Applying Water Repellent to Exterior Facades: Silane, Siloxane and Stone Breathability

A stone facade is the part of a building most exposed to rain, wind, sun and polluted air. Stone that absorbs rainwater gets dirty faster, carries salts along with the water it takes in, and becomes vulnerable to frost in cold climates. Water-repellent treatments are applied to reduce this absorption; however, a product that is wrongly chosen or applied to the wrong wall can block moisture from escaping the stone and make the problem worse instead of solving it. In this article we explain how silane- and siloxane-based water repellents work, why stone breathability matters, how to choose a product for your stone, the application conditions, and how a wrong application causes stone to crust.

In short: Silanes and siloxanes are water repellents that form a silicone-based structure on the walls of the stone's pores that repels water, without leaving a film on the surface; when chosen correctly, they reduce rain absorption while allowing water vapor to escape. Success depends on applying a product suited to the type of stone, under the right conditions, to a dry and repaired facade; on walls that are laden with salts or constantly take up moisture, application should not be carried out before the source of the moisture is resolved.

What Does a Water Repellent Do on a Facade?

When it rains, porous stone absorbs water through its surface. This water can travel several centimeters into the stone, binds airborne dirt and soot particles to the surface, and makes it easier for algae and lichen to take hold. As the water evaporates, it also carries dissolved salts from inside the stone to the surface; where the salts crystallize, white stains or surface loss occur. In cold climates, the water in the pores freezes and expands, and repeated freezing and thawing gradually loosens the stone's surface.

A water-repellent treatment noticeably reduces the stone's absorption of rainwater. As a result, the facade gets dirty more slowly, biological growth and salt movement decrease, and the effect of frost is weakened. However, a water repellent does not solve every problem: water entering through cracks in the joints, under window sills or at roof connections continues to reach the back of the stone. Rising damp from the ground and vapor coming from inside the building are also problems a water repellent cannot solve. For this reason, a water repellent should be regarded as the final protective layer of a sound, repaired facade.

Silane and Siloxane: Two Different Molecules from the Same Family

Silanes and siloxanes are water repellents based on silicone chemistry. Both penetrate the pores of the stone and react chemically there, forming a silicone resin network that coats the pore walls and repels water. This network does not clog the pores; it prevents the pore walls from being wetted by water. Since no glossy film forms on the surface, the appearance of the stone usually does not change.

Silanes

Silanes are small molecules. This small structure allows them to penetrate deep into the fine pores of stone and concrete. After entering the stone, they bond by reacting with moisture; an alkaline environment such as that in concrete facilitates this reaction. The small molecular structure also has a drawback: silanes are volatile, and part of the product may evaporate during application. For this reason, silane-based products are generally made with a high active ingredient content and are preferred especially on dense-textured concrete surfaces.

Siloxanes

Siloxanes are larger molecules formed by linking silane units together. They do not penetrate as deeply as silanes, but they evaporate less and form an effective water-repellent layer near the surface. Their reaction is not as dependent on the alkalinity of the substrate as silanes. Because of these properties, they are widely used on medium to highly porous surfaces such as brick, plaster and natural stone.

Blends and cream products

Many products on the market are made as silane and siloxane blends to combine the strengths of the two molecules. Some products are prepared as water-based emulsions or in a cream consistency. Cream products stay on vertical surfaces without running, remain in contact with the stone for longer and can therefore penetrate deeper; there is also less wind drift and waste during application.

Choosing by Stone Type

How well a water repellent bonds to stone also depends on the stone's mineral structure. Stones made of silicate minerals, such as granite, andesite and sandstone, bond well with silicone-based products. In calcite-based stones such as marble, limestone and travertine, bonding, especially of silanes, may be weaker; for these stones, you should choose products that the manufacturer explicitly states are suitable for calcareous stones.

Porosity also affects the choice. A very dense, polished granite facade absorbs almost no water and often does not need a water repellent. A porous sandstone or limestone, on the other hand, takes up plenty of product, and if a single coat is considered enough, protection may be uneven. The table below summarizes general tendencies for commonly encountered facade stones; the final decision should always be made on a test area:

Vapor Permeability: How Stone Breathes

In addition to the water it takes in from rain, a wall also carries moisture coming from inside the building and from the ground. This moisture leaves as vapor through the pores of the stone. What we call the breathing of stone is this vapor movement. One of the most important criteria in deciding whether a protective product is suitable for a facade is how well it maintains this movement.

Silanes and siloxanes that penetrate the pores make the pore walls water-repellent without clogging them; liquid water cannot get in, but water vapor largely continues to pass through. No protective product lets vapor through as freely as untreated stone, but with a good water repellent the difference is small. Film-forming coatings such as acrylic, polyurethane or some silicone resin paints, on the other hand, cover the surface with a continuous layer and can markedly restrict the escape of vapor. We explained the effect of film coatings on stone in detail in our acrylic varnish article in detail.

Crusting: Wrong Product or Wrong Wall

The most serious risk of water-repellent treatments is the surface of the stone lifting off in flakes or crusts. This problem usually arises in one of two ways. The first is applying a film-forming product to the facade: moisture and salt accumulating under the film lift the film, and sometimes the topmost layer of the stone, together. Freezing also speeds up this lifting by expanding the water trapped under the film.

The second way is more insidious and can occur even with the right product. When a water repellent is applied to a wall that draws moisture from the ground or contains heavy salts, the place where the salt crystallizes changes. Salts that previously appeared as white stains on the surface now crystallize inside the stone, right behind the water-repellent layer. The pressure created by crystallization can push the treated outer layer from behind and break it off as a crust. For a while everything looks fine on the surface, and then the stone starts to flake off unexpectedly.

For this reason, the moisture and salt condition of the facade must be assessed before applying a water repellent. If there is ground moisture, cracked joints, a faulty rain gutter or a salt-laden wall, these problems must be solved first. We discussed the causes of salt efflorescence from an interior perspective in our white efflorescence article ; the same mechanism applies on exterior facades. For historic buildings, this assessment should be made by an expert, and the application should be planned with the approval of the relevant authorities.

Before Application: Preparing the Facade

A water repellent preserves the facade as it is at that moment; it fixes a dirty facade as dirty and a cracked joint as cracked. For this reason, the facade needs to be cleaned and repaired before application. The main steps to take when preparing a facade for a water repellent are as follows:

• Cleaning: Algae, lichen, soiling and salt residues must be removed with methods suitable for the stone; a layer of dirt keeps the product from reaching the stone, and the product ends up trapped beneath it.
• Joint and crack repair: Open joints, cracks and broken-out fillings must be repaired before the water repellent is applied; a water repellent reduces capillary absorption but does not close an open gap.
• Eliminating the moisture source: Leaks from gutters, sills and roof junctions must be repaired, and if there is ground moisture, a solution for it must be planned separately.
• Drying: After cleaning and repairs, the facade must be left long enough to dry to the moisture level the product recommends; wet stone will not take up the product.
• Test panel: A trial should be carried out on an inconspicuous section of the facade, and once it has dried, the appearance and water absorption behavior should be checked.

We have also explained the effect that the methods used in facade cleaning have on stone in our article on black crust and air pollution separately.

Application Conditions

How evenly and deeply the water repellent penetrates the stone depends strictly on the conditions on the day of application. The stone must be dry; this is the first requirement. If it is applied right after rain, or in the morning before the dew has dried, the product cannot penetrate the stone. Rain in the hours after application can also wash away part of the product and reduce its effect, so the weather forecast should be checked for at least the next day or two.

Temperature and wind matter as well. On a facade heated by direct sun, the product evaporates very quickly and does not penetrate deeply enough; in very cold weather, the reaction slows down. The temperature range recommended for the product should be observed, and in hot seasons the hours when the facade is in the shade should be chosen. In windy weather, the sprayed product drifts and reaches windows, vehicles and plants.

Application is generally carried out in the following order:

1. Protect the surroundings: Windows, frames, metal parts and plants are covered; some products can leave marks on glass that are difficult to clean.
2. Start from the bottom: The product is applied with a low-pressure sprayer or a brush, working upward from the bottom of the facade.
3. Apply until saturated: It is applied until the stone stops absorbing the product and a brief sheen forms on the surface; depending on the product, the second coat is applied before the first has dried.
4. Remove runs: Product that pools or runs on tightly textured areas is spread out with a brush or wiped off before it dries.
5. Protect and wait: The treated surface is protected from rain, irrigation water and washing for the period recommended by the manufacturer, and then checked with a water test.

Checking Effectiveness and Renewal

Whether the water repellent is working can easily be seen by spraying water onto the facade some time after application. On treated stone, water beads up and runs off, and the stone does not darken. For a more measured assessment, a simple measuring device known as a Karsten tube, which shows how much water the stone absorbs from a given area, can be used; measurements taken at the same spot before and after application show the effect of the protection numerically.

The water-repellent effect is not permanent and decreases over the years. This period varies depending on the product, the stone, the orientation of the facade and the climate; protection weakens faster on south-facing facades that receive rain directly. For this reason, instead of giving a fixed renewal interval, it is better to carry out water tests at regular intervals on the parts of the facade that get the most rain and the most sun. When the protection weakens, the surface is cleaned again, dried and renewed with the same product or a compatible one. We have covered the factors that determine the lifespan of impregnation in our article on how long impregnation lasts in detail.

If the stone facade of your building has problems with water absorption, soiling or salt efflorescence and you are not sure whether a water repellent is suitable, the facade first needs to be inspected on site. You can reach us with a few photos via the service request form ; together we will plan the order of cleaning, repair and protection for the facade.