Finishing cob with plaster.

Hello everyone! I am very new to the cob building method and I recently saw some articles and images online that show a cob house or building being finished with some sort of plaster coating. I kind of assumed that the cob would be enough when smoothed, but these folks had very rough finished cob structures and the smooth texture of the walls came from a finishing plaster of some sort. I imagine the natural plaster threads will explain how this works, but I was wondering what others, who have used cob or have experience with the substance, think about finished vs. unfinished.

Any suggestions? I want to start working on a RMH design this summer as well as maybe building a small cob shed to get some practice before we go full tilt on a house one day. Thanks!

Are you inquiring about indoor or outdoor finish?

Lucas...I've plastered walls (interior walls, that is) with base coat and troweled it smooth, and never applied a finish coat. I like the looks of it, but I worked it with a trowel so it looked pretty. I have also left some walls unfinished...meaning I haven't troweled them smooth. I'm not that keen on the rough look...but that's a personal thing. I built a straw bale cottage at my educational center on our property in Missouri and hand applied the interior finish. It's pretty rough, and looks ugly to me. I am going to offer a workshop this summer to finish it. You can find out about the workshop on my website at www.evergreeninstitute.org. Just click on the schedule...Dan

Well, after seeing the pictures of the indoor plaster from the Natural Plasters film, I know that I am definitely interested in finishing the inside. So I guess my question is geared at exterior plastering. Like I said, I saw a cob building being finished and I always assumed that the final troweling made the surface smooth and beautiful. But this series of before/after shots showed a rough, straw and clay exterior wall that was not smooth or pretty. Then they covered that with some thick plastering and viola! Beautiful!

Fiber-rich finishes will generally hold up better to weather than raw cob. If the weather is bad and you don't have long enough eaves, sometimes a lime plaster will do it. (sometimes you need something like a porch roof or awning halfway down the wall, if it's a tall gable-end wall.) Graeme North from NZ has some good white papers on moisture detailing for maritime climates.

Never use cement-based stuccos or plasters on a cob wall, they will trap moisture in the cob (usually at the base of the wall) and can result in collapse of otherwise sound walls.

In some parts of rural England and Brittainy, it's not unusual to see the weather-side plastered and the other sides left unfinished, or re-plastered less often. I met a fellow from Brittany once who was on his way to Cob Cottage to learn earthen building techniques because his grandparents had an older section of their house/barn that hadn't been re-plastered in 80 years, and the family had decided someone ought to go learn how it was done in case it needed doing in another generation or so. So there's nothing structurally wrong with cob by itself, given adequate footings and eaves it will gently weather at a very slow rate.

I've seen cob 'burnished,' built smooth and then worked over with a steel float when it was leather-hard, to work all the straw stubble in. It can be worked over with a smooth stone or glass bottle, and then waxed or oiled if desired. It can be quite beautiful, you get little flattened strands of straw showing like a deckle glitter through the oiled surface, but I'd say plaster is easier.

You can do almost any level of plaster you want, from something really rough (say a screened cob mix, or cob made with graded builders' sand, with some horse dung instead of straw) just to patch and fill any dents. 'Harled' (thrown) plasters. Rough or textured troweling, like faux-old stuff. Smooth-floated plasters are the master-craftsmanship of which a good drywall job is the pale imitation. You don't need to do multiple layers at first; you can do a 'brown coat' and just leave it on there until you decide you want to upgrade.

-Erica W