I'm going to install a lean to shelter onto the house with timber plate bolted to the house and the other onto timber posts. Plates will be 6x2 and rafters 4x2. How do I best calculate the pitch? I understand how to do the calculations etc, but I'm getting confused because the top end of the rafter will be flush with the top of wall plate where bottom will be birds mouth. So if measuring from outside of bottom plate where do I take measurement to on the top wall plate on the house? Cant be as simple as going 3" down on the plate if birds mouth is 1" in at the bottom as the rafter will be at an angle so distance will be different? Hope someone is able to help! Thanks in advance.