When making your measurements, it’s important to take into account the height of your decking and joists. In our case, we measured 22mm down from under the sill to allow for the decking and added 90mm to allow for joists – a total 112mm. We then marked this at each end of the sill. We also measure from the ground to this point to establish the height measurement. We then run our string lines at this height around the perimeter of the area.