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Finishing the Interior (How to Build Additions Guide: Simple Room Additions)

The next step in finishing the interior is to trim the windows and doors and install baseboards. There may also be other trim to in stall, such as crown molding at the ceiling or picture molding around the top of the wall, but the techniques are similar

You will need basic carpentry tools, including a 16-ounce finish hammer, fine-toothed backsaw, miter box, bevel gauge, and try square.

Installing a Mitered Door Casing

Casings shouldn't be flush with the inside edge of jambs but should be set back ¼-inch to reveal some of jamb. This setback distance is called a reveal. Mark it on edges of jambs.

Square one end of a piece of casing. Stand it on floor and align inside edge along reveal. Mark top where reveal lines intersect.

At this mark, make a 45-degree miter cut and nail casing in place with 4-penny (4d) finish nails at jamb edge, 6d or 8d finish nails elsewhere.

Make miter cut at one end of an other piece of casing and hold this piece in position as head casing. Mark bottom edge where reveal lines intersect and make 45-degree miter cut outward from mark. Tack casing lightly in place.

Square end of third piece of casing and stand in place. Mark both inside and outside edges where they cross head casing. Marks should make a 45-degree angle. If they don't , ad just cut of head casing to complement angle of side casing and reinstall it. Then scribe side casing, cut it, and complete nailing.


Installing a mitered door casing: Mark “reveal”— 1/4” gap between frame and casing; Start miter cut where reveal lines intersect; Miter cut matches side casing cut; Side casing nailed in place; Start miter cut at mark on side jamb; Start miter cut at intersection of head casing

Trimming Out a Window

Measure and cut stool so that far edge fits between jambs and near edge extends beyond jambs and also ¼-inch beyond each side casing. Follow the same steps for installing side and head casings as described above for doors. Finish by cutting and nailing apron below stool to align with outside edges of casings.

Installing Baseboards

Make baseboards from one, two, or three pieces of molding. Leave a 1/2-inch gap beneath baseboard for floor covering if it's not already in place. If you are installing base shoe against baseboard, nail it into base board, not into floor. Use miter cuts to 5oin baseboard at outside corners.

At inside corners, use a butt joint for square-edged stock and a coped joint for curved stock. For a coped joint, square end of first piece of baseboard and install it. Cut second piece at 45-degree angle. Then make a second cut along contour of first piece, holding coping saw perpendicular to board. Fit second piece of molding over first and nail.


Parts of a window; 1-piece baseboard; 2-piece baseboard; 3-piece baseboard: Base shoe


Installing baseboards: Cut second piece with a 45° miter cut; Then cut away by holding saw to back surface and following the profile created by the miter cut

Installing Flooring

Install the finish flooring after door casings, window trim, and other moldings are in place and the room has been painted. (Leave off base boards if flooring will be tile, wood, or resilient materials.) Although you will probably hire professionals for most floor coverings, you may want to install tile, resilient flooring, or hard wood flooring yourself.

Preparing the Subfloor

The most important step in installing any type of flooring is careful preparation of the subfloor. If the floor covering will be hardwood strip flooring or thick carpeting, the only preparation the subfloor normally needs is scraping off any lumps of wallboard compound and sweeping the floor thoroughly.

If the flooring will be tile, lay down a vapor barrier of 4-mill polyethylene sheeting and then a second layer of plywood (minimum 3/8-inch thick).

Nail it with 2d deformed-shank nails, 6 inches on center.

If the floor covering will be resilient sheet or resilient tile, install a 5 underlayment of particleboard, ply wood, or hardboard over a vapor barrier of polyethylene sheeting. Nail with 2d deformed-shank nails, 6 inches on center. Leave a 1/16- to 1/8-inch gap between panels for expansion. Fill all gaps, holes, cracks, and nail depressions with a durable, fast- drying compound.

Installing Tile

Installation usually begins with full tiles along the most visually prominent wall and proceeds toward an exit so that you don’t have to walk on freshly laid tiles. Because each wall may not be straight or perfectly square with the others, it's necessary to snap chalklines to work from.

Measure out from the starting wall, at both ends, a distance equal to one tile plus two grout lines. Then snap a chalkline between these points. Repeat the procedure for the other three walls, using a carpenter’s square or a 3-4-5 triangle at each corner to keep the lines perpendicular. Double check by measuring the diagonals of the square.

After adjusting chalklines, lay out a dry run of tiles along the first wall. Use spacers between tiles for uniform grout lines. The dry run will help you decide where any cut tiles should go.

You may be able to widen the grout spaces slightly to avoid cutting.

After taking up the tiles, spread a small amount of adhesive according to the manufacturer’s instructions.

Observe all recommended safety precautions, such as wearing gloves and extinguishing nearby pilot lights. Set the first few tiles in place, with spacers between tiles. Repeat the process, working with small amounts of adhesive, until the entire floor is finished. Wipe off excess adhesive.

Allow the adhesive to set fully be fore applying grout. Remove the spacers as soon as you can walk on the tiles. Force grout into the cracks by spreading it diagonally with a rubber float or squeegee, working it into joints. Remove as much excess as possible while you work. Wipe off remainder with a wet sponge, rinsing it frequently. Then wait 30 minutes for a haze to form and polish the surface with a soft cloth. Follow manufacturer’s advice for curing the grout.

Installing Sheet Flooring

Whether resilient sheet flooring is laid loose, adhered only at the edges, or applied with adhesive under the whole sheet, it must be cut and fitted accurately. The most reliable method is to make a cutting pattern in the room out of 15# felt paper, roll out the resilient sheet in the driveway or similar large area, lay the pattern over it, and cut.

To make the pattern, roll out strips of felt building paper on the floor. Overlap rows 2 inches and tape them together. Cut out around cabinets and other obstacles. The paper should fit within ½-inch or so. To keep paper from shifting around, cut out small holes every 3 or 4 feet and stick a piece of masking tape over each one.

When the pattern is in place scribe a line around the perimeter, using a steel straightedge held against the wall. This line will be a guide for holding the straightedge when you cut the sheet flooring. After scribing a line around the entire room, roll up the pattern.

The resilient sheet, which comes in 6- and 12-foot widths, should be unrolled and allowed to “relax” at room temperature for several hours before cutting. Lay pattern over material, taping it down the same way it was taped to the floor. Then use the same straightedge to guide the cutting knife as you used for scribing the line around the perimeter. Hold the straightedge against the line and cut on the outer edge.

After cutting, discard the pattern, roll up the resilient sheet, and unroll it in the room. Adjust it to fit. If you are applying adhesive, roll back the resilient sheet to expose one edge at a time (or half of the room if you are adhering the entire sheet). Apply adhesive to floor with notched trowel, roll sheet out, and press it in place with your foot or a flooring roller.

Installing Wood Flooring

There are various types of wood flooring, but the easiest to install is prefinished 3 tongue-and- groove. Nails will not show because the tongue is covered by the groove of the next board.

If the floor isn't directly over a heated space, staple 1 5# building paper over the subfloor first. Overlap the edges 4 inches. Then snap a chalkline along the starter wall that's square to the other walls. (Check by measuring the diagonals of the room.)

Start by “racking,” or loosely arranging 6 or 7 rows of boards so the joints create a pleasing pattern. Mix long and short boards, staggering each joint at least 6 inches. Begin each row with the cut-off piece from the end of the previous row. Leave enough space against the starter wall to lay the first course.

Lay the first board against the chalkline with tongue edge toward the room, leaving a ½-inch gap at walls. Carefully face-nail this board with 8d finish nails at 8 inches on center. Complete the starter course with more boards, leaving a ½-inch gap at the end. Use the cut-off piece from the last board to begin the next row. Nail this row by driving nails through the tongue at a 45-degree angle. Set the nailheads. After the first 3 or 4 rows, use a nailing machine for rest of room.

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