What Really Keeps a Clay Tile Roof Waterproof in Portugal
A look beneath the tiles
Clay tile roofs are iconic in Portugal. From historic neighborhoods to modern developments, they’re one of the most common roof coverings in the country. But there’s a common misconception that can cause confusion for homeowners and buyers:
Clay tiles themselves are not the waterproofing layer.
Understanding what actually keeps water out of a tiled roof helps explain why some roofs perform well for decades—and why others develop moisture problems even when the tiles appear intact.
Clay tiles: a protective layer, not a seal
Clay tiles are designed as a discontinuous roof covering. Their job is to:
Deflect most rain and sun exposure
Protect the roof assembly from direct weather
Provide long-term durability when properly installed
However, due to their shape, overlaps, and natural tolerances, clay tiles are not intended to be fully watertight on their own. Under certain conditions—such as wind-driven rain, heavy storms, or aging materials—water can pass beneath the tiles.
This is not a defect. It’s an expected part of how tiled roofs are designed to function.
What actually provides waterproofing under a tiled roof
In modern Portuguese construction, manufacturers typically assume that a tiled roof includes a secondary water-shedding layer beneath the tiles, often referred to as a subtelha or underlayment.
This layer plays a critical role by:
Managing wind-driven rain that gets under the tiles
Channeling moisture safely toward the eaves
Protecting the roof structure from incidental water exposure
Helping control condensation when paired with ventilation
Common underlayment systems used in Portugal
1. Vapour-permeable waterproof membranes
The most common modern solution is a breathable, waterproof membrane installed beneath the tiles. These membranes:
Block liquid water
Allow water vapour to escape
Reduce condensation risk within the roof structure
Thy are typically installed from the eaves up to the ridge, with overlapping seams so water naturally drains downward.
2. Rigid “subtelha” boards or panels
In some homes—particularly renovations or traditional builds—you may find rigid subtelha panels beneath the tiles. These act as a continuous secondary roof surface and must be carefully detailed around:
Chimneys
Valleys
Skylights
Roof edges
When properly installed, they can be very effective.
3. Bituminous underlays (select applications)
In more exposed conditions or specific roof designs, bituminous underlays may be used. These tend to be more robust but less breathable and are usually selected based on the overall roof design rather than as a default option.
How the roof layers typically work together
A well-performing clay tile roof usually relies on multiple layers working as a system, not a single barrier:
Roof structure (rafters and deck)
Secondary waterproofing layer (membrane or subtelha)
Battens and, where used, counter-battens
Clay tiles and roof accessories
This layered approach allows water to be managed gradually while also allowing the roof to dry.
Ventilation: the hidden performance factor
Ventilation is a key part of how tiled roofs manage moisture. Many clay tile roofs in Portugal are designed as ventilated systems, which helps:
Dry moisture beneath the tiles
Reduce condensation
Extend the life of timber and membranes
Ventilation is commonly provided through:
Eave openings
Ventilated ridges
Air gaps created by battens and counter-battens
A roof can look fine from the outside but still perform poorly if ventilation paths are blocked or missing.
How clay tiles are typically fixed in place
Clay tiles are normally installed on battens and secured using:
Nails or screws
Clips or hooks designed for the specific tile profile
Mechanical fixing is especially important at:
Roof edges
Ridges and hips
Wind-exposed areas
While mortar may still be seen on older roofs, many modern systems now use dry-fix ridge solutions, which improve ventilation and reduce long-term cracking.
Why moisture problems still occur
From an inspection perspective, moisture issues beneath tiled roofs are often linked to:
Missing or damaged underlayment
No secondary waterproofing layer at all
Poor detailing at penetrations and valleys
Inadequate ventilation
Relying on tiles alone to keep water out
These issues are often hidden and may only become visible after damage has progressed.
Final thoughts
Clay tile roofs can perform exceptionally well in Portugal’s climate—but only when the entire roof system is working as intended. The tiles you see are just one part of that system.
Understanding what’s happening beneath the tiles helps explain why proper detailing, ventilation, and secondary waterproofing matter just as much as the tiles themselves.