Is a packed garage keeping you from buying
your dream motorcycle? Or maybe you just
need a little more space for lawn equipment,
bikes or your woodworking tools. A bump-out addition
may be the solution to your overcrowded garage. And in
this article we’ll show you how to build one.
If you have basic carpentry skills and experience building
a deck, shed or other structure, then you can build
this garage addition. In this article, we’ll focus on the
tough parts of the project, like installing the beam and
building the roof. Study Figures A – C for construction
details. A Materials List is available in Additional Information below.
Figure A: Bump-Out Details
Start with a traditional pressure-treated deck, then add the walls and roof.
Tools, time and cost
In addition to a basic set of carpentry
tools, you’ll need a hammer drill to bore
holes for the concrete anchors, and ladders
or rented scaffolding to finish the
roofing and other exterior details. While
not essential, a reciprocating saw will simplify
the wall tear-out.
We spent roughly $2,000 for materials
for this 5 x 12-ft. garage addition. Your
costs will vary depending on the type of
siding, roofing and windows you choose.
With a helper, plan to spend about half
a day installing the beam and a long weekend
framing the addition. Then plan to
spend another weekend finishing the exterior.
The time it takes to complete the
project will depend on whether you finish
the inside as we did. Keep in mind that
you can hire a siding, roofing or drywall
contractor to complete parts of the project
you’re not comfortable with.
This addition is large enough to accommodate
a garden tractor, a large motorcycle,
or a workbench and a table saw. If
you’d prefer a larger size, contact an architect
or structural engineer to specify the
size of the header, floor joists and rafters.
Garage additions usually require a
building permit. Contact your local building
department to see what’s needed.
Make sure to call 811 a few
days before you dig the footing holes to
have underground utilities located and
Support the roof with a new header
Before removing a section of the garage
wall, you’ll have to add a header to support
the weight of the roof above it. The
gable end of this truss-framed garage roof
doesn’t support much weight, so we were
able to add the double 2x10 header and
remove the wall section without adding
temporary supports. Gable end walls that
support a second floor or ridge beam will
require a larger header and temporary
support. If you have a second floor above
the garage or aren’t sure how the roof is
framed, contact an architect or engineer
to calculate the header size.
Start by locating the center of your
addition and marking the size of the
opening on the bottom plate of the wall.
We centered the bump-out on the garage
wall, but this isn’t necessary. Remember to
make the opening 7 in. narrower than the
width of the addition. Next make marks
3 in. beyond the opening marks on each
side to locate the inside edge of the king
studs. Cut king studs and nail them to the
top and bottom plates. For a 12-ft.-wide
addition, the distance between the king
studs should be 11 ft. 11 in. Cut the 2x10
headers to this length and nail them
together with 10d nails. Nail a 2x4 to the
bottom of the 2x10s to complete the
header (Figure B).
Cut out the top section of the wall studs
with a reciprocating saw to make a path
for the new header. Saw the studs 11-1/2
in. down from the top plate and knock out
the short stud sections with a hammer.
Then cut off the nails with a nail nipper or
reciprocating saw. You may also have to
cut off a few sheathing and siding nails so
they don’t interfere with the header. After
nailing in the king studs (Photo 1), set the
header in place on the cutoff studs. Wedge
the header tight to the top plate by driving
shims between the cutoff studs and the
header at each end. Cut a pair of trimmer
studs for each end of the header and nail
them in place with 16d nails (Photo 1).
Remove the wall
If you have vinyl, aluminum or steel siding,
it’s probably easier to remove it from
the entire garage wall and reinstall it after
the addition is built. We cut the wood siding
2-1/2 in. beyond the width of the
addition to allow for 1/2-in. wall sheathing
and 2-in. inside corner boards. Pry off
the siding and remove the wall (Photo 2).
In many garages, the walls rest on a
block or concrete curb that has to be
removed to create a continuous floor. We
used an angle grinder fitted with a diamond
blade to score the concrete block
flush to the cutoff bottom plate on both
sides of the opening. Then we broke out
the concrete blocks between the trimmers
with a sledgehammer. If you have a solid
concrete curb, removing it will be a
tougher job. Consider renting a concrete
saw to score the entire length of the opening
before breaking it out. Photo 6 shows
how to patch the concrete after the plywood floor of the addition is in place.
Build the platform
Start building the platform by bolting the
ledger to the garage. Position the top edge
of the ledger 1-1/2 in. below the garage
floor in the center of the opening and level
it, using temporary stakes for support.
Attach it with 1/2-in. concrete sleeve
anchors located 12 in. apart. Add two
extra anchors at each end.
Next locate and mark the center of the
footings (Figure A). Dig the footing holes
and pour 8-in.-thick concrete pads into
the bottom of each. After the concrete
hardens, set treated 4x4 posts into the
footing holes. At this point, you’ll know
whether you need to remove soil in the
area under the bump-out to make room
for the joists. Make sure there are at least a
few inches of clearance between the bottom
of the joists and the ground.
The next step is to cut the posts to the
correct length and build the platform. Use
a level resting on a straight board to mark
each post level with the bottom of the ledger board (Photo 4). Cut the posts at
the marks and set them back into the
holes. Complete the floor frame by cutting
and assembling the floor joists and attaching
the frame to the posts with metal post
caps (Figure A). Finally, pack dirt around
the posts and nail one layer of treated plywood
and a second layer of BC plywood
to the floor framing. If the surrounding
soil is above the bottom of the joists, build
a three-sided 2x6 dam and pack the soil
against it. Don’t attach the 2x6s to the
joists. The goal is to prevent dirt from getting
under the joists while allowing the
2x6s to move with the soil.
Build and stand the walls
Build the walls on a flat surface like the
garage floor. Start by cutting the top and
bottom plates for the long wall and marking
the stud locations on them. Use
Figure A as a guide. Lay out the window
opening to match the rough-opening dimensions provided by your window
supplier. Assemble the wall by nailing the
studs to the plates with 16d nails. Stand
the wall on the platform, straighten the
bottom plate and nail it to the rim joist.
Plumb and brace the outside corners.
Now measure from the new front wall to
the existing garage wall at the top and bottom
on each side and cut the plates for the
short walls accordingly. If the existing
garage wall isn’t plumb, this procedure
will ensure that the short walls will fit
Build the two short walls and set them
in place on the platform. First position the
walls and nail through the bottom plate
into the floor with 16d nails. With a level,
plumb the stud that’s against the garage
wall before nailing it to the wall. Align the
corner studs on adjacent walls and nail
them together. Finally, use a level to make
sure the corner stud on the long wall is
plumb and nail a diagonal brace to the
inside of the wall (Photo 7). Complete the
wall construction by adding the tie plates,
making sure to cut them so they overlap the top plate at the corners.
Figure C: Rafter Details
It’s easier to visualize and mark the roof rafter cuts if you mark them in place on the wall.
Cut the rafters and build the roof
For the best appearance, match the slopes
of the addition and garage roofs. In Photo
8, we show a simple method of marking a
2x6 to use as a guide for making a pattern
rafter. Figure C shows how to modify the marks to create a pattern rafter. Use the
pattern to mark the remaining rafters.
Photo 9 shows how to cut the siding
using the rafters as a saw guide. We added
a 3/8-in. shim under the rafters to allow a
1-3/4-in. gap between the roof framing
and the siding cut (Photo 9). This provided
enough room for 1/2-in. sheathing,
two layers of dimensional shingles and a
3/4-in. space for the step flashing to slide
into. Adjust the cut in your siding to correspond
to the thickness of your roofing
and sheathing material. Set the saw just
deep enough to cut through the siding.
When you’re done cutting both sides,
remove the rafters and pry off the siding
in the area of the new roof.
Frame the roof using Figure A and
Photos 10 and 11 as a guide. Nail
through the ridge into the rafters and toenail
the bird’s-mouth to the tie plate.
Reinforce the connection between the
rafters and the tie plate with metal hurricane
ties. Then complete the roof frame
by adding the 2x6 subfascia and building
the side and end overhangs. Match the
overhangs to the overhangs on the garage.
When you’re done with the roof frame,
cut 1/2-in. sheathing to fit and nail it to
Back to Top
Finish the addition with trim, roofing and siding
At this point in the project, your garage
addition will probably vary considerably
from what we show here. In general, you’ll
start by finishing the trim on the overhangs,
including the soffit and fascia, with
wood or metal to match your garage.
Then install the roof shingles according to
the manufacturer’s instructions. The key
to a leak-proof roof is proper step flashing
(Photo 13). We slid the flashing under the
siding. But if you’ve removed the siding
entirely, then simply install the step flashing
with the shingles. Then install the siding
over the flashing.
Install the window before the siding,
being careful to flash around it with building
paper or special self-adhesive window-
flashing tape according to the manufacturer’s
instructions (Photo 12).
Finally, install siding to match your
On the inside we added a few outlets
and recessed ceiling lights. Then we insulated
the walls and ceiling before hanging
and taping the drywall.