Penetrating Damp and Driving Rain: How Cavity Face Brick Walls Safeguard Buildings
October 9, 2026
Heavy summer storms across Gauteng, KwaZulu-Natal, and the Cape coast do not just fall straight down. Wind pushes rain sideways against a building, forcing water into every crack, joint, and pore in the wall. When that water makes it through to the inside, it shows up as penetrating damp. Here is how it happens, and how a properly built cavity face brick wall is designed to stop it.
How Penetrating Damp Gets In
Penetrating damp is water that enters a wall from the outside. Unlike rising damp, which stays low, it can appear at any height and typically gets worse during or soon after rain. We introduce the difference in our guide to what causes damp in brick walls, and the walls most at risk are the ones with just one layer between the weather and the room.
Single-Skin Walls
A single-skin wall is one layer of brick thick. There is no gap, so any water that works its way through the brick, the mortar joints, or a crack in the finish has nowhere to go but inward. When that single skin is also plastered, as we describe in why cement plaster acts like a sponge, the plaster soaks up the rain and passes it to the wall behind.
Poorly Plastered or Cracked Finishes
Hairline cracks, thin patches of plaster, and unsealed gaps around windows and doors all let wind-driven rain past the finish. Once it has reached the brick behind, an absorbent brick pulls it further in. Our piece on why cement stock bricks cause damp explains why a more absorbent brick makes this worse.
How a Cavity Wall Works
A cavity wall is built from two layers, or skins, of masonry with a gap between them. In South Africa, SANS 10400-K, the national building regulation for walls, requires a cavity in a cavity wall to be no less than 50 mm and no more than 110 mm wide. The outer skin faces the weather, and the inner skin carries the interior finish. The two are held together with metal wall ties.
The Outer Skin Takes the Weather
A face brick outer skin is made to be exposed to rain, so it does not rely on plaster or paint for protection. Some water may pass through the outer skin during a long storm, which is expected and allowed for in the design.
The Cavity Breaks the Path
Because there is an air gap, any water that does pass through the outer skin runs down the inside face of that skin instead of crossing to the inner wall. The wall ties are shaped so that water cannot follow them across the gap. The inner skin stays dry, and so does the interior.
Weep Holes Let Water Out
At the base of the cavity, small openings are left in the vertical joints of the outer skin. These are weep holes, and they let any water that has collected in the cavity drain out instead of building up. Because weep holes are formed in the vertical joints, the joint itself matters. SANS 10400-K requires the gap in the butt joint between adjacent bricks to be between 10 mm and 12 mm for walls built with burnt clay masonry units, and not more than 12 mm for concrete masonry units. For a cavity wall to work, the cavity itself must stay clean. Mortar droppings that fall into the gap can pile up on wall ties or block the weep holes, creating exactly the bridge the cavity was meant to prevent.
Single-Skin Walls Versus Cavity Face Brick Walls
| Feature | Plastered Single-Skin Wall | Cavity Face Brick Wall |
|---|---|---|
| Layers between rain and room | One | Two, with an air gap |
| Where rain ends up | Soaks toward the interior | Drains down the cavity and out |
| Protection depends on | Plaster and paint staying intact | The outer skin and a clean cavity |
| Upkeep | Repainting and crack repair | Occasional pointing repairs |
| Typical weak points | Cracks and unsealed gaps | Blocked weep holes, bridged ties |
Why Cavity Face Brick Keeps Interiors Dry
The strength of a cavity face brick wall is that it plans for water getting in rather than pretending it will not. The outer skin sheds most of the rain, the cavity catches whatever gets past, and the weep holes send it back outside. Inside, the plaster sits on the inner skin, where it stays dry. If you are weighing up a single-skin build instead, read what a plastered wall can hide before you commit.
Your location affects which face brick you need, since coastal and high-rainfall areas are tougher on walls than dry inland ones. Our guide to South African exposure zones helps you match the brick to the site. If damp is also coming from the ground, read how plaster bridges the damp proof course.
When you are ready to price a wall, browse our face brick range or use the brick calculator.
Frequently Asked Questions
What is penetrating damp?
Penetrating damp is water entering a wall from outside, usually through cracks, poor mortar joints, gaps around windows, or an absorbent finish. It can appear at any height and typically gets worse during or after rain.
How wide should a cavity in a cavity wall be in South Africa?
SANS 10400-K requires a cavity in a cavity wall to be at least 50 mm and no more than 110 mm wide.
What do weep holes do?
Weep holes are small openings at the base of the outer skin that let water collected in the cavity drain out, so it does not build up and cross to the inner wall.
Can a cavity wall still get damp?
Yes, if the cavity is blocked by mortar droppings, the weep holes are covered, or wall ties are fitted badly. A clean cavity and careful workmanship are what make the system work.
Is a cavity wall better than a plastered single-skin wall for heavy rain?
A properly built cavity face brick wall is designed to handle wind-driven rain, because it breaks the path water takes to the inside. A plastered single-skin wall depends on its finish staying crack-free.
Get a Quote From Dezyno Bricks
Dezyno Bricks supplies premium clay face brick, semi-face brick, and plaster brick across South Africa from our yard in Springs, Gauteng. Call us on 061 538 5968, email sales@dezynobricks.co.za, or message us on WhatsApp to get a quote for your next project.
