Ultra High Quality and strong frame. All units are constructed from either 18mm OSB Board , or High Quality 18mm WISA Spruce Structural Plywood.Cross section of 100mm deep allowing you to fill the cavity with insulation.All frame arches when built are 36mm in thickness making them a very professional sturdy construction.Frames are pre drilled at precise points allowing for you to run electric cables etc through the cavity if required.