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How far can you cantilever a bumpout?

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Q: How far can you cantilever a bumpout?
How Far Extend Bumpout
(c) Lee Wallender; Licensed to About.com
Q: "I want to build a bumpout to my 2nd story kitchen. As you can see, it would be impracticable to extend the bumpout downward and build foundation footers. So I'm interested in cantilevering the bumpout so that I don't have to build a foundation. How far can you cantilever a bumpout?"

A: A bumpout is a micro-addition to a house that adds a few square feet and enhances an existing room. It does not create a separate room (though some will define a bumpout as such).

Normally, when you design any kind of addition--bumpout or otherwise--you decide what you need and then you build around it. But when you choose to cantilever your bumpout, you are letting the dimensions of your house's joists dictate the size of the bumpout.

Generally, bumpouts can be cantilevered a couple of feet. More than two feet is unusual.

To calculate how far you can go:

  1. Go into the basement, crawlspace, or any place where you may have exposed joists.
  2. Measure the depth of a joist. By depth, we mean the vertical measurement. For example, if a joist is a 2x10, it is the 10" part we are referring to. Stated and true measurements (also called "dressed dimension") of dimensional lumber are not the same. In this case, the true depth of a 10" joist is 9.25".
  3. Multiply joist depth by 4.
  4. 9.25 x 4 = 37. The resulting number is a rule of thumb for how far you can cantilever your bumpout.
For every foot that you cantilever outward, you'll need to sister twice that length along an existing joist. For example, if your bumpout cantilevers 2 feet, you have a minimum of 4 feet running alongside an existing joist.
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