View Full Version : Calculating the radius and angle for Bow-roof greenhouse

03-10-2017, 07:18 PM
I am trying to draw the arches on sketchup. I decided to have 6m wide structure using 5 meters long one piece timber and followed the instruction on here (http://www.codesmiths.com/shed/workshop/techniques/arches.htm).

The classic Gothic arch is known as the quinto acuto or "pointed fifth". The arcs are 4/5 of the span. The centres of the arcs are inside the span of the arch.


I ended up with this:


The problem was the joints, they did not align, so I had to rotate the arch.


I looked into different calculations and drew 5 different arches:



My question is: Should I stick with the suggested calculation method (4.8m radius and 59.68310 degree angle) and force the bottom joints and post to stick together or go for the wider angle and lower radius (4m radius and 71.61972 degree angle) so that the bottom joints will attach to the post by itself?

Gib Etheridge
03-10-2017, 09:52 PM
Getting to technical.

The second one looks much better.

Just bevel the bottom of the beam so the arch mates to it's inner and outer faces. Make the beam narrow enough that it will fit between the inner and outer bands of the arch.

03-10-2017, 10:03 PM
I would just go to shamrock aquaponics, Spend the $25 usd, and buy the engineered plans with instructions and be done with it.

My building just survived 81mph winds two days ago and I got my plans from them.

I appreciate if you want to design it yourself but damn, that's getting really technical when you're dealing with fractional degrees.

David G
03-10-2017, 10:49 PM

03-10-2017, 10:55 PM
Wow, you are really overthinking this. Bigly. Just get some wood and build that thing!

03-11-2017, 12:08 AM
I built my bow-roof boat shed with a vertical post attached to each arch, instead of a separate knee wall, to avoid that issue and make a stronger structure. When you set up a form on the shop floor for the first one it will take a shape that works. I posted some photos here in a thread a couple years ago. Good luck! Tom

03-11-2017, 01:41 AM
Do as Gib said.

when i built mine, i took a 2x4 and set it upon the knee wall. I beveled the outside edge to match the arch (approx) and made it narrow enough to set inside the arches. I made it so the outer strapping of the arches ended up flush with the outer edges of the knee wall.

good luck

Skiff Man
03-11-2017, 07:13 PM
These bow structures are designed as frames, not arches. So the bottom of the bow-frames should be vertical or you will get a lateral arch thrust. You could angle your knee walls to match the angle at the bottom of the bow-frames. Also, your ridge beam should be taller than it is wide, and add a steel tie across the top and and a collar tie at each bow-frame.

03-11-2017, 08:58 PM
Instead of a ridge beam I made a cap out of 2 1x4s. The cap fits over the top of the bows. That made it really easy to erect. And gives great support for the tarp/plastic/whatever. I connected the bows together on the ground and swung them up into place with a push pole.