Light clay straw walls are a great surface for finishing using natural plasters. Because the straw-clay is tamped inside of a stud wall, you have a fairly flat surface to start with, compared to something like straw bale which needs a lot of trimming and often extra thick plaster to achieve an even surface.

Here I’ll give some tips and suggestions for how to prepare a light clay straw wall to receive clay plaster, using our recent outhouse project as our demonstration site. In this post, I’ll talk about how to make and install wood trim, which should be the first step in preparing your newly built wall to receive clay plaster.

There are a few tricks you’ll need to ensure a quality job. Read ahead for my method…

Clay Plastering Light Clay Straw Walls

Ok, let’s start from the top and talk briefly about the advantages and disadvantages of light clay straw, as it pertains to ease of finishing. Like I stated above, a well-packed wall is quite flat to begin with, which means you don’t need nearly as much plaster material (or time spent) to correct the wall to be nice and even. That’s a good thing, since mixing and applying plaster can be very labor-intense.

However, the studs can pose a challenge, since you need to bridge each and every piece of framing with the clay plaster, and this will require an extra step to ensure that the plaster doesn’t delaminate over the bare wood. Note that just because the plaster may seem to stay adhered initially doesn’t mean that it will over the long term.

Also, light clay straw is often a “soft” wall, especially when your wall is only 4″ thick and lightly packed. Without a plaster coat, light clay straw doesn’t have a whole lot of integrity, so you need to be careful about material shifting inside of each stud cavity. Knowing that, let’s get into things…

Benefits of Wood Trim

The very first step in preparing your walls for plaster is making and installing all of your wood trim around windows, doors, corners, etc. This is a very time-consuming process, but the benefit of having all of your trim in place will be obvious. Wood trim provides an excellent plaster stop — a place to cleanly end your plaster at various transition points. It’s also a great visual aide for applying an even thickness of plaster. Where you install trim is somewhat a matter of personal preference, but some obvious places include the following:

• All windows and doors
• Outside corners
• Baseboard
• At the top of the wall

You might also trim inside corners and divide large sections of wall into “panels” for ease of plastering. It depends on what you want.

A recently trimmed window in the outhouse, prior to plastering

How to Make Wood Trim

Here are some tips for how to prepare the trim to be compatible with clay plaster.

Clay plaster over light clay straw walls should be in the neighborhood of 3/4″ thick. That thickness includes both the bases coat and the finish coat. Knowing that, your trim should be approximately 1/4″ thicker than the total depth of your plaster. Let’s say we’re aiming for a 3/4″ thick plaster. That means we should use a true 1″ thick board for making our trim.

Basically, it’s very desirable to have a wood reveal — that means the wood should always be thicker than the finish surface of the wall. For the record, I used 1″ thick rough sawn oak for all the trim in our outhouse. I tried to maintain as much thickness in the wood that I could, and I usually ended up with about 15/16″ once it was planed down.

Clay plaster has a tendency to pull away from wood. Because we don’t want to see a crack between our plaster and all of the lovely wood trim we’re about to install, a key trick to preventing this is creating a rabbet on every edge of wood that comes into contact with the clay plaster. A rabbet is nothing more than the corner cut out of the wood, like this:

Here’s an example of a 1/2″ by 1/2″ rabbet cut into a piece of trim

The rabbet’s purpose is this — when you bring plaster up to and behind the rabbet, any shrinkage that is likely to occur is totally hidden. That means there’s no chance of seeing a crack develop between the plaster and wood. Pretty nice, eh? Planing, ripping, and prepping trim is already a lot of work, and it’s not that much more effort to create the rabbet.

The rabbets I cut in my 1″ thick trim were about 1/2″ wide by 1/2″ deep, which leaves a 1/2″ reveal — the visible portion of wood that projects out past the plaster. If you want a thicker reveal, you’ll need thicker wood for your trim. I’d say this is about as tight as you can go — I don’t think you’d want any less than 1/2″ for the thickness of your rabbet.

Looking ahead for just a minute, here’s what the trim with 1/2″ reveal looks like before and after the base coat of clay plaster has been applied. Note that the shadow in either photo is the void between the wall and the wood.

Clay plaster will get tucked in the void that is created by the rabbet

Here’s a look with the base coat plaster applied

UPDATE: 9/2017 — I don’t necessarily think cutting rabbets in the trim is absolutely essential. In fact, it’s a lot more work than leaving the trim intact. I’m not sure that I recommend it anymore, but I’m leaving the instructions above in place in the event that I change my mind again. 😉

Again, cutting and installing trim work takes a ton of time… don’t underestimate it. However, it’s worth all the time you have, since once it’s done you have very clean plaster stops at all of your major wall transitions. A well-executed wood trim job can advance the aesthetic qualities of your natural wall to the next level.

Looking ahead… clay plaster going up on the outhouse wall

In my next post, I’ll continue with my instructions and describe how to make and apply a clay slip to your light clay straw walls.