The Self Build Self Help Site Questions answered
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How do I get rid of efflorescence on a wall?
If building a wall in facing brick then you must really keep the bricks dry before use. This will considerably reduce the likelehood of efflorescence developing. A sulfacrete cement should also be used.

Efflorscence can be brushed off once it has developed, certainly prior to rendering. But don't just take our word for it, find some advice on the FAQ page of the Brick Development Association site.

Efflorescence can be confused with lime staining, which produces much harder deposits which are very difficult to remove and may require acid treatment - best done by a specialist. The calcium hydroxide can come from the cement or from water in the blocks or ground water, which is why it is often seen on retaining walls.

Examples of Efflorescence

Photo of window with slate sill

Photo of window with slate sill

Photo of window with slate sill


(Click on pictures for a larger view)

Examples of Lime Staining

Photo of window with slate sill

Photo of window with slate sill

Photo of window with slate sill

Should weep holes be provided in a rendered external wall, where there are cavity trays?
With a rendered wall, there is very little likelihood that water will penetrate into the cavity. It may happen if there are cracks in the render. As the likelihood is minimal, then weep holes are not usually necessary. It may be a good idea, however, to provide them where there is a long cavity tray, or run of cavity trays such as in roof cavity trays, where the final tray, a catchment tray, is situated.

On fair faced walls, there is a higher likelihood that rain may penetrate, for example, through the bed between brick courses. It is usual practice to provide weep holes for all cavity trays in fair faced walls. The weep holes do not usually show in the perps in brick walls. If weep holes were to be put into a rendered wall, everywhere that there is a cavity tray, the wall would look a mess with weep holes all over the shop!

Should I use over fascia vents if I use a vapour permeable membrane underneath roof slates?
It is critical to ensure that there is good ventilation in the roof space, to prevent any damp rot setting in. OK, this may take a long time, but houses are there for a long time. So, to ensure good ventilation use both a vapour permable membrane and over fascia vents. The former is only slightly more expensive than a non-permable membrane and the vents are only just over a pound per metre.

The membrane will also ensure continued ventilation if the vents get blocked up by overenthusiastic stuffing of the insulation material into the eaves space.

Also, when a house is being built there is a lot of water about in the new plaster which gives rise to a lot of damp air. This is better dispersed from the roof space by over fascia vents.

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