Framing the Floor
The floor of a large addition may require special techniques in addition to basic joist framing. There may be step-downs for sunken rooms or multiple levels. If tile floors are in stalled over a mortar bed for a bath room or entry floor, joists will have to be dropped to accommodate the thicker finish floor. Joists should be doubled or even tripled under concentrated loads like a bathtub, partition wall, or fireplace. For floors with large spans, truss joists are stronger than conventional lumber joists and also make it easier to run plumbing and duct work under the floor.
Don’t overlook outside access, foundation vents, and insulation when framing the floor over a crawl space. Be sure to get any subfloor utilities inspected before laying insulation and subfloor.
Framing the Walls
A large addition is likely to have some walls over 20 feet long. Build and raise such walls in sections rather than all at once.
As with any remodeling, verify all dimensions on the plans with site conditions before framing the walls. Consider the following questions.
+ Do dimensions in plans refer to centers of walls or edges?
+ Are wall heights standard, using conventional studs, or do they match existing nonstandard wall heights in the house?
+ Should window and door headers be set at standard heights or at heights to match existing ones?
+ Do new walls line up with existing walls that are temporarily hidden be hind the exterior wall of the house?
Framing the Roof
A large roof is beyond the skills of most homeowners and should be contracted out, especially if it involves dormers, angles, or complex overhangs.
If you are doing a fairly simple roof yourself, study the plans and familiarize yourself with the existing roof structure in order to match angles or join roofs smoothly. Be prepared to spend extra time on details such as lookouts, soffits, fascia boards, barge rafters, and skylight openings. They can easily take as much time and effort as main rafters.
If you use manufactured trusses, arrange for delivery to the top of walls. Have three or four helpers available to help tilt them up.
A large addition will include many framing details to handle after the basic shell is erected—for example, a built-in metal fireplace. Get installation specifications from the manufacturer and observe local code requirements such as minimum clearances and location of firestops.
You may need backing for cabinets or towel racks. Gable louvers, foundation vents, and scuttles to the attic are other special framing needs that are easy to overlook.
A skylight installed over a drop ceiling requires an enclosed shaft, and stairs demand meticulous carpentry to be attractive and safe. Closets need to be framed if they were omitted from initial wall framing.
Closing in the Shell
As the addition takes shape during framing, the push to close in the shell makes it easy to leave out critical steps in installing sheathing, windows, siding, and roofing, or to install them out of proper sequence. Generally the sequence is roofing, windows, siding, but it can vary depending on the material used.
One of the most important features of a weatherproof shell is metal flashing for certain joints and openings. Itemize your flashing needs as early in construction as possible and have pieces on site during framing. Some are stock items, such as valley and vent flashings for the roof, louvers for end walls, and angle flashings for siding joints. But others may need to be specially fabricated, such as chimney or skylight saddles and special flashing over windows or doors. For a large project it's advisable to have a sheet metal contractor take care of all flashing, along with duct work and other heating needs.
Roof sheathing and roofing usually go on as soon as the rafters are framed. Be sure plumbing vents, chimneys, flues, hood ducts, and flashings are completed before you install roof covering.
Windows will vary with design, but most go on directly over framing or sheathing, before any siding is in stalled. They can often be done while you are waiting for the plumber or sheet metal contractor to finish roughing in.
Before the exterior shell is completed, schedule the stocking of wallboard inside the shell with your wallboard installer. You may need to leave out a window or a section of siding until the wallboard is stocked.
If the siding material is stucco, make sure interior wallboard is nailed off before stucco is applied. Nailing may disturb fresh stucco.
Installing Plumbing, Heating, and Wiring
The plumbing and electrical wiring in a large addition are similar to those in any addition with a bathroom or kitchen. But large and small additions are significantly different in their heating and air-conditioning needs.
Unlike a one-room addition, a large addition needs a major heating and /or cooling system. The options are to install a new separate system for the addition, to extend an existing central-heating system to the new living spaces, or to replace and enlarge the existing system with a completely new one. The easiest and most efficient solution is to install a separate system for the new addition. This avoids the expense of replacing components and allows you to zone the house into two separate areas. You can save energy by heating or cooling only the area you are using.
If you are using a fireplace or wood-burning stove for heating, have it installed by the vendor. The cost is usually reasonable, and you are assured of having a safe installation that meets local code requirements.
Finishing the Interior
When rough plumbing, mechanical, electrical, and framing inspections are completed, you can install insulation and finish interior walls and ceilings with wallboard or other approved surface material. The techniques are the same as for any new construction. After proper inspections you can proceed with trim, painting, finish wiring, fixture plumbing, cabinets, floor covering, and other finish operations.