Multistory Additions: Second-Floor Additions (How to Build Additions Guide)

Adding new rooms over your present home or garage is a major project. You must tear off the roof reinforce the foundation, alter the first-floor structure, build stairs in existing living space, and carry all materials up one floor. None of these challenges is insurmountable, but all demand a higher level of planning, determination, and ingenuity than most other additions. Be ready for setbacks and have a large work crew available for key phases of the project.

Reinforcing the Foundation

If your present foundation isn't adequate to support two floors, your plans will include details for strengthening it. Much of the work occurs in an enclosed crawl space or basement, so you can begin this phase while the weather is still too unstable for building the addition itself.

Following are ways to alter an existing foundation.

+ Pour reinforced concrete pads, usually 2 to 4 feet square and 12 to 18 inches thick, under portions of existing foundation where loads from upstairs addition are concentrated. This method disturbs the existing foundation least, but requires a structural design with few bearing points.

+ Pour reinforced concrete grade beams, 1 foot wide, along exterior side of existing foundation to widen it. Tie new beam to existing footing with rebar dowels every 18 inches.

+ Design second-floor support system to rest on new foundation walls placed inside or outside existing foundation. This is easier to do where first floor has odd shapes or features.

Foundation reinforcement using concrete pads: New concrete pad under old foundation at stress points, New concrete pad; Foundation using concrete grade beam: New reinforced concrete grade beam, Rebar, Footing

Shoring up the house: Shoring beams; Knock post into position with sledgehammer

For instance, you could push an exterior wall out even with an existing bay window along entire length of the proposed upstairs addition, and build new foundation on outside of existing foundation.

+ Replace existing foundation with new foundation designed for two stories. This involves shoring up floor joists, trimming or removing cripple studs, removing old foundation, building forms, placing rebar, and pouring concrete for the new foundation. The work can be done in sections or all at once.

Getting Started

Doing foundation work under the house requires familiarity with the floor structure, knowledge of shoring techniques, and skill in working with concrete. It also involves working in a cramped space, often in a horizontal position. To make the work easier, have the following items on hand.

+ Work lights with extra bulbs.

+ Extension cords plugged into GFCI outlets, such as an upstairs bathroom or properly wired outdoor outlet.

+ Dust masks, goggles, gloves, and hard hat.

+ Knee pads.

+ Rope and shallow sled for pulling materials in and out of tight areas.

Finally, if you are excavating in a tight area for a large footing, rent a conveyor to bring the debris out.


To provide light and access, remove a strip of siding from kneewall above foundation before beginning shoring.

Install shoring about 2 feet from foundation, which is close enough to support house walls but far enough to allow working room and avoid under mining foundation by excavation.

Start by lifting 4-by or 6-by beam up against bottoms of floor joists. Temporarily hold each end in place with a loop of wire or cord nailed to joists. Place 2 by 10 or larger pads on ground under ends and center of beam. Set a jack on each pad and jack beam up against joists, forcing pad down into dirt as far as it will go. Cut 4 by 4 posts to fit snugly next to jacks. Tap posts into place with sledgehammer, using a small level to make sure they are plumb in both directions. Remove jacks. Toenail posts to pads and beam. Repeat process with next beam in line. Posts at end of each beam can share a pad. If posts are longer than 5 feet, brace them to joists diagonally with 2 by 4s.

Removing Old Foundation

If there are kneewalls above the foundation, trim studs to allow enough room for new foundation height, or remove studs altogether. If not, proceed with breaking up foundation.

Remove mudsill by loosening anchor bolts and prying it off. Unless old foundation is already crumbling you will need a demolition hammer. Rent a 60-pound hammer with pointed bit, and work from outside. Cut rebar with a reciprocating saw or backsaw. If concrete is deteriorating, a sledgehammer will probably do.

Building New Foundation

Once the concrete and rebar are removed you can widen the trench for the new footing, build forms, place rebar, and pour concrete. New rebar should be tied into old foundation using the dowel technique for an inverted-T foundation described earlier.

If floor joists rest directly on foundation, there are several methods for placing concrete in forms and installing mudsill. You can remove rim joist to get concrete into forms. Then slip mudsill into place, with holes predrilled for anchor bolts, while concrete is wet. Insert anchor bolts down through sill.

Or you can leave outside form board off: Nail new mudsill, with anchor bolts in place, up into joists before pouring concrete; tilt form board outward at an angle while placing concrete; and then force board upright when there is enough concrete in forms to fill all the spaces.

Strengthening Underfloor Framing

If the existing one-story house has kneewalls between foundation and floor, the studs are most likely 2 by 4s. The new second story will require 2 by 6 (or 3 by 4) kneewalls. This means either replacing the 2 by 4 stud wall with a 2 by 6 wall or doubling up all 2 by 4s.

When doubling up studs, cut new stud to fit snugly and tap into place with sledgehammer—rather than framing hammer—to prevent split ting. Then nail two studs together with 1 6d nails every 16 inches and toenail new stud into mudsill and top plate with three 8d nails at each connection.

The plans may also call for tying interior bearing walls on the first floor to the floor joists below or to the foundation for shear strength. If walls line up over floor joists, wall framing and joist can be connected by 3-foot metal straps every 4 feet. Strip part of wall away to expose studs. Cut a slot in the floor so a strap can extend down the face of the stud, through the floor, and against joist below. Nail with 16d nails.

Preparing the Downstairs Walls

Any exterior and bearing walls on the first floor that will be supporting the second-floor addition must have at least 2 by 4 studs spaced no farther apart than 16 inches on center. The walls must have double top plates, lapped at the corners, and have headers over doors and windows large enough to support the new loads. Your plans may also require some walls to have hold-downs, metal straps, plywood shear panels, or other structural devices.

Replacing the foundation: Build new foundation forms according to local code; Floor joists; Shoring beams

If is difficult to tell if existing walls are adequate without stripping off siding or interior wall covering. Consider remodeling a downstairs kitchen or other room with strategic walls as part of your project, or insulating exterior walls by stripping off siding if they need it. Or make exploratory holes in interior plaster or wallboard to check framing conditions of key walls (double top plates, spacing of studs, size of headers, and so on). If you find any deficiencies you will have to open up the wall to check further and make alterations.

If studs are undersized or spaced too far apart, strip interior wall covering and double up existing studs or place new studs between them. You may have to move wiring or plumbing temporarily to do this. If walls have single top plates, trim tops of all studs 1½-inches and slip in another plate (after shoring ceiling joists); or remove all ceilings when you remove the roof and double up the plates from the top before laying new floor joists. Seek professional advice be fore undertaking either project.

To enlarge a header, strip interior wall covering around it and shore up ceiling joists. Cut cripple studs to accommodate the new, larger header. Remove cut ends of cripple studs and existing header. Then slip new header into place and nail it securely with 1 6d nails. Finally, toe-nail cripple studs into top of header with four 8d nails at ends of studs.

To install plywood for shear panels, strip wall covering on one side of wall. Double up studs every 4 feet where plywood seams will fall. Ex tend all electrical boxes so that front edges will be flush with finish wall. Get framing inspection if required. Then cut panels to fit, including cut outs for electrical boxes and other obstructions, and nail them to wall framing according to the nailing schedule in your plans. Plywood should extend from soleplate to top plates, with nails spaced at 4 inches along them.

If your plans include putting in new downstairs walls to carry some of the upstairs floor load, build them the same as any interior bearing walls. Remove enough ceiling material to nail cap plate directly to bottoms of joists overhead.

Special reinforcement for shear walls: Finished wall; Plywood shear panels, Regular framing

Tearing Off the Roof

Early in your preparations you should determine whether your roof framing is conventional rafters or prefabricated trusses (found in many newer homes). If it's trusses, consult an engineer or experienced contractor before attempting to remove the roof.

In general, the following preparations will help a roof tear-off go quickly, safely, and efficiently.

+ Have a large tarp ready to cover exposed ceiling at night. Buy 20 to 30 rubberized cords for attaching tarp to side of house.

+ Arrange for debris removal. If possible, have debris box set next to house, directly below work area.

+ Tack plywood over downstairs windows where debris will be dropped.

+ Have safety ropes available if roof slope is greater than 5 in 12.

+ Place sheets of plywood over ceiling joists to give you a place to stand and to keep debris from falling through ceiling. If possible, have sheets in position before starting tear-off. If ceiling joists are weak or feel springy, install shoring in rooms below to support long spans while you work.

+ Arrange for an electrician to move power lines, phone wires, and cable TV lines attached to roof where you will be working.

+ Move obstructions that will interfere with new floor framing — for example, heating ducts or obsolete flues in attic.

+ Line up two or three helpers who are enthusiastic but careful workers.

There are several ways to remove a roof. You can do it in complete sections by sawing through the roofing, sheathing, and rafters all at once. This method is quick but difficult and dangerous: The sections are heavy, you might sever a vital structural member, and it's hard on saw blades.

A slower but more reliable method is to rip off shingles first. Use flat shovels or wide wrecking bars. Start from ridge and work down. Be careful: Loose shingles make the roof slippery. Do the section above the debris box last.

After removing the shingles, pry off sheathing boards and remove rafters or trusses.

If you aren't removing the entire roof you will have to establish a line where roof removal should end. If the new addition lines up over a down stairs wall, find the wall plate in the attic. Use a plumb bob to mark bottom of roof sheathing at 4 or 5 points. Drive a nail up through each mark so it will be seen on the roof. Snap a chalk line on shingles, using nail points as guides, and cut through shingles and sheathing along this line. Use a cheap new carbide blade and cut slowly. Wear goggles.

Framing the New Floor

Building a level floor platform without disturbing the ceiling below requires careful work. Your plans will show which downstairs walls support floor joists and how high joists should be raised above wall plates. In some designs they are raised just high enough to clear any obstructions between ceiling joists, such as wiring or plaster keys. Another method is to run floor joists above ceiling joists.

Either way you will have to nail short blocks between ceiling joists on top of wall plates to support floor joists. The thickness of these blocks varies with the design, from single 1 by 4s to stacked 2 by 4s. Existing wall plates may not be level, so you may have to adjust block thickness as you install each one. Use the first block as a control for leveling all others with a transit, water level, or long straight edge and level.

Your plans may include one or more beams, instead of blocks, with joist hangers nailed to them to sup port floor joists. They may bridge the ceiling and rest on supporting posts let into exterior walls, or they may rest directly on ceiling joists if the bearing wall below is adequately supported. When using such beams, be sure to account for differences between depths of beams and floor joists as you install blocks or trim posts.

If ceiling has to be removed to double up wall plates or take down roof trusses, blocks are unnecessary and floor joists can rest directly on exposed wall plates. If plates aren't level, cut long tapered shims out of 2 by 4s to nail on top of plates.

Once all blocks, shims, beams, and plates are ready, install floor joists. The techniques are the same as for any floor system, except that footing may be more treacherous; you may have to adjust layout to clear ceiling joists and obstructions; and you may have to install solid blocking between joists at the ends instead of nailing them to a rim joist. If floor is out of square because edges line up over downstairs walls that aren't square, begin joist layout along whichever wall has at least one square corner. That way, all joist bays will be parallel and square except for the last one. Align perimeter edges of floor framing with exterior wall framing below.

Get inspections for framing, plumbing, and mechanical lines that will be concealed once subfloor is in place. Install insulation against exterior joists if cavity will not be accessible from below. Then add subfloor.

Completing the Shell

Once the platform is completed you can frame the walls. For safety, spray-paint a bright line 5 feet from the edges of the subfloor to warn workers that they are close to an unprotected dropoff. Then erect the walls.

The installation of sheathing, windows, and siding will require scaffolding, unless you install them before tilting up the walls. (See Completing the Shell.)

Lap sheathing down over floor joists so it abuts downstairs siding. If sheathing is also finish siding, install metal flashing along lower edge to create a weathertight joint and conceal it with trim.

If the addition does not cover the entire house it will have to be joined to the old roof. Use step flashing to seal the joint between roof shingles and new wall. (See Tying New Walls to Old Roof above.)

Upstairs walls that line up over downstairs walls that encroach on legal setbacks will probably have to be fireproofed. A common method is to cover them with conventional sheathing, nail a layer of wallboard on the outside, tape joints, and cover wall board with siding. If sprinklers are also required, consult your city fire inspector or a plumber to see if sup ply pipe must be run all the way back to your meter or if it can be tapped into existing water pipes.

Techniques for framing, sheathing, and covering the roof are the same as for any new construction.

Build stairs as soon as possible to facilitate further construction, using the techniques noted above.

A deck built over living space will require careful roofing techniques beneath it for long-term waterproofing. One method is to construct a plywood floor platform with a slight slope, at least ¼-inch per foot. Cover joint between floor and intersecting walls with metal angle flashing, nailing it to wall sheathing and subfloor. Nail a metal drip edge to downslope edge of floor. Cover plywood surface and metal flashings with elastomeric roofing compound designed for ex posed decks. Several types are avail able. Some require priming the surfaces first. Others consist of a solid rubberlike membrane. Follow manufacturer’s instructions carefully. After membrane cures, apply siding to walls and , if desired, install a wood deck over roofing membrane.

Installing Utilities

Basically, plumbing and wiring techniques for a two-story addition are similar to those described earlier in guide. However, there are some special factors to be considered.


Locate upstairs fixtures so drain- pipes can be run through downstairs space unobtrusively. Try to connect them to an existing 3- or 4-inch soil stack or conceal them within a 2 by 6 wall, a closet, or a special chase. Lack of space may require hanging lateral drainpipes below the downstairs ceiling to avoid notching critical joists. If so, keep the pipes close to a down stairs wall and box them in with a soffit. To prevent noise, strap drain-pipes securely and wrap insulation around them.

Once drain lines are installed, sup ply pipes can be fit inside walls and run around obstructions. Consider distance from water heater when running hot water pipes.

A good location for a new water heater is inside the old attic space. The space for a gas heater must be well vented. According to codes, the flue must either be at least 10 feet away from the new upstairs roof or be extended 2 feet above the new ridge.

A bathtub requires an access panel to plumbing and , for whirlpool tubs, to the motor. Conceal access panel in a closet, or install it in the ceiling directly beneath the tub.

Removing an old roof: Remove roofing from the ridge down; Keep roof clean by sweeping often,

Framing a second-floor addition: New floor joists, New rim joist rests on blocks, Existing ceiling, Existing ceiling joists, Existing double top plate, blocks support new floor joists, Existing wall

Gas Appliances

Gas lines must also be planned care fully because the pipes are rigid and require threaded fittings. Install any long risers inside existing downstairs walls before putting down the subfloor so that upstairs framing won’t interfere with poking risers down inside walls.


Even if your home has central forced air heat, it may not be easy to expand it into the upstairs addition because new ducts are difficult to install in downstairs walls or beneath new floor joists. It is usually worthwhile to install a second furnace either in the addition or in the original attic and run ducts to upstairs rooms through the new attic. This also allows you to divide your home into upstairs and downstairs heating zones for greater energy efficiency.

Electrical Wiring

Wiring techniques for a second-floor addition are the same as for any new construction. Run new circuits from service panel downstairs or install new subpanel in the upstairs area to keep distances shorter.

Deck over living space: Roofing membrane

Completing the Addition

Installing insulation, wallboard, trim, finish fixtures, and flooring is the same as in any room addition. To eliminate carrying wallboard up stairs, try to arrange for a lift truck to deliver it through an upstairs window.


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