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Our one-story house has a very large window on the first floor -- appx. 72" high by 130" wide. I'd like to add a basement egress window to the room below. I'm worried that having more glass than wall on this side will somehow compromise the structure. Is that a realistic concern? E.g...

  • Does cutting the foundation for an egress window put more lateral stress on the foundation above/below the egress window?
  • Does having an egress window put more lateral stress on the above-grade wall?

In case it matters, the wall is a gable wall. This wall supports a beam in the ceiling of the 1st story and the foundation beneath this wall supports a beam in the floor of the 1st story, but in both cases the beams are a few feet away from the window opening and the proposed egress window. Floor joists and ceiling joists run parallel to the wall in question.

The house is 60+ years old and there is evidence of some small displacement (1/4") between the corners of the very large first floor window and the floor/ceiling.

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    You haven't said what type of foundation you have. That's really the only concern. If you properly head the new opening nothing really changes. – isherwood Dec 20 '17 at 19:05
  • Unsolicited pedantry: "lateral" means sideways or horizontal (across the primary axis, whatever that may be--in our case gravity). Is that what you meant? – isherwood Dec 20 '17 at 19:06
  • I did intend to say lateral, but I welcome cautionary advice along any axis, so pick your favorite! I assumed that a header would distribute vertical load, but I really don't know how important the walls are in keeping perpendicular walls upright. If you cut enough cardboard out of one side of a shoebox, it tends to get a bit floppy -- the other sides buckle, the top sags, the bottom bows. Same thing in a house? – hoosteeno Dec 20 '17 at 20:55
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    Lateral stress isn't usually a concern with foundations except with respect to backfilling. Your questions as they're written are a bit nebulous. Really, it's a matter of engineering a header/beam to carry the span across the window. Consult a local expert for that. – isherwood Dec 20 '17 at 21:06
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    Not sure how an "egress" window would be a major concern under an 11-foot-wide window. In most jurisdictions the egress only needs to be 20 inches wide. E.g., NFPA 101 (2015): 24.2.2.3.3. – Upnorth Dec 21 '17 at 6:03

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