Hydrophobic treatment of a facade: making masonry permanently water-repellent
Contents
- What you need: shopping list
- Why apply a hydrophobic treatment – and what it cannot do
- Step by step: hydrophobic treatment of a facade
- Consumption by substrate
- Waiting times at a glance
- Common mistakes – and how to avoid them
- Frequently asked questions about facade hydrophobic treatment
- Further in the shop
In short: A facade hydrophobic treatment stops rainwater from penetrating the building material, without impairing the breathability of the wall – moisture inside can continue to dry outwards. The procedure is always the same: check the substrate for absorbency, remove moss and algae completely and rinse them off, close cracks wider than 0.3 mm, then apply the impregnation from top to bottom, generously, wet-on-wet. What decides the result is not the product but the penetration depth – and you only achieve that with a generous application on an absorbent, clean substrate.
What you need: shopping list
Basic set (always required)
Optional – depending on the installation situation
- Soudal Cement Repair Express 300 ml – as soon as cracks are wider than 0.3 mm or holes are present in the masonry
- Otto anti-mould spray – for mould, fungal, moss or algae growth; sprayed directly onto the growth
- Schuller Allround FSP facade brush 75 mm – if you brush instead of spray or treat only partial areas
- KIP 313 window protection film – self-adhesive, stays in place even in wind, weather-resistant for up to three weeks
- Tesa Tesakrepp 4319 – masking tape with a high stretch reserve, for edges, curves and frame junctions
- Telescopic painter's pole – for higher surfaces without changing ladders
Tools
- Stiff brush or root brush for brushing off loosened green growth
- Water hose or high-pressure cleaner for rinsing (not too high a pressure on soft joints)
- Cartridge gun or putty knife for crack repair, damp filler for smoothing
- Safety goggles and protective gloves, respirator for spraying work
- Ladder or scaffolding with secure footing
Why apply a hydrophobic treatment – and what it cannot do
Mineral building materials absorb water by capillary action. An externally wet wall insulates less well, soils faster and offers algae and moss the moisture they need. The siloxane impregnation penetrates the building material and makes the pore walls water-repellent: moisture entering from outside is prevented, but moisture inside can still escape outwards. According to the manufacturer this improves the thermal insulation of the treated outer wall, reduces the tendency to soiling and prevents salt efflorescence as well as moss and algae growth.
The limit is just as important: a hydrophobic treatment is not waterproofing. It works against water entering a vertical surface from outside – not against rising damp from the ground and not against pressing water on parts in contact with soil. If moisture rises from below into the wall, only a damp-proof course or waterproofing of the area in contact with soil will help. So first identify the cause: Damp cellar wall – finding the cause and repairing it.
Step by step: hydrophobic treatment of a facade
Check the substrate and its absorbency
Suitable are concrete, aerated concrete, fibre cement, brick masonry, organic renders, natural and artificial stone, and mineral paint coatings – on vertical surfaces. Horizontal surfaces such as wall copings or window sills are outside this scope of application.
Why: the impregnation only works if it can penetrate. On a dense, painted or varnished surface it stays on top and does nothing.
Check: spray water onto the wall. If it soaks in within a few seconds and the surface darkens, the substrate is absorbent. If the water already beads off, a further hydrophobic treatment is pointless. At the same time mark all cracks: anything over 0.3 mm must be closed in step 4.
Remove moss, algae and green growth completely
Apply the green growth remover undiluted and evenly with a pressure sprayer, brush or scrubbing brush, and leave it to act for at least 24 hours. For heavy growth, brush off the residues or repeat the application. According to the manufacturer the 5-litre jerrycan is enough for around 50 m².
Why: the manufacturer of the impregnation expressly requires moss, mould and algae growth to be removed completely. Organic residues act like a separating layer – underneath, the building material remains untreated.
Check: no green or black film is visible any more. Important: if the surface is to be impregnated or painted afterwards, according to the manufacturer it must first be rinsed off with water. Do not let residues get into waterways, gullies or drains – use biocidal products carefully.
Pre-treat efflorescence and weathered areas
The substrate must be dust-free, grease-free, load-bearing and as dry as possible. Pre-treat weathered surfaces with efflorescence by sand or steam blasting, or by washing with suitable cleaners.
Why: salt efflorescence and powdery layers later come away along with the impregnation.
Check: wipe the dry surface with the back of your hand – if dust or powder remains, the substrate is not yet load-bearing. For heavily weathered mortar joints the manufacturer recommends a trial on a small area to see whether a hydrophobic treatment is still effective there.
Close cracks and holes
The impregnation bridges hairline cracks up to a width of 0.3 mm. Anything wider is filled from the cartridge with Cement Repair Express – suitable for static joints up to 5 cm wide, processing temperature 5–30 °C. Smooth with a damp putty knife or filler; for wider cracks apply a second layer after about 30 minutes.
Why: an open crack is a water path behind the facade. No impregnation in the world closes it.
Check: the mortar does not shrink as it dries – after curing there must be no shrinkage gap along the crack edges. Surface flush, not tacky, and can be painted over and plastered over after a few hours.
Cover adjacent surfaces
Cover glass, window and door frames made of wood and metal, gutters and elastic expansion joints. This also applies to adjacent facades: their colour can be deepened by the impregnation. Plastics in the facade or flat roof area must be checked for compatibility before the hydrophobic treatment.
Why: splashes on glass and frames can hardly be removed later, and an elastic expansion joint has no business with the impregnation.
Check: walk round before the first spray – no open glass edge, no exposed frame surface, sheeting pressed tightly at the edges.
Apply the impregnation generously, wet-on-wet
Shake the pack well before use. Otto Siloxan is applied from top to bottom by spraying or brushing, in one or several passes – always very generously, i.e. wet-on-wet. The substrate may be slightly damp but must not be wet from rain. With the pressure sprayer work at a maximum of 3 bar: fill the container no higher than the 5-litre mark, build up pressure, test the spray pattern on an inconspicuous spot and then work in overlapping strips.
Why: the effect comes from the penetration depth, not from a film on the surface. Whoever applies it sparingly or lets it dry between passes creates exactly such a film.
Check: after application the surface must have a uniform dark, wet sheen, with no dry stripes between the strips. Take up any run-off from above immediately with the strip.
Check the effect, clean the equipment
Check: after drying, place water drops on the surface at several points. If they bead off and the substrate does not darken, the hydrophobic treatment has taken. If water still soaks in at one point, there was too little material there – rework that spot.
Afterwards, release the residual pressure on the sprayer, empty the container, flush all parts carrying the medium and remove and clean the dirt filter. Store the sprayer empty and frost-free. Seal opened packs well and use them up soon; at 5–25 °C, unopened packs keep for around 12 months. The treated surface can be painted over with dispersion paints.
Consumption by substrate
Consumption depends almost entirely on absorbency – between fibre-cement board and brick masonry the factor is seven. For highly absorbent surfaces use the upper value and determine the actual requirement on a trial area of one square metre.
| Substrate | Consumption |
|---|---|
| Concrete | 0.25–0.5 l/m² |
| Render | 0.5–1 l/m² |
| Calcium silicate brick | 0.40–0.7 l/m² |
| Brick wall | 0.40–2 l/m² |
| Aerated concrete | 0.50–2 l/m² |
| Fibre-cement boards | 0.10–0.3 l/m² |
| Natural stone | 0.05–3 l/m² |
Waiting times at a glance
| Work step | Waiting time | Check before continuing |
|---|---|---|
| Green growth remover left to act | at least 24 h | no green or black film any more; otherwise brush off or repeat the application |
| Surface after rinsing | until visibly dry | substrate largely dry, certainly not wet from rain |
| Crack mortar skin formation | approx. 30 min | surface no longer tacky; apply the second layer now for wide cracks |
| Crack mortar can be painted over | a few hours | repair spot firm, no shrinkage gap at the edges |
| Impregnation, several passes | no intermediate drying | work wet-on-wet; surface evenly wet and glossy |
| Effect check | after drying | water drops bead off, substrate does not darken |
Details from the manufacturer information for the linked products. For processing temperature and waiting time before a follow-up coat, always follow the current data sheet of the manufacturer.
Common mistakes – and how to avoid them
- Applied to non-absorbent surfaces. On dense concrete, tiles or a paint coat the impregnation stays on top. Do the water test first.
- Green growth only sprayed off. The manufacturer requires complete removal of moss, mould and algae – not just rinsing the surface.
- Cleaner residues not rinsed off. Before impregnating or painting, the surface must be rinsed off with water.
- Cracks wider than 0.3 mm left open. Only hairline cracks are bridged. Anything wider remains a water path.
- Applied too sparingly or not wet-on-wet. The result is a surface film without penetration depth – visually inconspicuous, but ineffective.
- Worked from bottom to top. Run-off material dries on the untreated surface and leaves joint marks. Always work from top to bottom.
- Worked in rain or on a rain-wet wall. Slightly damp is acceptable, rain-wet is not.
- Frames, glass and expansion joints not covered – and plastic parts not checked for compatibility in advance.
- Hydrophobic treatment used against rising damp. Wrong product in the wrong place: only a damp-proof course or waterproofing of the area in contact with soil helps against that.
Frequently asked questions about facade hydrophobic treatment
What does a hydrophobic treatment actually achieve?
It prevents moisture from penetrating the building material without impairing its breathability. According to the manufacturer, this improves the thermal insulation of the treated outer wall, the surface soils more slowly, and salt efflorescence as well as moss and algae growth are prevented.
On which substrates does it work?
Specified are concrete, aerated concrete, fibre cement, brick masonry, organic renders, natural and artificial stone, and mineral paint coatings – in each case on vertical surfaces. What matters is that the substrate is absorbent, load-bearing, dust-free and grease-free.
Does a hydrophobic treatment replace waterproofing?
No. It prevents moisture entering from outside and lets moisture inside escape outwards. Against rising damp or pressing water on parts in contact with soil it is the wrong means – there you need waterproofing of the area in contact with soil.
How much material do I need?
Between 0.05 l/m² on dense natural stone and 3 l/m² on highly absorbent natural stone – the consumption table above gives the values for each substrate. Only the trial area is reliable: treat one square metre generously, wet-on-wet, and measure the consumption.
May the wall be damp?
Slightly damp substrates are acceptable, rain-wet ones are not. The substrate should be as dry as possible, load-bearing, and dust- and grease-free.
Can I paint the facade afterwards?
Yes, the treated surface can be painted over with dispersion paints. The waiting time before the follow-up coat follows the data sheet of the paint manufacturer.
Does it help against algae and moss?
Indirectly: because the building material no longer soaks through, moss and algae growth as well as salt efflorescence are prevented. Existing growth must, however, be completely removed beforehand – the impregnation is not a cleaning agent.
Further in the shop
All products in this guide can be found in the category Hydrophobic treatment & impregnation; cleaners for preparation are listed under Facade cleaners, mould & green growth removers.
Relevant guides: Damp cellar wall – finding the cause and repairing it · Sealing a cellar wall from outside with KSK-Bahn · Sealing slurry or bitumen – which goes where?
Questions about the make-up of your facade? Baustoffheld, Lindenstrasse 56, 47623 Kevelaer, phone +49 (0) 2831 - 916 77 03.
Basic set: you always need these products
3 variants
Content: 5 Litre (€7.59 / 1 Litre)
Content: 5 Litre (€3.39 / 1 Litre)
Optional – depending on the installation situation
Content: 0,3 Litre (€30.97 / 1 Litre)
2 variants
Content: 0,5 Litre (€17.90 / 1 Litre)
3 variants
Content: 100 Metre (€0.17 / 1 Metre)
2 variants
Content: 50 Metre (€0.23 / 1 Metre)
More guides on this topic
All guides on “Wood, paint and façades”
Comparison
Guide: GORI 33 or GORI 88 – which wood stain suits your project?
GORI 33 or GORI 88: thin-film or medium-film wood stain for which timber component
Problem solving
Guide: brown stains in wall plaster – finding and fixing the cause
Where brown stains in fresh plaster come from and how barrier primer blocks them permanently
Instructions
Guide: refreshing, cleaning and caring for WPC decking
Clean WPC decking, refresh its colour and impregnate it, with a realistic care plan