Repairing Cracks in Natural Plaster

I recently responded to an email about cracks in lime plaster. The person reported seeing cracks through all three coats of plaster along the joint between the top of the bales and the wood box beam. She has tried a lime wash patch, which made things worse or at least more noticeable, and she was asking what to do. My response is below.

It sounds like the joint between the wood box beam and the plaster is opening up. This usually happens when the wood has not been separated from the plaster. In other words, if the wood was not covered with roofing felt, the plaster would be adhered to the wood. The problem here is that wood moves at a different rate than straw so you get stress fractures at the joint between them. Another potential problem is that the joint was not reinforced with plaster lath or some other structural mesh.

For the same reason stated above you need to span the joint with a structural mesh so that the plaster can hang on to that and the different rates of movement are absorbed and moderated by the mesh. The mixing ratio of the mud could definitely have an affect on things as well. My first assumption, not having seen or heard about the construction of the building, is that it is related to the items above, not the mixing ratio. If so, the only fix is to deal with the problem. If it is a lack of roofing felt, you may need to tear out the plaster and start over in that area.

Shy of such a drastic fix, you could try using fiberglass mesh tape (like drywall tape but get the higher quality plaster tape) placed over the crack and then embedded in fresh mud. This should give the plaster enough strength to withstand the movement of the plaster beneath it. I would certainly try that first.

4 Responses to Repairing Cracks in Natural Plaster

  1. alan  franks
    alan franks Thu, September 13, 2007 at 1:47 pm #

    How do I know if the plaster is cracking because of poor detailing or because of my plaster mix? What if it was just the ratio of sand to lime? You seem pretty sure in this posting that the problem is in the detailing, not the mix. How can you be sure?

  2. Andrew
    Andrew Thu, September 13, 2007 at 1:47 pm #

    That is a great question. Sorry I wasn’t more detailed. The reason I think it is based in the construction detailing and not the plaster is that the crack seems to be very straight. Plaster born cracks are usually random and spread out like spider webs where as cracks born from poor detailing tends to follow the line of the sub straight or joint in question. That is what we are seeing in this situation: a straight line along the no-reinforced joint between box beam and bales.

  3. JB
    JB Thu, September 13, 2007 at 1:48 pm #

    Is stucco the only thing that can be used on the outside of a straw bale house? How about wood, bricks, or stone? What about the inside of the house?

  4. Andrew
    Andrew Thu, September 13, 2007 at 1:48 pm #

    Other finishes are possible on a straw bale home; however, they often require more work than they are worth. For example, adding wood siding would require that the bales be plastered with at least one scratch coat to reduce the fire risk to the bales. In addition, nailers will have to be installed every 16″ to attach the wood siding to. The siding itself increases the risk of moisture damage to the bales so a vented air space is also a good idea between the bales and the siding. All of this is extra work that is otherwise not required if you use plaster. Stone and other masonry products are a bit easier as they can be embedded in the mortar which is placed directly on the bales and no additional nailers or ventilation are required. In general, I find it easiest and most time and cost effective to stick with plaster on both sides of the bales.

Leave a Reply