"Where a window has a greater projected area than it does a developed area its surface area ratio should be entered in the relevant windows 'Surface area ratio'. This can be found in the corresponding envelopes Envelope Details screen. The surface area ratio is defined by: ""Surface area ratio – This is the “developed area to projected area” ratio for the window or rooflight. The developed area is the total area of the glass plus the frame, and the projected area is the area of the opening in the wall/roof. Therefore, for domed or conical rooflights, for example, this ratio would be larger than 1. It cannot have a value which is less than 1.""