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. 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
in a bump-out of your own.
If, after working through all these considerations,
you decide a bump-out 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 The
Family Handyman.
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!”
Charlie Avoles
Before you cut a hole in your house...
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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.
Bump-out addition details
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
2x8 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?
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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 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 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.
Roof lines
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.
