Roofing a Geodesic Dome Greenhouse

Since we are planning to try to make this a year around greenhouse in a colder climate, we’re going to install two types of roof system. A standard roof on the north side and corrugated polycarbonate on the south side. This allows us to insulate the north side where we don’t get much sun and it saves us money because polycarbonate is not cheap.

Assembling the polycarbonate

Besides the process of cutting out every triangle, we need to do a bit of assembly work on each panel. A lot of sources on the internet say to just angle the channels down so the water will run out, but the channels will grow mold and you lose the insulating quality.

First we clean up all the edges from cutting, then we apply foil tape to each edge to seal the panel. We then cover the tape with poly U-channel to protect the seal.

During this assembly process we don’t want to remove the printed side plastic film from the poly until the panels are installed. This side has the UV resistant coating that will prevent these panels from yellowing.

Installing the polycarbonate

After we finished assembling all the panels, installation moved quickly. There are a few steps to our install process:

  1. Apply double sided glazing tape to the inside of the panel.
  2. Align panel and stick to frame.
  3. Apply foil backed butyl rubber tape to seams
  4. Cap seam with a strip of pvc attached with screws with washers

The goal of this process is that we never actually puncture the polycarbonate panel. We run the screws between panels down the seam. The glazing tape is really just there to cushion the panel on the frame and allow expand/shrinking movement along with making it easier to install the panel.

We made a mistake

Against my better judgement we used tape to seal the seams. This tape was highly recommended as a permanent sealing product. However the catch is that it is not self sealing like the ice and water shield we used to waterproof the exterior. This means that when we attached the pvc strip and put screws through the tape, it did not create a water tight seal around the screw. Needless to say when it rained we had a few leaks…. ok a lot of leaks.

How we fixed it

We were forced to remove all the pvc strips, cut the tape down the seams (we weren’t getting it off), and fill the gap with silicone. We used about 3 cases of silicone to plug all the leaks. We made sure it was the exterior silicone type 2. This stuff is made to be weather proof, UV proof, and last for years.

We’ll do it better next time

In hindsight if we skipped the tape and used silicone down all the seams, this would be bulletproof. Also note that it would probably be a good idea to add a bead of silicone inside the U-channel of every panel. The U-channel doesn’t quite seal tight enough to keep water out during a super heavy rain.

Installing shingles

We did a lot of research into various roofing techniques thinking that we could abstract a method out of standard practices. The outcome of that was that we though that we would need to add ridge caps or ridge shingles down every seam. As you an imagine, this would not result in the most attractive solution.

After a lot of searching we found a few examples of shingling each continuous triangle, then overlapping the triangles above those. I know I did a crappy job of explaining it, but check out the image below. The colors of the triangles aren’t important, but you can see the continuous sections I’m talking about. This way you always have a downward overlap all the way down the roof and it looks really nice.

We started by taping all the seams with foil backed butyl rubber tape. This is most likely unneeded because we caulked all the seams. However, we kept reading about how domes typically leak, so we figured better safe than sorry. We also installed proper flashing around the base of the panels and remembered to use roofing cement under every shingle tab. The roofing cement is needed when you’re close to vertical on your roof pitch.

We also installed floor level vents and a large exhaust fan. This most likely won’t be enough air circulation, but we thought it would work for now. The exhaust blower is rated at 1500CFM which is almost triple what is needed for the space.

We then applied the underlayment and spent the next 48 hours up on a lift installing the shingles. Worst. Job. Ever. My dad was with me the whole time up on the lift and I really couldn’t have gotten through this without him. I can honestly say that I would rather have spent the money on more polycarbonate than have to roof this side of the dome again, but damn it looks pretty.

Well that’s it! The building is sealed, the roof is done, and the grass is back, but we’re far from done. At least we’re enclosed and ready for winter. We started early June and finished around early September.

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