How to Calculate the Size of Egress Openings

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You can calculate the size of egress openings.
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If a fire breaks out and spreads rapidly while you are home, getting out of the building is your number-one priority, but what happens if you're in the basement, and the windows are too small to fit through them? This is why the International Residential Code requires egress windows in the basement and in bedrooms on the first three stories of any building. Apart from the fact that climbing out of a fourth-story window doesn't provide a safe escape route, few homes have more than three stories anyway.

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The code specifies the minimum height and width of an egress opening as well as the minimum amount of open space for both egress doors and windows. This last requirement is important because if a casement window has the proper length and width but the window doesn't open far enough to allow a person to fit through it, it isn't much good as an egress opening. The code also specifies the maximum height of the window sill from the floor because the opening isn't helpful if it's too high for someone to reach.

IRC Code Requirements for Windows

According to code, the minimum height for an egress window is 24 inches, the minimum width is 20 inches and the window must have a total open area of 5. 7 square feet. A window that measures 24 x 20 inches from the outsides of the frame doesn't qualify because when you measure its open space when fully open, it will be less than the required minimum, often only 3.33 square feet. Further requirements include:

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  • The window must open from the inside without the use of a key or tools. If there are bars or grilles, they must also open from the inside.

  • The window sill can be no farther from the floor than 44 inches.

  • Ground-floor egress windows can be smaller, with only 5 square feet of open space, because no space will be taken up by rescue ladders.

Note that when a room is converted into a bedroom during a remodel, a new window may have to be installed to meet code requirements. The amount of free open space depends on the window style and frame material, and window manufacturers provide calculators on their sites. You input the rough opening size and choose a window style and material, and the calculator tells you whether that window provides the required space.

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Requirements for Basement Windows

When a basement windowsill has to be below ground level to keep it 44 inches from the basement floor, the window needs a window well, which is a dug-out space outside the window to allow egress. The well must allow the window to be fully opened, and it must have a floor with a minimum area of 9 square feet. If the well is deeper than 44 inches, a ladder or stairway must be provided. The ladder must be at least 12 inches wide and project above ground level no less than 3 inches.

When a basement egress window is placed under a deck or porch, the top of the window well can be no less than 36 inches from the bottom of the deck or porch floor joists. Because that leaves a bare minimum of space for crawling out of the basement, firefighters and building inspectors may increase the minimum to as much as 60 inches.

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Requirements for Egress Doors

Every dwelling unit must have at least one egress door. The door opening must have a minimum width of 32 inches as measured from the face of the door and the stop when the door is open 90 degrees, and it must be at least 78 inches in height as measured from the threshold to the stop on the top jamb. It must be easy to open from the inside, requiring no key or special knowledge.

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Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker.com.