Cement renders


In Our Opinion

Cement does not bond well with straw, you will need to prove a suitable medium such as chicken wire to the bales prior to applying the cement render.  This takes inordinate time and resources.   Cement does however have a long life span and is weather resistant to a certain extent. You will need to consult your local plasterer as to the best type of mix for your area, taking into account sand types and finishes that you require. As a guide you may consider the following.

Scratch coat

• one part cement
• one part lime/plaster master
• five parts sand

Brown coat

• one part cement
• half part lime/plaster master
• five parts sand

Colour coat

• one part off/white cement
• quarter part lime/plaster master
• six parts sand
• oxides to colour

The mixes should not be too rich in cement, otherwise the wall could be susceptible to cracking and shrinkage. Apply the scratch coat, and leave until the coat has set. This will vary according to the temperature and humidity at the time of rendering. The brown coat is applied as soon as the scratch coat has set. It may be necessary to spray the walls with water to help adhesion of the brown coat.

The colour coat can be applied anytime after the brown coat has set, and you will need to keep the walls moist until this happens. This will avoid cracking and shrinkage. We also recommend that you apply a coat of a silicone-based water repellent once the walls have completely dried and set (this could take up to 28 days).

We are very much against the use of cement renders in strawbale building for many valid reasons. The high cost to the environment is one. However, the overwhelming evidence is that straw and cement are not compatible. Some recent work suggests that even in drier climates cement-rendered walls not protected with wide verandahs or eaves are not allowing moisture that gets in to get out, leading to the subsequent deterioration of the strawbales.

Having said that here is a photo of Lethbridge Winery near Geelong that was rendered with cement and sand as well as colour.  The render was pumped on in one coat premixed at the cement depot.  The average depth is 45mm.  The jumbo bale walls were netted with thick gauge chicken wire and sewn through the walls.

Lethbridge Straw Bale Winery
Web site: Lethbridge Straw Bale Winery