Traditionally this would be done with tar paper, but I think that's increasingly falling out of favour, making way for more modern fabrics. I opted to install HydraShell by Marco Industries.
A testament to the simplicity of this phase of construction is that we completed it in a single morning with two of us snapping away on our staple guns.
Five sheets allowed us to complete the roof with a minimum of 3" overlap. We tucked the ends around the boards at the gables and stapled them up underneath. Grandpa at first thought we would lay some board up and down each edge, but they went down tight enough, and with the wrap at the ends, we decided to forgo this precaution.
That night we were treated to some of the heaviest rains I have experienced in the yurts, and the shell stood up fine.
We have subsequently experienced rain every single day since it has been completed, and aside from rain blowing in from the sides and wetting the base layer of my logs and the window sills, the interior of the sauna appears to be dry. I am looking forward to getting the steel installed on the porch and roof sections though, as that will give us a large, dry work area to complete construction.
Apologies for no "in progress" pictures, but things went up just that quickly that Donna didn't even have a chance to put her skills to use.