White deposit on outdoor stairs and window sills
A white deposit is most often efflorescence: water passes through the stone or mortar, dissolves salts and leaves them on the surface when it evaporates. Brush off the dry deposit and wash off the rest with water. To stop it coming back, you have to stop the water under the stone: correct the slope, seal the joints and drain the water away.
What it is
Efflorescence forms when water passes through stone or mortar, dissolves natural salts and then evaporates at the surface. The salts crystallise and leave a white, powdery deposit.1
Where the water comes from
- Moisture from the ground under the stairs.
- Rain getting into the joints.
- Snow melting on the steps.
- Mortar that is still drying after construction.1
How to remove it
- Sweep off the dry deposit with a soft brush. Do not wash it off straight away with lots of water, or the salts will soak back in.
- Wash the rest with water and a neutral cleaner and dry thoroughly.
- A stubborn deposit is removed with a dedicated efflorescence remover suited to the stone. Do not use acidic products on marble or limestone.
Why it comes back
Efflorescence is a symptom, and the cause is moving water.1 As long as water passes through the structure, the deposit will keep coming back.
How to stop it
- A slope on steps and window sills so water does not stand.
- A drip groove under the exterior window sill so water does not run down the wall.
- Tight joints and flashings at the wall.
- Damp-proofing under the stairs.
For a new installation we plan this from the start. For old stairs we will tell you where the water is coming from.
(Sources)
-
Rose Restoration, Efflorescence on Stone: Why It Turns White and How to Fix It
(Questions)
Will the deposit disappear on its own?
Sometimes, once the structure dries out after construction. If it comes back every season, the cause is water under the stone.
Will sealing help?
Not if the water comes from below. A sealer can even trap the salts under the surface.
