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  Phil Voice March 20, 2009 at 7:54pm

  Susan
  
  I am not sure if this would be a problem if the right materials had been used.
  
  This sometimes happens if a scratch coat isn't applied correctly or the render is put on without any form of bonding additive such as PVA.
  
  I would remove the render, clean up the surface and then apply a key coat of PVA (30% PVA 70% water) and then apply a scratch coat with PVA added (you will have to check the rates for the mix because I cannot remember)
  
  Make sure the scratch coat has the right integrity and then set the surface with the final render.
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  Gareth March 20, 2009 at 8:41pm

  I agree with Philip, best to start over. If the wall is damp I would prime the background with SBR bonding agent, PVA is not so great in damp conditions. also add a waterproofer to your scratch coat.
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  Kevin Munt March 20, 2009 at 10:02pm

  If the wall is not soaking wet try hydrated lime NHL 3.5 and a sand sharper than soft builders sand. The render should be applied in 3 coats of no more than 5mm thick, the first being a scratch coat. Lime mortar is breathable and will allow some moisture through the wall thus stopping any back pressure that will blow a cemednt based mortar. If the wall is to be painted after use a lime based breathble paint.
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  Jackson's Landscape Design March 21, 2009 at 10:10am

  Any weep holes within this 500mm high wall? If not bore through the wall to realease the water and pressure, when dry at least take a look at this product, Vandex BB75.
  
  Vandex BB75 is a cementitious, efflorescence-free surface waterproofer (tanking slurry) with hydrophobic properties, resistant to water and moisture.
  
  Vandex BB75 is applied by trowel or spray to surfaces such as concrete, masonry, and render, which have to be sealed and protected against the influence of water and moisture. Typical areas of application include:
  
  Basement / cellar refurbishment
  Underground car parks
  Retaining walls
  Lift pits
  Tanks / reservoirs / water treatment works
  Sewer / sewage treatment plant waterproofing (BB75Z)
  Fish ponds & water features
  
  Once cured, Vandex BB75 tanking slurry forms a waterproof coating (tested up to 7 bar). It can be employed against active and passive water pressures, and is approved for use in contact with potable water. The initial and final bonding capability of Vandex BB75 is excellent, making it suitable for application to horizontal as well as vertical surfaces. It is durable, resistant to frost and heat after setting, whilst remaining vapour-permeable. The properties of Vandex BB75 can be further enhanced by using the "Vandex PK75" or "Vandex Cemelast Liquid" polymers as a guaging solution - the resulting products being referred to as Vandex BB75 E and Vandex Cemelast.
  
  Vandex BB75 is not intended to provide a decorative finish.
  
  This will save the 500mm dig
  
  Kerryc
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  Susan March 21, 2009 at 10:27am

  Thanks a lot - this is really useful info.
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  David Channon March 21, 2009 at 7:43pm

  When I was involved in the chemical damp course business, we used to inject a chemical course into damp victorian brick walls and it was very effective.
  However, you will need to replace the render with a waterproof one. Look for a tanking mix. The product of choice about 15 years ago was called SIKA and added to the render mix. The DPC injection usually contained silicone is some form.
  Just so that your aware, its ground salts that cause the render to blow. The water migrating through the wall evaporates but the ground salts dissolved within it get left behind. They build up in the brick and any gaps and renders. They are hydroscopic and attract moisture from the air when humidity levels permit. These spots can freeze in cold weather and the expansion caused spalling in the brick and forces render off the wall.
  
  Hope this helps?