How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
How much can I cut out of a floor joist?
Friday, September 28, 2018
The rules are divided into the categories of notching (cutting away an area of wood at the top or bottom) and bored holes (drilling through the center).
- Notches in the top or bottom not to exceed 1/6 of the height of the joist/rafter.
- The length of the notch cannot exceed 1/3 of the depth of the joist/rafter.
- No notches in the center 1/3 of the span.
- Notches at the end of a rafter/joist cannot exceed 1/4 of the the height
- Notches and bored holes cannot be in the same area.
Bored hole requirements:
- Holes cannot be more than 1/3 of the height of the joist/rafter.
- Holes must be a minimum of 2 inches from the top, bottom and other holes in the joist/rafter.
The worst offenders who ignore these safety standards are plumbers and HVAC contractors struggling to retrofit piping, ducts, or an air handler through a space obstructed by a structural member, like in the photo at the top of the page.
Also, see our blog post How much of a roof truss can I cut out to make a storage platform in the attic?
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To learn more about exterior walls and structures, see these other blog posts:
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