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Bathroom Additions (How to Build Additions Guide: Simple Room Additions)

An extra bathroom is a bonus for any home, whether by itself or as part of a suite of rooms. Designs may range from spartan simplicity to extravagant luxury, but the construction basics are the same. Thorough design and careful preparation will ensure a successful project.

Preparing for Construction

If you are enlarging an existing bath room, you will not be able to use it for several days, even weeks. If your home has no other bathroom or only a half-bath, make alternate plans.

Also, before you get any permits or begin construction, study your plans carefully and verify all details and specifications, including the following points.

Walls. It is important to verify the thickness of existing walls, especially where plumbing will be installed. Plastic 2-inch pipe and fittings and 3-inch pipe without fittings will fit within a 2 by 4 wall, but larger fittings or pipe will require furring or re building with wider studs.

Check existing walls that will be come bearing walls to be sure they have foundation support, double top plates, large enough headers over doors and windows, and studs centered on 16 inches. You may not be able to inspect walls until demolition, but be prepared for the unexpected.

Plumbing. Verify direction of floor joists in case plumbing must be in stalled between them.

Verify the exact location of plumbing fixtures and check manufacturer’s rough-in dimensions, especially for bathtubs and prefabricated shower units. Be sure the drain location is specified for bathtubs (either right-hand or left-hand). Check whether washbasin is recessed into countertop or is surface mounted with a separate or built-in rim.

Verify the location of plumbing lines in attic and under floor, and measure clearance between drain lines and floor joists to see if proper slope is possible in extending lines.

It is also important to plan locations for access panels required for certain plumbing fixtures. They can affect the finish decor.

Framing details. Go over framing details in plans, such as connections between addition and house or special walls for enclosing a shower or tub. They may require steel brackets or other devices.

Backing requirements. See if your plans specify backing material for tile and for other finish surfaces. If they don't , consult a tile setter about backing requirements.

Roof Check roof slope and eave height carefully if plans show roof extended over new addition.

Delivery of prefabricated items. Measure to see whether doorway is wide enough for delivering a prefabricated shower unit or large bathtub. If not, plan to leave part of an addition wall unfinished until after such items have been delivered.

Also, decide now on locations for all accessories, such as towel racks, toilet paper holders, and soap dishes.

Water heater. Check size and location of water heater. Is it adequate for the new bathroom? Consider a larger heater or a secondary unit closer to the new bathroom.

Windows. If you are installing new windows or glass doors, check to see if safety glazing, such as tempered glass, will be required near the bath tub or other critical areas. If so, count on extra time for delivery.


Checking roof slope: New bathroom addition; Optional roof slope; Continuation of existing roof slope may not provide enough ceiling height. If not, you will need to alter roof plan.

Sequence of Construction

After obtaining permits and arranging for financing, order any materials that take longer than two weeks for delivery.

Next line up any subcontractors you want to hire, such as a plumber and an electrician.

Begin site preparation and foundation for the addition. The basic techniques are the same as for the ground level addition described previously. However, a bathroom addition will involve these further steps.

+ After completing framing for the foundation and the floor, install underfloor plumbing, wiring, and mechanical ducts. Be sure to get them inspected before installing insulation and laying the subfloor (unless there is a full basement).

+ When framing walls, leave an opening large enough for demolition debris to be removed and , if necessary, for a prefabricated bathtub or shower unit to be delivered before the walls are closed.

+ Run plumbing vents and other rooftop pipes or ducts through the roof sheathing before roofing.

+ If you are enlarging an existing bathroom, begin demolition after completing the addition shell. When performing demolition work, observe the same safety precautions and techniques as described for kitchens.

With shell and any demolition completed and plumbing and wiring run under the floor, you can proceed as you would in constructing any new bathroom, following the guidelines in the next section.


Installing prefabricated tub or shower unit: Width of enclosure specified by manufacturer; Studs spaced for faucet assembly.

Rough Framing for a Bathroom

Bathroom floor, walls, and ceiling are framed using basic rough carpentry techniques: studs centered on 16 inches, double top plates, and headers over doors and windows. How ever, some framing details are unique to bathrooms. Some of them involve plumbing, so consult a plumber early in the framing process to go over special needs. Details to consider include the following.

Floor joists. Bathtubs, particularly large luxury tubs or cast iron tubs, can be very heavy. Ideally, the tub should be located over or near a foundation wall, girder, or bearing wall. If it's not, the joists should be strong enough to support a tub perpendicular to them as long as they are properly sized for normal loads, aren't weakened by notches or holes, and have end support over foundation or girders.

If tub lies parallel to floor joists or is perpendicular to them but centered over a long joist span, double up joists under edges and /or feet.

A sunken bathtub requires different floor support altogether: either a concrete slab on the ground for bath rooms over shallow crawl space, or dropped floor joists supported by downstairs walls for bathrooms over basement or first floor.

Prefabricated units. A prefabricated bathtub or shower unit needs to be supported by framing around the walls. Build the walls using standard stud framing techniques. How ever, adjust the stud spacings where the faucet will be installed so that a stud will not be in the way.

The width of the enclosure must be accurate, so check manufacturer’s instructions or measure the unit itself at the base.


Framing for bathroom fixtures: Headers and framing for medicine cabinet, Support for wall-hung lavatory, Double joist under edge of tub, Shelf at foot of tub shortens alcove to fit tub; Sunken tub over shallow crawl space:

A toilet may need special floor framing to provide enough room for the closet bend. If a joist is in the way of the closet bend, cut the joist and support if with a header. Double the header and joists that support it if you have to cut more than one joist.

Plumbing walls. Some plumbing can't fit inside a 2 by 4 wall, such as a 4-inch soil stack, fittings for a 3-inch stack, 2-inch cast-iron fittings, or large shower valves. It may be possible to fur out part of the wall by nailing 2 by 2s to studs, but it's easier to frame the entire wall with 2 by 6s or even 2 by 8s to give a more uniform appearance.

Wing walls. Short walls that en close the end of a tub, shower, or similar area are framed like ordinary partition walls if they extend to the ceiling. However, they may be designed to terminate just below the ceiling to create an open feeling. If so, the wing walls will have no sup port along the top and one edge, and must be stabilized with one of the methods described for kitchen wing walls shown earlier.

Pocket doors. Sliding doors recessed into the wall don't take up valuable floor space, an advantage in tight bathrooms. They are available in kits that include the pocket frame. When planning the header over a pocket door, include door frame as well as opening in span dimension (for example, header for 30-inch door has to span 60 inches, requiring a 63-inch header). Avoid plumbing vents, recessed towel racks, and other intrusions into wall where pocket frame is located.

Blocking. Nail 2 by 6 or larger blocks between studs where towel racks, toilet paper holders, grab bars, or other accessories will be installed.

Recessed alcoves. Small nooks in shower walls or other locations can be framed between studs as simple boxes with 2 by 4s and then lined with wallboard or tile backer board.

Larger alcoves, for wide medicine cabinets or recessed towel bays, may require cutting a stud and using 2 by 4s for header and sill.

Soffits. Soffit framing can be built after walls and ceiling are covered with wallboard, but if soffit contains recessed lighting it's easier to frame it before, so wiring can be installed and inspected with the rest of the bathroom.

Using 2 by 2s will save lumber and make nailing easier, but if 2 by 4s are more readily available they are fine. Build an L-shaped ladder frame on the floor, lift it into position, and nail it to wall studs and ceiling joists.

Skylight. A bathroom skylight need not be large to be dramatic, as long as the light shaft flares out enough to bring direct light to most of the room. Install skylight in roof with conventional framing techniques. Frame ceiling opening larger than skylight. Then frame a light shaft by nailing studs between roof opening and ceiling opening. Some stud ends may need to be cut with compound bevel angles. Mark these by holding stud material in place and scribing along ceiling or rafter framing to mark both sides of each cut.

Plumbing for a Bathroom

Whether you are enlarging an existing bathroom or adding a new one, rough plumbing will be a major expense and an important phase of construction. Installing it in the new space shouldn't be too difficult, but be ready for complications when altering old plumbing or tying new plumbing into old.


Toilet framing: You are allowed to remove part of one joist if it’s in the way. You must secure the cut joist to adjacent joists with headers. If you want a handrail, set a 1x6 board into studs beside toilet to hold rail. Soil stack, Cut joist, Closet bend, Joists

Drain- and Vent-Pipes

A bathroom requires a 3-inch drain. It may seem simple enough to run a new pipe from bathroom to main drain, but certain restraints complicate the task. First, the drainpipe must have a minimum slope of ¼-inch per foot. There may be a limited number of places where new pipe can be connected to old, either at an existing fitting or where a new one can be installed easily. Codes restrict which fittings can be used to join branch and main drain, usually requiring a Y, a combination, or a similar long-sweep fitting to ensure proper flow. Even when all these conditions are satisfied, obstacles such as heating ducts or structural members may have to be moved or may force you to move the pipe.

Drainpipes for tub, shower, and washbasin are usually connected to the same branch drain, but if it creates a more convenient run, some of the fixtures may be connected to a different drain.

All fixtures must be vented to the roof. If clustered near each other, they can share a common vent, which must be a minimum of 2 inches in diameter. When connecting a horizontal vent to the roof vent, try to change direction in the attic rather than below the wall plates to avoid boring holes through too many wall studs. Any change in vent-pipe direction from vertical to horizontal must be made at least 6 inches above the flood line of the fixture (approximately 24 inches above the floor for a bathtub, 40 inches for a sink).

Inspection of completed rough plumbing includes a water test for leaks. For this test, all stub-outs must be sealed with temporary plugs, and some means must be provided for holding water in new pipes and keeping it from flowing into the existing plumbing system. Plumbers in stall a tee and cleanout at this point and insert an inflatable test plug to hold water back. Proper test pressure is achieved when the system, including vent pipes, is filled with water to a specified height above the horizontal drain (usually 10 feet). If the test is made for underfloor pipes, before the subfloor is laid down, it's necessary to install one vent pipe to get the 10-foot head of pressure, even before the walls are framed.

Water Supply Pipes

Run water supply pipes after drain- and vent pipes are installed. If you are connecting new copper pipes to existing galvanized pipes, use a special dielectric union or intermediate brass fitting to prevent corrosion caused by joining dissimilar metals that are constantly exposed to water.

Water supply pipes are normally tested by capping all stub-outs and pumping air into the pipes to a certain pressure. If you connect new pipes to existing pipes, however, water pressure from the house system may be enough for the test.

Rough-in Dimensions

Each fixture has rough-in dimensions for drain and water supply stubs. They are fairly standard for most fixtures, but check manufacturer’s specifications for special items such as luxury bathtubs, custom toilets, or pedestal sinks. Also be sure to verify whether the dimensions are from finish floor or subfloor.

Freeze Protection

If you live in a cold climate, take the precautions listed earlier to prevent frozen water pipes.

Hot Water

Hot water systems are usually sized by the number of bedrooms in a home (not bathrooms), so adding a new bathroom may not require an added water heater. However, you may want to consider an additional heater for your bathroom to provide more capacity and quicker hot water due to shorter runs. If the heater will be gas, check the local code for allowable locations and additional venting requirements.


Rough plumbing dimensions: Toilet: Centerline, Supply stub; Washbasin: Rim or countertop; Shower; Bathtub

Luxury Bathtub

Large tubs or tubs with whirlpool features require special installations. Follow manufacturer’s instructions. Drain and overflow connections are the same as for conventional tubs, except that drain hole may be located in center or at edge of tub rather than at end. Faucets and spout may be deck-mounted on a ledge around the tub or installed on the wall. In some cases they may be built in by the manufacturer. If the tub includes a spout mounted below the rim, you will need a vacuum breaker in the water supply lines to prevent back flow. You will also need access to motor and controls: either a removable front panel or access panel at the end of the tub (located in adjacent room or closet).

If tub is fiberglass, set it in place on a ½-inch layer of mortar or quick- setting plaster compound to provide maximum support. For heat retention, it's a good idea to place fiberglass blanket insulation around the tub before closing in walls.


Built-in GFCI outlet; GFCI circuit breaker; GFCI breaker installed in service panel: Branch circuit to bathroom outlets

Electrical Wiring for the Bathroom

The presence of water and grounded pipes in the bathroom area makes safety as well as convenience a vital concern. Be familiar with local codes and with the wiring scheme of your working drawings. Here are some typical electrical requirements for bathrooms.

+ A separate circuit is required for each permanently installed appliance, such as whirlpool bathtub, ceiling fan, electric wall heater (120v or 240v), radiant heat lamp, and electric water heater.

+ All bathroom receptacles, and any motors for bathtub equipment, must have ground fault circuit interrupter (GFCI) protection, which automatically deadens the outlet if a potential exists for electric shock. Although the GFCI can be a circuit breaker at the main panel or other house outlet that protects all outlets downstream from it (including the bathroom), it's better to install a GFCI receptacle right in the bathroom to make it easy to reset.

Bathroom receptacles don't re quire separate circuits. Still, it may be easier to run a new circuit for them than to tap into an existing one if you are already running new circuits required for individual appliances.

+ Bathroom lighting must not be on the same circuit as individual appliances. It is also desirable, although not required, to make it not subject to GFCI. As with receptacles, it may be easier to run a separate circuit for lighting than to tap into an existing circuit. It depends on the electrical capacity of existing circuits for an additional load and the accessibility of nearby fixtures.

+ For safety reasons, no fixtures, switches, or outlets can be located within a certain distance of a bathtub or shower. The minimum is usually 5 feet horizontally and 7½-feet vertically, but check local building codes, especially if you are installing lights in a drop ceiling located over a shower or tub. You may be required to protect them with a GFCI device.

Evaluating Your Present System

In planning new circuits for a bath room addition you must make sure the present electrical system can meet the new demand. Unless you are adding electric wall heaters or your home’s wiring is antiquated, the present system can probably handle the new bathroom. As with kitchen additions, the circuits will require new circuit breakers in the service panel and a main service rating high enough for the total load. (See Installing Wiring section).

Bathroom Heating and Ventilation

A bathroom addition can be heated by extending the existing forced air or hot water (hydronic) systems, or by adding an electric baseboard or wall heater. Because bathrooms have limited wall space, you may have difficulty finding a good location for a forced air register. The most common solution is to locate it under the toe kick of the vanity cabinet by bringing a duct up through the floor. Use a transition elbow to connect the register to a 3¼-inch by 10-inch rectangular duct that fits horizontally under the cabinet. Cut a hole into the toe kick for the duct, set the vanity in place, and cover the hole with the register grill.

Most electric wall heaters can be wired with a 120-volt or a 240-volt circuit. The 240-volt circuit requires 3-wire cable and a double circuit breaker, but the cost is offset by more efficient use of electricity.

An exhaust fan is required in bath rooms with no openable windows and is a good idea for any bathroom. All fans should be ducted to the outside through the roof or a nearby wall. To install a bathroom fan, set housing in place between ceiling joists. Install a termination cap nearby in wall or roof by drilling a hole 4½-inches in diameter through sheathing and attaching cap from outside. Run 4-inch flexible duct from fan housing to termination cap, using tightening band or sheet metal screws to connect it at each end.

Run a 2-wire feeder cable (#12) from circuit breaker panel to fan junction box. Then run cable from fan to switch box on wall, using 2- wire, 3-wire, or two 2-wire cables (all with ground wire), depending on how many switches are needed to control fan motor, radiant heat lamp, light, and so on. All wires in this switch loop will be hot, so mark any white wires with black tape or paint.

Finishing the Walls and Ceiling

After plumbing, wiring, duct work, and framing are inspected you can install insulation and cover the walls and ceilings.

Install insulation the same as for a basic room addition. Then install polyethylene sheeting over insulation and framing as a vapor barrier, stapling it to studs and joists. After stapling the vapor barrier in place, use a utility knife to cut it out around electrical boxes and plumbing stub-outs. Be especially cautious with window sills above tub or shower, wrapping them with plastic so moisture will not get trapped on wood framing.

To minimize sound transmission, in stall insulation in interior as well as exterior walls. For maximum sound deadening, frame walls with double studs, apply sound-deadening board beneath wallboard, or install wall board over metal resilient channels.

Use water-resistant wallboard or special tile backing board in wet locations, such as shower walls or around the bathtub. Use fiberglass mesh tape and moisture-resistant joint compound to seal joints. You can also use Type I tile mastic to seal the joints of the cement backing board. Don’t hang water-resistant wallboard on the ceiling; it's too heavy to span normal joist spacings without sagging.

Paint walls and ceiling with alkyd (oil-based) rather than latex paints. Although special vinyl paints are often recommended for sealing fresh wallboard, use a durable alkyd primer to create sufficient tooth for oil-based finish paint. You can do final painting after cabinets and fixtures are installed, but it's easier to do beforehand.

Installing Flooring

Installation of floor covering must be coordinated with installation of bath tub, toilet, vanity, and other fixtures. Although flooring can be installed around fixtures, it's much easier to do it beforehand (except carpeting).

The bathtub is usually installed be fore the finish floor. This creates a problem with most flooring materials because the seam between flooring and tub is vulnerable to moisture. If you are doing all the work yourself and don't need to accommodate subcontractors, try to install the flooring before installing the bathtub so that you can run it a few inches under the tub. This eliminates the possibility of ugly peeling around the tub.

If the finish flooring is of a type that requires underlayment (such as resilient materials or tile), avoid using particleboard. It will expand and deteriorate more rapidly than other materials if moisture should seep through to it. Use underlayment grade hardboard or plywood instead.

A ceramic tile floor should be in stalled over a mortar bed or on stable plywood (a 1 layer over the subfloor). Use epoxy-based adhesive to lay the tile.

Completing the Bathroom

You’re now ready to install vanity, sink, shower doors, lights, electrical fixtures, trim, doors, and window coverings. The process is the same as in any new bathroom.

Next: Complex Ground-Level Additions: Preconstruction

Prev: Kitchen Additions

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