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Internal cob walls

We live in a house with an original cob section that was extended with rendered block work extensions during the 1980s and 1990s. As a result, the external face of the front if the building is a cob wall and the former cob external walls are now internal walls.

Before the extensions were added, the house had been rendered with a cement based render and the internal faces of the walls had been plastered with gypsum or cement plaster.

We had the external render on the front of the house removed and replaced with lime render.

My question is: is it necessary for us to remove the existing plaster from the internal faces of the cob walls and replace it with lime plaster or can internal cob walls reasonably be left covered with cement or gypsum plaster surfacing?

10 thoughts on “Internal cob walls”

  • Adam Brown

    Good Morning

    This is a good question and one I am asked regularly, in terms of performance and moisture management the short answer would be yes, however in reality it is not always done, at least not straightaway. You have rendered the external face with lime and this is always the area we would recommend you tackle first, as this is where the majority of water/moisture can enter the wall and also escape.

    The existing plaster can cause problems by holding moisture within the wall and the material (cement, gypsum etc.) is too hard for the cob and can cause further problems, however removing this material can also cause issues so if you are removing this you would have to take care not to damage the cob. In a lot of cases removing the plaster can pull away the face or sections of cob so you may need to make allowances to repair/replace sections. As i mentioned above the technical answer would be to remove the plaster and re-plaster with lime but this can be a large undertaking. It is highly likely that the current plaster will be subject to issues with moisture at some point, but a lot of people will wait until the issues arise and then look to undertake the work. If you wanted to give me a call on 01208 79779 I can discuss this in much more detail.

    Kind Regards

    Adam Brown.

    Reply
    • Peter Paul

      Hi, we are planning to build an extension which will result in a cob external wall into an internal one. Do we need to plaster this with lime plaster?

      Many thanks

      Reply
      • Adam Brown

        Hi Peter

        Ideally both sides of this wall should have lime plaster to allow for breathability, but also the strength of the plaster is much weaker and more suitable for the softer/weaker nature of the cob.

        If you would like to contact me on 01208 79779 I can discuss this in more detail.

        Kind Regards

        Adam

        Reply
  • Peter Paul

    Hi, we are about to build an extension which will make a currently external cob wall into an internal wall. As this wall is cob do we need to lime plaster this wall?

    Reply
    • Adam Brown

      Hi Peter

      Ideally both sides of this wall should have lime plaster to allow for breathability, but also the strength of the plaster is much weaker and more suitable for the softer/weaker nature of the cob.

      If you would like to contact me on 01208 79779 I can discuss this in more detail.

      Kind Regards

      Adam

      Reply
  • Hazel Matthews

    We have recently bought a house that has a cob wall that has been faced with cement blocks and pebble dash rendering. A surveyor has asked that the block and pebble dash be removed but we are worried that this might cause more problems than it solves. Can you advise us what to do.

    Reply
    • Adam Brown

      Hi Hazel

      Unfortunately this is not an easy question to answer, especially without seeing the property or the walls in question. I can see why it is being advised that the cement and blocks are removed, but removal can also present its own issues. If you are able to contact me on 01208 79779 we can discuss this is more detail.

      Kind Regards

      Adam

      Reply
  • Neil Taylor

    Hello,
    I need to drill through an internal cob wall to put in some gas pipe and electric cable. What's the best way to go about it?

    Reply
    • Adam Brown

      Hi Neil

      There is no right answer here as far as we aware. We would likely recommend a drill with an appropriate sized bit for the pipe, looking to cause the minimal amount of disturbance within the wall.

      Kind Regards

      Adam

      Reply
      • mike williams

        Hi Neil

        I've only just seen your question so you've probably solved it by now but I came across the same problem myself. As soon as I pulled the drill out, the hole filled up with debris.

        Use a long masonry bit to go right through the wall (go slow as the bit exits the far side otherwise you might lose a large chunk of plaster or render). Disconnect the bit from the drill and leave the bit in the wall. Gently drive a length of galvanised conduit over the bit (you should earth bond the conduit) and pull out the bit.

        Easy when you know how.

        Reply

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