Build Strong and Stylish PorchesDesigning the structure to complement a home

Unlike decks, porches provide shelter from rain and relief from sun. Screened, they keep out insects. The roof that makes it all possible raises the level of difficulty of the build, whether you're talking about engineering footings to handle extra loads, trimming out the walls and ceiling, or framing and shingling the roof itself. Even so, most of the skills required to build a porch you already use when you build a deck.

The covered space also provides opportunities for you to make profitable add-on sales, such as fireplaces, lighting, ceiling fans, and custom trim-out. Porches account for at least half my annual sales dollars.

Architectural Limits
A porch addition should look like it was part of the home's original plan. When I visit a house to sell a job, I measure carefully and pay attention to architectural details such as wall features and trim, roof pitches, windows, doors, and fireplaces. For example, if there's a 12-on-12 gable near the planned porch, I'll mirror that pitch with the porch roof, if possible (Figure 1). I might also need to account for existing skylights, vent pipes, and level changes within the house.

Figure 1. Matching the roof pitch is one way of grafting a new porch onto an existing house.

Outside, I match up the porch overhangs with those on the house. Inside a new porch, I remove the house's cornice overhangs so you see nothing but new ceiling. If the second floor cantilevers out into what will be the new porch, I incorporate the cantilever in the ceiling finish (Figure 2).

Figure 2. Covering a house's second-story cantilever with the same material that's on the porch ceiling helps to blend it in.

Beyond style considerations, the location of existing windows and doors can physically limit where the porch can be attached to the house. Upper-story windows near the intersection of the porch roof and the house wall may affect the roof pitch or height.

Porches vs. Decks
I generally design both porches and decks to be wider than they are deep, for stability. Unless it's a simple front entry, a porch should extend 12 feet or more out from the house because a shallower porch doesn't allow enough room to walk around furniture. An acceptable minimum size is 12 feet by 16 feet.

Floor framing and footings are somewhat different for porches than they are for decks. Roof loads need to be factored in, which means the footings will probably be larger than they would be for a deck. Unless you run continuous columns from the roof to the footings all along the side of a porch, side joists under the eaves of a gable roof have to be designed as beams to support the columns that carry the roof (Figure 3). To attach these beams to the house, I use an approved inside-corner bracket at the ledger. As the 2009 IRC comes into force, we may no longer be able to hang beams from ledgers -- then the end of a beam would require its own footing. Check with your local inspector.

Figure 3. Steel strapping at the corners and at post connections helps keep a porch tied to the ground in severe winds. Gravity loads from the roof are carried to the posts through a continuous perimeter beam, and then to the ground.

Positive footing-to-column, column-to-beam, and beam-to-joist attachments are even more important on porches than on decks because of the increased wind-lift potential created by the roof. On decks, I commonly cantilever joists past the outer beam, whereas on porches, I place outer beams below the very end of the joists. This lets me place columns at the outer corners of the structure, simplifying the roof support and tie down, and allowing downspouts to run straight to the ground.