July 2017 - Home in the Earth

Stuccoing the skylight curbs

Posted on July 1, 2017 by Simon

Contents

The Video
The Mix
Improved mix
The Gallery

While we were waiting for the shotcrete crew, we continued on some side jobs. One of which was insulating the skylight curbs and protecting them with stucco. Detail on our mix recipe, etc. later, but first, the video.

The Video

The Mix

Here, we used the scratch stucco recipe of

5 buckets (25 gallons) of sand
94 lbs (1 bag) of portland cement
17 lbs (1/3rd bag) of hydrated lime

We mixed this all dry and then used buckets of the mix, mixed with water (about 20 cups) using a drill mixer. After applying the stucco (it naturally forms a layer about 3/8ths of an inch thick), we used a “scratch” tool to give it texture so the second layer would grip it well. Then we left it for a few weeks to cure and watered it often to increase the strength during the cure period.

The recipe for the brown coat was very similar, except with 10% more sand and lime to weaken it a little bit. You want the brown coat weaker so that if it tries to shrink as it cures, it will crack instead of cracking the scratch coat off the wall.

Before applying the brown coat, it is a good idea to wet down the wall. Otherwise the moisture from the fresh stucco is sucked into the scratch coat and it is difficult to work with.

After applying the brown coat, we continued to water it for another couple weeks, again to improve the cure and the strength.

Improved mix

As I mention in the video, along with our improving stucco skills, we also experimented with the mix and found that adding a quart of thinset after mixing in the water really improved the workability of the stucco. It also improved the stickiness (important for ceilings) and gave it some waterproof characteristics also.

The Gallery

Again, sometimes the easiest way to tell a story is as captions on pictures.

: The skylight concrete would have leaked a lot of heat. So we needed to wrap them in insulation. But first we needed some wood to screw the insulation too… Necessity is the mother of invention.

: This side of the skylight needed more insulation because it included some vertical ducts. I decided I would need to extend the electrical box a little. This was not enough.