We decided to put an earthen floor in the cottage because we wanted to minimize the amount of concrete used in the build. Earthen floors are softer under foot than concrete and have an aesthetic that we like. I also tried installing an earthen floor in our shed several years ago. It developed some cracks after a while and was still functional for a shed but I felt like putting an earthen floor in the cottage was a shot at redemption.
An earthen floor is kind of like a layer cake made of rock and dirt. The cottage’s foundation is a rubble trench topped by a concrete grade or perimeter beam. The top of the grade beam is about 8 inches tall. The goal is to have the finished floor height be just over the grade beam. We start with a drainage layer of gravel 3 – 4” thick followed by vapor barrier, then more gravel, road base and finally a ¾” layer of cob sealed with several coats of linseed oil. For the most part we followed Sukita Reay’s methodology for building an earthen floor.
The Drainage Layer
To prevent water wicking up from below we put down about 4” of gravel. I started with 1” river rock gradually shifting to 1/2” inch crushed limestone. I tamped it down as I went. In retrospect I think it would have been better just to use all crushed lime stone. River rock is rounded and doesn’t tamp down as well as the crushed lime stone.
After unloading several yards of rock I put down a 6 mm plastic liner as a vapor barrier. I followed up the plastic liner about an inch and a half of more crushed limestone. I tamped it down and tried to get it fairly level before bringing in the road base.
Roadbase is a mixture of dirt and small aggregate that can be be compacted to make a foundation roads, paths or in this case an earthen floor. I did a lot of raking and tamping. I also misted the road base to get better compaction.
After putting down most of the road base I used my laser level to put down grade stakes in a 2 foot by 2 foot grid. The top of the grade stakes are level with the top of the concrete grade beam. I then filled in low areas with the remaining road base.
For the cob layer I used a mix of 80% masonry sand, 20% C&C Ball Clay and finely chopped straw. C&C Ball Clay is a very light gray which is why it looks like concrete. I could have added tint in the cob mix but I’ll add it to sealing oil instead.
To maintain a consistent level I ripped ¾ inch plywood into strips and laid them across the grade stakes. After a section was filled in and level I slid out the strips and filled in the void with a little more cob.
Creating cold seams with cob is not as detrimental as it would be say with concrete but my goal was to finish the main area of the cottage in a single day. The main area is roughly 130 square feet and took about eight hours.
I tried to inset a pattern in the floor using a different color cob. This experiment ultimately proved unsuccessful. I think that if I had let the surrounding cob dry and be sealed before infilling the pattern it would have worked.
I let the floor cure for a little over a week. There was some cracking and I wanted to apply a skim coat before sealing the floor but I hadn’t decided on a final floor cover so I put down cardboard to provide some protection and went about prepping the cottage for interior plaster. See link above for that video.
So about a month later, after finishing the interior plaster Jean and I put down a ¼ inch layer of sand, clay and really finely chopped straw filling in cracks.
A few days later I started the process of applying linseed oil. Linseed oil seals and waterproofs the earthen floor. I mixed red iron oxide powder in with the linseed oil to stain the floor a deep red. I just poured out small amounts and then spread it around with brush. About half an hour a later I came back and wiped up the excess. You can see there are still some fine cracks in the skim coat. During the clean up I rubbed out the cracks and that seemed to close them up pretty well. I took about a gallon of linseed oil for ~150 square feet of floor.
After the first coat the stain was pretty uneven. Jean applied a couple of more coats of the next couple of days and it evened out quite nicely. With the subsequent coats I cut the linseed oil with thinner so help with penetrating the floor. Here’s a picture after the third application.