How NOT to Plaster a Wall: Avoid Cold Joints Whenever Possible

Written by Andrew Morrison

Partially Plastered Wall with Cold JointsIt’s not unusual for people to get excited about plastering their straw bale structure. After all, this is when a simple structure starts to look like a home. When the plaster goes on, suddenly, the building gets a new lease on life and it really begins to feel like completion is near. This is also a very difficult aspect of the construction process and one that will be very visible for the life of your home.

When people come to visit, the plaster is likely the first thing they will notice about the home. It will either look great and wow them, or it won’t. Of course, you’re likely hoping for the wow factor! Okay, back to my point. Take a look at the photo above. What problems do you see? I see one major problem that happens far too often.

The plaster has been applied by several different people and there are a series of wet edges all drying at once. Here’s the biggest problem: this picture was taken during a lunch break and so the “wet edges” were left to dry for a while. These dried edges will ultimately result in cold joints, areas where plaster of different wetness is in contact. The fresh, wet plaster will not fully bond to the drying plaster shown here and a cold joint will result. That’s not a huge deal in this case because we’re looking at a scratch coat and cold joints are common throughout the field on scratch coats. That said, it is always best to limit the cold joints in your scratch coat as best you can.

People plastering a straw bale house

As you can see in the picture to the left, working with lots of plastering hands is easy to accomplish while keeping a wet edge. Each person simply sets up near each other and works from the bottom up (or top down if you prefer) so that they overlap on the edges. Section by section, the wall is completed with a constant wet edge in contact with a wet edge. Should you break for lunch or some other reason, it’s best to stop in a straight line that will be located differently in each coat.

In other words, don’t always stop 1′ from the ceiling when you take a break as the cold joints will then travel through all of the coats in which you stop in that location. You can imagine though that if this crew stopped for lunch now, their cold joint would be much more manageable than the one shown in the first picture.

If you’re interested in really learning how to build with bales and you want to have perhaps one of the best weeks of your life in the process, then come to one of our workshops. We ALWAYS have a good time and you will gain the confidence to build your own house too. CLICK HERE to see what workshop locations and dates we have available this year!

21 Responses

  1. Yes you do. It’s a thing of beauty, eh? I spoke with Ben yesterday and they are starting the brown coat today. They were mixing plaster last night and looking forward to a long, tiring, yet fun day with a group of about a dozen people.

  2. Wetting the edges slightly does help; however, if you’re too aggressive, you can wash the lime out of the mix. If this were earthen plaster, then wetting would work very well. Lime plaster is different in that the curing process and the drying process take place simultaneously and wetting the edges will not give you any advantage on the curing in terms of relieving the cold joint. Like I mentioned earlier, the plaster is not ruined by any means, but it’s a best practice to minimize your cold joints.

  3. Andrew- is it possible to use fiberglass drywall tape (or some kind of mesh like that) to overcome the weak areas caused by cold joints? I suppose you wouldn’t want that on the finish coat, but if you planned properly, stopped on a straight line and didn’t have the scratch & brown joints on top of each other, would that work?

    Unfortunately, our new home will be the first structure I’ve actually seen a brown & finish coat put on, and so am trying to anticipate the times we won’t have a large crew handy, but still must get the work done on a schedule. Obviously, if the issue is more aesthetics than structural strength, the tape might not make a difference.

    Ted T.

  4. Hi, Just wondering if anyone can give some good information about the top coat in an earthen render and how to give it some sort of resistance to the weather (we have a verandah but driving rain can still blow in and could cause some problems,) Cheers Lee 🙂

  5. I noticed that in the photo there was no sheetrock on the ceiling. When I built my house I sheetrocked all the walls and ceilings that were going to intersect the plaster BEFORE I plastered. This made the sheetrocking much easier because I had no wiggly surfaces against which to fit the sheetrock.The finished product looks seamless.

  6. Yes. This is the proper way to do it too Gilberto when the time line is open for it. In a week long workshop, we don’t have time to install the drywall after the bales are raised and before the plaster is applied. As such, the drywall will be hung before the brown coat of plaster. It can still be cut to a straight line and the brown coat will cover any variances in the wall to ceiling transition.

  7. Oh yeah, and it sure pays to plaster one room at a time even if you have to either work late or quit early before starting on the next room. When we plastered the living room we started early and worked late. Two years later we have only four hairline cracks throughout the house. I still have not figured out why these lines appeared.

  8. Re: weather protection, I am building a hybrid cob/strawbale house now, using earthen plaster. We are planning to apply a lime wash, with pigment, over the earthen plaster. I guess that is slightly different from having an earth plaster top coat, but we had the same concerns about weathering, so this is what we decided to try. Will report back with the results after a pacific north-west winter! 🙂

    Re: cold joints – is the result of this that you get cracks in the plaster? Any other suggestions for preventing cracks in earth plaster? We are misting the wall with water before and after applying plaster, but still get cracks. I know it’s an art to get a nice looking plastered surface, but any suggestions, things to look for or try, would be great, thanks.

  9. Earth plaster is different from lime plaster. If you are misting the walls, keeping a wet edge while plastering, and doing everything else “right,” you may want to consider the mix itself. It may be too clay rich and thus when it dries, it cracks. Too much clay causes cracks. Too little clay makes the plaster weak. It’s a balance and, as you said, an art.

  10. Hi Ted. The best thing is to plan for a stopping point. As long as you are smart about where you have your cold joints in the scratch and brown and don’t line them up as you mention, you will be fine. For the finish coat, a plaster stop is a great idea on the face of a big wall. It’s best to install that stop before you start plastering, so that may not be an option for you now. I don’t think the mesh is needed in the scratch and brown in this situation and it would not work in the finish.

    Remember to send me pictures of your place Ted!!! Send them to [email protected].

  11. I have been a plastering contractor for 32 years and I give 3 guarantees with sand and cement plaster.

    1. It will get Hard.
    2. It will be Gray
    3. It will crack.

    Of course I am being fecitious, but given that plaster is an architectual coating and not structural it does seem only reasonable

    I have plastered over many different building systems including Strawbale, and inevitably cracks appear. Even when I go overboard in using all the techniques I have learned over the years to eliminate them they still “Mysteriously” appear in illogical places. Sometimes homeowners and contractors have unrealistic expectations about Portland cement plaster and don’t realize that cracks pose no threat to the structure.

    There are some very effective Crystaline cement additives that will waterproof cement based finishes. KIM (Kryton Internal Membrane) is one I have used with excellent results (On my own homes)

  12. Hi there,
    What is the best way to get a smooth plaster edge where the wall & ceiling join? We have used a product called econodek for our roof-ceiling-in-one, which is actually colorbond (steel) on the ceiling surface. After the first coat of plaster has gone on, im’ realising its very difficult to get a straight, smooth line where the wall joins this metal ceiling! Any ideas?
    Many thanks,
    Rosie – Perth, Australia

  13. Thanks Kevin. I could not agree more about home owners sometimes having unrealistic expectations about cracking. It’s important that we all (home owners that is) realize that cracks WILL appear in the plaster over time. Keep an eye on them. Make sure they are not telescoping all the way through to the bales. Maintain them as necessary, but in most cases, don’t worry. In fact, in almost all cases, the cracks are merely cosmetic and likely don’t penetrate through the brown coat of plaster.

  14. Hi Rosie. I don’t fully understand the material you are joining to the bale walls at the ceiling line. Sorry. Is it exposed metal? Can you trim the wall/ceiling line out with a finish trim? Perhaps the plaster will turn the corner, but I don’t get the material clearly. Any more detail or a link to an example would be great.

  15. Hi Lee and Andrew

    “Just wondering if anyone can give some good information about the top coat in an earthen render and how to give it some sort of resistance to the weather (we have a verandah but driving rain can still blow in and could cause some problems,)”

    Silicate paint seems to be the bee’s knees for this. Extra $ but well worth it, according to some builders around here (Moab).

  16. I am a fan of the silicate paints as well. They are your best bet for protecting earthen plasters in my opinion. I recently recommended (on another comment) that folks interested in earthen plaster contact Bill and Athena Steen at the Canelo Project as they are true experts on the subject.

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