How to Add a Load-Bearing Wall on Top of a Garage Slab

Planning and Cutting

• 1

  Find a point of reference to determine your load-bearing wall location. Measure over from the edge of the slab or an existing wall and make a mark to notate the placement. Repeat several times along the approximate length.

• 2

  Measure the length of the wall location and make a large X to indicate the end of the wall. Connect the location marks to outline the exact wall length and placement.

• 3

  Cut three 2-by-4-inch boards to match the length of the wall needed. These will form your bottom plate and the two top plates, which are the boards on either end of the vertical studs. Non-load-bearing walls use a single plate on either end, in contrast.

• 4

  Lay the bottom plate and one top plate side by side with the ends flush. Measure down the length of the plates, marking a straight line across both studs at once with a carpenter's square. Make the first mark at 1 1/2 inches to indicate the end of the first stud.

• 5

  Mark the plates again at 3 inches, 15 ¼ inches and every 16 inches thereafter until the end. End with two studs side like the beginning of the plate. The marks made show the edge of each stud. The center of each regular stud, excluding the ends, will be 16 inches apart, which is the preferred stud spacing.

• 6

  Find the wall height necessary. Consult the architectural plans or decide the height you prefer for new construction; the standard garage-ceiling height is 8 feet. For existing garages, measure from the concrete slab to the ceiling to determine the height.

• 7

  Subtract 4 1/2 inches from the wall height measurement. Cut one stud per mark on one of the plates. You must reduce the wall height by 4 ½ inches to allow for the thickness of the plates on either end. While made of 2-by-4-inch boards, the plates are actually 1 ½ inches thick. Three plates are thus 4 ½ inches thick. Studs cut 4 ½ inches shorter than needed, combined with three plates, match the ceiling height exactly.

Building and Erecting

• 8

  Set one plate on its side, with the longest face running vertically. Align a stud with the end of the plate, so that the edge of the stud aligns with the 1 1/2-inch mark. Drive two 16d nails through the plate into the stud end. Stagger the nails slightly to avoid splitting the wood. Work down the length of the plate, nailing additional studs aligned with stud marks. Flip the wall over and attach the opposite plate to the free stud ends.

• 9

  Omit the studs wherever you want a door or window. Instead, find the rough opening required for the window or door: typically 1/2 inch, both height- and width-wise, for windows and 2 inches for the height and width of a door. Add an additional 3 inches to the rough opening to accommodate the studs framing the opening, called “king studs.” Measure and mark the bottom plate to reflect the total opening required. For example, a door 32 inches wide requires an opening of 32 plus 2 inches and another 3 inches for a total of 37 inches.

• 10

  Position the king studs between the plates, aligned with the door or window opening marks. Nail through the top and bottom plates into the stud ends to secure.

• 11

  Measure the width of the opening between the king studs and cut the header board to fit. Consult a structural engineer, your local building department or architectural plans to determine the lumber needed for the header. Unlike non-load-bearing walls, a simple 2-by-4 will not be sufficient. Load-bearing walls withstand tremendous pressure from the roof alone. When coupled with ice, snow, storage or other possible loads, a 2-by-4 is likely to bow or collapse. The size of header board needed is calculated based on many factors. Builder's use a rule of thumb that two 2-by-12 inch boards with a strip of 1/2-inch-thick plywood sandwiched between is adequate for most doors and windows up to 4 or 6 feet wide.

• 12

  Nail the header boards together and insert, at the proper height between the king studs. The height needed is the door or window dimension plus an additional 2 inches for doors or ½ inch for windows. Toenail the header in place, driving the nails at an angle through the header into the king studs. Cut a 2-by-4 board of the same width and nail at the appropriate height below the header to form the sill plate for a window. A door doesn't require a sill plate.

• 13

  Cut two additional 2-by-4s to run between the header and top plate and from the header or sill plate down to the bottom plate. For a window, make two additional boards to run between the sill and header. These boards, called "cripples," spread the weight of the door or window to the surrounding studs. Nail the cripples in place, to both the king studs and the header, sill and plates as applicable.

• 14

  Insert blocks between the header and sill vertically and nail in place to substitute for missing studs. Use one every 16 inches.

Anchoring the Wall

• 15

  Nail the extra top plate to the top of the wall construction to create a double plate. Erect the wall, using a helper, and position according to the placement marks previously made.

• 16

  Secure the wall to the concrete pad. Drill pilot holes, following the product instructions, through the wall into the concrete beneath. Insert concrete anchors or powder actuated nails, one every 6 to 8 inches, for a secure installation.

• 17

  Tie the walls in with adjoining walls by driving nails through the end studs into adjacent studs. Place a steel plate on top of the top plate, half on the new wall half on the old. Drive six 8d nails through each side into the respective plate to reinforce the wall stability. Nail the top plate to the joists above if adding a load-bearing wall to an existing building.