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Net clear opening vs rough opening: what it means on an inspection

Rafter Tie editorial teamUpdated 3 min read

A rough opening is the hole left in the wall framing for a window or door. The net clear opening is the smaller space you can actually pass through once the frame and the open sash are accounted for. Code limits on egress openings apply to the net clear opening, not to the rough opening or the window label.

A window drawn from the inside with a dashed rough opening, a grey frame, an open casement sash, and a green rectangle for the smaller net clear opening that remains
The rough opening is the framing hole. The net clear opening is what is left to use.
Use this diagram

How to measure a net clear opening

  1. Open the window fully as it normally operates. Open it only the way a person inside would, without lifting a sash off its track. The code looks at the opening in normal use.
  2. Measure the clear height. Measure from the lowest edge you can pass over to the highest point of the open space, ignoring frame, sash and hardware that block it.
  3. Measure the clear width. Measure the narrowest width of the open space. A sash that stays in the opening, such as the fixed half of a slider or a casement sash hinged at the side, reduces it.
  4. Compute the area. Multiply clear height by clear width in inches and compare the result with 820.8 square inches, which is 5.7 square feet. A double-hung window with a 36-inch wide opening that clears only 20 inches in height gives 36 x 20 = 720, which falls short of 820.8 square inches. Its 20-inch height is also under the 24-inch minimum.
  5. Use the manufacturer's data as a check. Windows sold for egress list their net clear opening. That label is a good starting point, and your own measurement still decides.

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Public Safety and Special Construction

In a new bedroom, the only emergency escape window has a keyed sash lock, with the key hanging on a hook beside the window. What should the inspector do?

Press A-D or 1-4 to choose an answer while this question is on screen.

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Common mix-ups

  • Net clear versus rough opening. Rough opening is a framing dimension for the carpenter. Net clear is a use dimension for the inspector. They differ by the frame, the sash and the hardware.
  • Net clear versus glass size. Glass size says nothing about the opening. A big pane in a window that opens only a few inches fails.
  • Window type changes the result. A double-hung or single-hung window can open only about half its height. A casement window can open nearly its full height. A slider opens about half its width.
  • Net clear opening is a measurement, not a product name. A window sold as egress may fail if it is installed in a way that blocks the sash or if a lock stops it opening fully.

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Clear openings on the B1

This distinction decides many egress questions in public safety and special construction, 17% of the B1. The question gives window dimensions and asks whether it meets the limits, or asks which dimension the code measures. The wall header page relates to the framing side of the same opening, since the header spans the rough opening.

Practice it

Use the egress opening check to test combinations of height and width against the full set of limits. Then read wall header for how the rough opening is framed. The IRC terms glossary links the rest.

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