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Protecting the Facade From Water Staining

Sto external wall insulation (EWI) systems protect the facade from the elements, but if water is present then it may be susceptible to staining.

Kevin Davies, Senior Product Manager for Sto, says, "The management of water is a fundamental issue for EWI systems on new build construction. Robust detailing will prevent staining and enable the building to weather homogeneously."

There are three separate areas of consideration for designers when detailing an EWI system: the top, middle and bottom.

The Top – Eaves detailing


When detailing the eaves, you must ensure the roof overhang is large enough to throw water clear of the facade. This minimises streaking or staining of the render.

Care must be taken to achieve thermal continuity of the EWI system with the roof insulation to prevent a cold bridge forming at the junction.

The Middle – Openings and pipework


It is important to ensure water does not penetrate around openings, such as windows and doors. Sto has developed a sill detail featuring stop ends with an up-stand onto which the render can be finished. This prevents rainwater that collects on the sill from coming into contact with the render. The sill projects a minimum of 35mm clear of the facade to throw water clear. Also consider thermal expansion and contraction of the sill, as cracks can form at the junction with the system where water can enter.

Pipework projections through the facade and supports for rainwater downpipes require careful detailing to prevent water penetration. "Where normally it would be easy to attach a downpipe to a wall, you cannot screw into the face of a EWI system. Instead you have to incorporate fixing blocks on which to attach the downpipe and ensure that these details are weathertight," says Davies.

Sto manufactures two types of fixing block to help installers:

  • Type 1
    A high density polystyrene block, suitable for lightweight fixtures such as rainwater pipe brackets and entrance door lights.
  • Type 2
    Manufactured from high density polyurethane for heavier items, such as CCTV cameras and satellite dishes.


The Bottom – Splash zones


It is important to prevent rain run-off from splashing dirt from the ground onto the building, creating areas where algae can grow. The use of a French drain or other soft, vegetation-free surfaces can reduce splashback, keeping the base of the facade clean.

The use of StoSilco hydrophobic render or StoLotusan superhydrophobic render can help facades shed water quickly and efficiently. This also reduces algae growth conditions, keeping the EWI system performing at its peak for years to come.

A library of illustrative details is available on the Sto website.