I built a 12' wide greenhouse to see how hard it would be to join shorter boards to make the arches. It was a hassle and ultimately I made an arch by cutting strips off a 12' 2x6. This greenhouse can be extended in 4' increments. The incremental cost is likely less than $1 per square foot and possibly as low as 50 cents per sqft. Planning information here: https://www.youtube.com/redirect?even...