Window Sills: Where the opening of an operable window's sill component is more than 72" above the finished grade, the lowest part of the opening must be at least 24 inches above the completed floor surface. For windows that are less than 72", the minimum opening height is 36". Sill heights should be measured from the bottom of the opening to the highest point on the outside of the building. If you want to know the sill height for your house, use a measuring tape and measure each window sash separately. Add up all the recorded measurements and divide by two for the total window siding height. The National Fenestration Rating Council (NFRC) provides guidance on window sill requirements in its publication "A Guide to Window Safety Features." The guide states that sills must be able to support the weight of people or objects that might fall through the opening. Therefore, sills must be high enough to prevent someone from falling out of the opening. The NFRC also recommends that sashes be glued and screwed instead of nailed together for security reasons. The organization says this method prevents screws from being removed by vandals who may find them disturbing when camping in close proximity to your home. The sill height requirement is not one that will surprise you if you have ever tried to open a tall window.