14 A note about that threshold Given that the greenhouse door is not typically built with a threshold, it is suggested to place one continu- ous row of 4x4’s or 6x6’s all the way around the pe- rimeter of the greenhouse. For those greenhouse gar- deners wishing to add a raised wall or some height to the greenhouse foundation, the bottom row of timbers or concrete footing would remain lower. This allows the door to open and close easily, prevents gravel or dirt spilling out of the doorway and creates structural strength so that the door opening will not shift. Place your pressure treated wood beams and add a barrier between the wood and the aluminum frame. Staggering timber joints on a multi-layered timber foundation is essential for a solid wood foundation.