the Actual Defect
One of the most important principles in water-ingress investigation is that the place where water becomes visible is not necessarily the place where it entered the building.
Once inside the building envelope, moisture can travel:
- Behind cladding
- Along structural elements
- Through insulation
- Around window frames
- Along membranes
- Through cavities
- Across ceilings
- Down internal construction elements
For example, water may enter through a failed façade joint above a window and only become visible around the window opening.
Similarly, moisture entering through a parapet defect may travel downward before producing staining on an internal ceiling or wall.
This is why simply sealing the area nearest to the visible damp patch does not always resolve the original problem.
Why Water Ingress Can Be Difficult to Diagnose From Inside
Internal inspection is useful because it shows where moisture is becoming visible, but it may not reveal the external defect responsible for the problem.
The actual entry point may be higher on the building, positioned behind a façade component or located on an elevation that cannot be examined properly from inside.
From ground level, obvious defects may sometimes be visible, including:
- Major cracks
- Overflowing gutters
- Loose flashings
- Severe staining
- Damaged cladding components
Smaller failed joints and localised defects can be much harder to assess without close access.
How Weather Conditions Can Help Identify the Source
The circumstances surrounding a leak can provide useful diagnostic information.
Property managers and occupants should note:
- Whether the problem occurs during every rainfall
- Whether strong wind is required
- Which direction the wind is coming from
- Which elevation is exposed
- How long it takes before water becomes visible
- Whether the leak continues after rainfall stops
- Which floors or rooms are affected
For example, a leak that repeatedly appears only during rain accompanied by wind from one direction may justify closer inspection of the corresponding elevation.
How a Water-Ingress Investigation May Be Carried Out
The objective of an investigation should be to identify likely entry points and understand how water could travel from the external defect to the internal symptom.
Review the Leak History
Information about previous incidents, weather conditions and earlier repair attempts can help narrow down the investigation.
Inspect the External Building Envelope
Relevant areas may include:
- Window perimeters
- Sealant joints
- Masonry
- Pointing
- Cladding interfaces
- Parapets
- Flashings
- Balconies
- Roof edges
- Drainage systems
Document Visible Defects
Photographic documentation can help property managers understand the condition of areas that are difficult to view from ground level.
It can also support:
- Repair planning
- Quotations
- Comparison of defects over time
- Maintenance records
- Post-repair documentation
Carry Out Targeted Testing Where Appropriate
Where visual inspection does not identify a clear cause, controlled testing may sometimes form part of a wider investigation.
The appropriate method depends on the building construction, suspected defect and agreed scope of investigation.
When Is a Close-Up External Inspection Useful?
Close-up inspection can be useful where the suspected defect is located:
- Several storeys above ground
- Around high-level glazing
- At roof or parapet level
- Above a courtyard
- On a difficult external elevation
- Around cladding or façade joints that cannot be assessed from street level
Rope access can allow trained technicians to reach specific sections of the façade without installing extensive temporary access infrastructure around the entire building.
The most appropriate access method will depend on the building, location of the suspected defect and scope of the investigation.
What Should You Do If Your Building Leaks Only During Storms?
Intermittent leaks can be difficult to investigate because the property may appear completely dry between periods of severe weather.
If possible, keep a simple record each time the problem occurs.
- Date and approximate time
- Rainfall conditions
- Wind direction if known
- Room or area affected
- Where water first became visible
- How long the leak continued
- Photographs or videos of the problem
Patterns in this information can help identify which parts of the building should be inspected first.
It can also be useful to establish whether several occupants on the same elevation experience similar symptoms during the same weather conditions.
Warning Signs of External Water Ingress
Water ingress does not always appear as an active stream of water. Some problems develop gradually.
Internal warning signs can include:
- Damp patches
- Peeling paint
- Bubbling finishes
- Staining around windows
- Mould or recurring moisture
- Discoloured ceilings
- Musty smells
- Repeated damp after storms
External signs can include:
- Cracked sealants
- Open joints
- Damaged pointing
- Façade staining
- Damaged render
- Overflowing gutters
- Loose flashings
- Cracked coping stones
These signs should not automatically be treated as confirmation of the leak source, but they can help narrow down the investigation.
Why Repeated Temporary Repairs May Not Solve the Problem
Water-ingress problems can continue when repairs are based only on assumptions.
Sealing the most visible crack may not help if rainwater is entering through a different joint or travelling from another part of the façade.
A more useful approach is to consider:
- Where water could enter
- How water could travel through the construction
- Which weather conditions trigger the leak
- Whether surrounding building elements show deterioration
- Whether previous repair attempts changed the symptoms
The objective should be to address the identified defect rather than repeatedly covering the location where water becomes visible.
Reducing the Risk of Future Water Ingress
Planned inspection and maintenance can help identify external defects before they develop into larger water-ingress problems.
Areas worth including within an appropriate maintenance strategy can include:
- Roofs
- Gutters and rainwater outlets
- Window perimeters
- External sealants
- Masonry and pointing
- Parapets
- Balconies
- Cladding systems
- Flashings
Inspection frequency should reflect the building’s construction, condition, exposure, previous defects and any relevant professional or manufacturer recommendations.
For wider remedial requirements, see our building maintenance and repair services .
Water Ingress Problems Should Be Investigated, Not Guessed
A building that leaks only during severe weather can be easy to overlook once the rain stops and the affected area begins to dry.
However, the external defect responsible for the problem may still remain. Repeated moisture exposure can affect sealants, masonry, insulation, concrete and internal finishes depending on the construction of the property.
Understanding when the leak occurs and inspecting the relevant external areas can help narrow down the likely source and allow more targeted remedial work.
Does Your Building Leak Only During Heavy Rain?
Information about when the leak appears, wind direction, the affected elevation and previous repair attempts can help narrow down possible external entry points.
If the suspected area is located on an upper floor or difficult-to-access façade, City Rope Access can provide close external access for leak investigation and appropriate remedial work.
Frequently Asked Questions
Why does my building only leak when it is windy?
Strong wind can drive rain against façades, windows and external joints. Existing defects that remain relatively dry during lighter rainfall may become exposed when rain repeatedly reaches the same area during windy conditions.
Can wind-driven rain penetrate brickwork?
Water may penetrate through cracks, open mortar joints or other masonry defects. Prolonged exposure to wind-driven rain can make these weaknesses more noticeable.
Can failed window sealant cause water ingress?
Yes. Cracked, detached or deteriorated external sealant can create a route for rainwater around a window system. However, other façade defects should also be considered during investigation.
Why can a leak appear away from the actual entry point?
Water can travel through cavities, behind cladding, around window systems or along other building elements before becoming visible internally. The visible damp patch may therefore be some distance from the original external defect.
Can blocked gutters contribute to water ingress?
Yes. Restricted gutters and outlets can cause water to overflow or collect around roof and façade details, increasing exposure around existing defects.
How is the source of a façade leak investigated?
The investigation may include reviewing when the leak occurs, inspecting relevant external building elements, documenting visible defects and considering possible routes through which water could travel.
Can rope access be used to investigate water ingress?
Rope access can provide close access to many high-level windows, façades, parapets, roof edges and other difficult external areas where ground-level inspection does not provide sufficient detail.
Do you always need scaffolding to investigate a high-rise leak?
No. The appropriate access method depends on the building and location of the suspected defect. Rope access may be suitable for localised high-level investigation, while other projects may require different access arrangements.
Should a building be inspected after severe weather?
Additional inspection may be appropriate where severe weather has resulted in new leaks, visible external damage, overflowing drainage or changes in previously identified defects.
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