Fibre Reinforced Facade Encyclopedia: Finishes
Weathering is the long-term change in the appearance of a facade caused by rain, wind-borne dirt, pollution, biological growth and ultraviolet light. On a cementitious facade it is not a defect and it cannot be designed out, only directed. The panels will not look on day 3,650 the way they looked on day one, and the difference between a facade that ages well and one that ages badly is almost entirely a matter of how water is made to move across the surface.
Two identical panels, one under a coping with a 30 mm drip and one under a flush coping, will look completely different after three winters. The drip is worth more than any coating.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Primary mechanism | Differential washing by rainwater | Dirt deposits evenly and is removed unevenly |
| Onset of visible change | Typically 12 to 36 months | Faster in urban and industrial exposure |
| Sheltered surfaces | Accumulate dirt, go darker | Soffits, reveals, upward-facing returns |
| Washed surfaces | Stay cleaner, go paler | Exposed faces in prevailing rain |
| Streak source | Concentrated run-off from an edge | Sills, copings, transoms, joint ends |
| Minimum drip projection | Typically 25 to 40 mm past the face below | With a throat or groove on the underside |
| Carbonation | Progressive reaction with atmospheric CO2 | Slightly lightens and hardens the surface |
| Biological growth | Algae and lichen on persistently damp faces | Shaded elevations in temperate Australia |
| Design life reference | 40 to 50 years for GRC | Appearance change sits well within that |
Why facades streak
Airborne dirt lands on a facade roughly uniformly and rain removes it non-uniformly, and that mismatch is the entire mechanism. Where water runs as a thin even sheet it washes the surface evenly. Where a horizontal edge collects water and releases it at a point or along a short line, it runs down as a concentrated stream that scours a clean path while the surface either side keeps its dirt, producing light streaks on a dark ground or dark streaks on a light one.
The second mechanism is dissolved load. Water that has stood on a coping, sill or ledge picks up dirt, salts and, on a cementitious surface, calcium compounds. Running down a vertical face it evaporates and deposits what it was carrying, leaving a dark dirt streak or a white calcium deposit, which is the run-off form of efflorescence. Both are far more visible than the general soiling around them.
The third is shelter. A deeply recessed reveal or the underside of a projecting band receives dirt but no rain, so it darkens steadily and never washes. That is not a fault and is sometimes used deliberately to emphasise modelling. It becomes a problem only where a sheltered zone sits immediately beside a well washed one with no transition, because the eye reads the boundary as a stain rather than a shadow.
Detailing that controls run-off
- Put a drip on every horizontal projection: project 25 to 40 mm clear of the face below and cut a throat or groove on the underside so water releases rather than tracking back
- Slope every upward-facing surface, copings, sills, cornices and horizontal reveals, so water leaves in a chosen direction instead of pooling
- Direct sill run-off sideways to a joint or drainage point rather than over the front edge onto the panel below
- Terminate horizontal joints at a vertical joint that can take the flow, rather than letting them release water at their ends
- Keep fixings, brackets and exposed metal out of the run-off path, because metallic staining is permanent
- Break long vertical runs with a recessed joint or shadow line so streaking is interrupted
- Use texture deliberately: a textured or exposed aggregate face breaks a sheet of water into many small paths
Self-cleaning, and what it actually means
Hydrophobic self-cleaning is real: a surface that repels water sheds it as droplets that roll off carrying loose dirt, and because the water does not soak in, dirt is not drawn into the pores. A silane or siloxane impregnation gives this, producing a facade that weathers more evenly and stays lighter, but it does not clean itself in any absolute sense; it reduces the rate at which dirt fixes to the surface.
Photocatalytic self-cleaning is also real: a titanium dioxide containing surface breaks down organic dirt under ultraviolet light and becomes highly wettable so rain sheets off the residue. Its limits are that it needs ultraviolet exposure, so it does nothing on the sheltered and shaded surfaces that soil most, and that its effect diminishes as the active surface weathers. The claim to treat carefully is that any facade will stay looking new. None will, and a specification promising a uniform appearance for the design life is setting up a dispute.
Setting expectations honestly
A cementitious facade will change in three ways over its life. It will soil, differentially, according to how rain reaches each surface. It will carbonate, which slightly lightens and hardens the outer layer over years and is entirely benign. And it may show minor efflorescence in its first year or two as free lime migrates to the surface and is washed away, which is normal and self-limiting.
None of this is failure and none of it is covered by any reasonable warranty, which addresses defects in manufacture and materials rather than appearance change in service. What can be managed is the rate and pattern, through detailing first, finish selection second and a defined cleaning regime third. A matte, textured, mid-tone surface hides differential soiling far better than a flat smooth pale one, which is why very pale smooth facades in urban locations demand the most from detailing and from the maintenance budget.
Standards and references
- ISO 15686 parts 1 and 2, Buildings and constructed assets, service life planning and prediction
- AS/NZS 4284 Testing of building facades, including water penetration testing of the assembled system
- GRCA Specification for the Manufacture, Curing and Testing of GRC Products, and GRCA guidance on appearance
- PCI MNL-117 and PCI Architectural Precast Concrete, guidance on weathering, staining and surface appearance
- AS/NZS 1170.2 for exposure and prevailing wind driven rain assessment (check the edition of the NCC adopted in your jurisdiction)
- EN 1504-2 class H hydrophobic impregnation, where a water repellent forms part of the strategy
Related terms
Questions specifiers ask
Why do facades streak below sills and copings?
Because a horizontal edge collects water and releases it at a point or short line rather than as an even sheet. That concentrated stream scours a clean path down the panel and, if it has been standing on the horizontal surface, deposits the dirt and dissolved calcium it picked up. A drip of 25 to 40 mm with a throat prevents both.
Can weathering be prevented?
No. It can be directed and slowed. Detailing that controls where water goes does most of the work, finish selection some, and a defined cleaning regime the rest. Any specification promising a uniform appearance across the design life is promising something no facade material delivers.
Do self-cleaning facades work?
Two effects are real. A hydrophobic impregnation sheds water as droplets that carry loose dirt away and stops dirt being drawn into the pores. A photocatalytic titanium dioxide surface breaks down organic dirt under ultraviolet, but needs sunlight, so it does nothing on the sheltered surfaces that soil most. Neither keeps a facade looking new.
Is efflorescence in the first year a defect?
Usually not. Free lime migrating to the surface and being washed away in the first year or two is normal for a cementitious panel and is self-limiting. Persistent recurring efflorescence points to a continuing water path through the panel or joint, which is a different problem.
Working out how a moulded facade will age, and what the drips and reveals need to do? We review geometry for run-off before the moulds are cut. Talk to us about detailing
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.