Bump-out addition overview: Benefits and planning
Building a “bump-out” is a big project that adds only a few square feet to a room. (Here are some other ways to add square footage to your home.) But in some rooms—especially bathrooms—an extra 6 or 7 sq. ft. allows for a complete transformation. With this wee addition, you can actually turn a half bath into a full bath, install a luxury two-sink vanity or even install a spa. The bump-out shown here allowed Charlie Avoles to expand a bathroom and replace a small tub with an oversized, easy-access shower.
A bump-out is a home addition. And though a lot smaller than most, it raises many of the same planning issues. This article covers the main issues Charlie had to wrestle with; they’re the same things you’ll have to consider if you’re interested learning how to build a room addition yourself.
If, after working through all these considerations, you decide a DIY home addition is worth the effort, get a building permit. That might seem like a pain in the neck. But think of it this way: An expert will check your plans in advance to make sure your bump-out will be built right. Then, during construction, another pro will make a house call to inspect your work. Not bad for $50 to $100 (not including plumbing and electrical permit fees).
Meet a Pro
Charlie Avoles is one of the licensed master plumbers who checks and double-checks every bit of plumbing info that appears in We do everything by our own hands. When he’s not nitpicking magazine articles, Charlie is co-owner of St. Paul Pipeworks in St. Paul, Minnesota, and sometimes wears a general contractor hat, as he did for the bump-out shown here.
“A bump-out doesn’t add a lot of square footage, but think of it this way: This bathroom grew by more than 25 percent!”
Before you cut a hole in your house…
Install a shoring wall
Install a temporary shoring wall to hold things up while you cut the hole, extend the floor and set the header.
In most cases, you’ll need to build a “shoring wall” (a temporary stud wall) inside the house, a couple of feet from the exterior wall. The shoring wall will support the ceiling and walls above while you cut a hole in the wall and install the header.
A Header Supports the Weight Above
When you remove a section of wall, you need to add a header in its place. Charlie’s bump-out called for a doubled 2×8 header, but there’s no easy rule of thumb for sizing a header—check with your building inspector. In some (rare) situations, no header is needed.
How far can a bump-out bump out?
Finished bump-out addition
Cantilevered bump-out distance is limited by the size of the floor joists.
Floor joists can typically cantilever up to four times their depth. So if your existing joists are 2x8s (7-1/4 in. deep), the new joists can extend 29 in. outside (7-1/4 x 4 = 29). Check with your building inspector. A bump-out set on footings is limited only by setback requirements and other local codes.
Two ways to support the floor
Charlie’s bump-out addition rests on joists that protrude from the house and have no support under them—kind of like a balcony. These “cantilevered” joists are nailed onto the sides of the old joists (called “sistering”) inside the basement. This approach works only if existing joists run the right direction, of course. You can also set a bump-out addition on footings, just like you’d build a deck. Charlie chose the cantilevered method to avoid digging holes to frost depth (42 in. in southern Minnesota) and pouring concrete footings. Footings are much less work in warmer climates.
New joists must reach far inside
In most cases, cantilevered joists must extend 2 ft. inside the foundation for every 1 ft. outside, so joists that cantilever 2 ft. must reach 4 ft. inside. That may mean moving plumbing, electrical or heating lines that run between existing joists. (So figure out what you’re getting into before you start!) One more thing: You’re going to have to cut out the rim joist to access the joists for sistering in the new ones.
Consider a lower ceiling
Sometimes, a header extending down from the ceiling creates an attractive break in a room. In this case, the header would have been an eyesore centered above the shower. Charlie’s solution was to lower the whole ceiling. That created a flat, uninterrupted ceiling. Plus, a small room feels bigger with a lower ceiling.
A lean-to roof like the one on Charlie’s bump-out is the easiest type to build, and Charlie was able to connect to the existing roofline, for a neater appearance (See Photo above). On many homes, the solution is even simpler: Run the new exterior walls right up to the existing roof soffit and you won’t need any roof at all. You’ll have to limit the depth of the bump-out to the depth of the roof overhang, of course.