Building A Porch
The porch has withstood the test of time as an icon of American architecture, adding outdoor comfort, distinction and value to the home. Today's home designs incorporate the porch as a natural extension of the family's living space. If adding a front porch as part of a remodeling project, carefully select a design that enhances curb appeal. Match the porch to the architectural style of home so that it becomes a unified part of the home and not something tacked on as an afterthought.
Porch Flooring Selection
Unlike decks, porches are fully-covered outdoor floor systems. Tongue and groove (T&G) flooring is traditionally installed and since porches are only partially protected from weather, preservative treatment is typically recommended. Porch flooring is similar in sizes, grades and patterns to flooring for interior use.
Typically, nominal T&G thicknesses are 1-1/4" (1" actual) and are available in widths of 4" to 6" nominal (3-1/8" to 5-1/8" actual). The grade of C&Better is most often specified for porch flooring applications. See Table below for complete grade descriptions. The homeowner may also opt for standard lumber (2x4" or 2x6") or 5/4x6” radius-edge decking, but will not get the traditional “outdoor room” ambiance and utility of the solid surface, no-gap T&G floor.
To combat the deteriorating effects of outdoor exposure, moisture, decay and termite attack, pressure treatment with a waterborne preservative is recommended for all wood components of the porch. Most building codes require that wood in close proximity to the ground be pressure-treated to specific preservative retention levels. This requirement applies to flooring and floor joists within 18" of exposed soil and structural materials such as columns, posts, girders and beams within 12" of exposed soil.
Make sure the wood is treated for its intended exposure. Check plastic end tags or ink stamps affixed to the lumber for “above ground” or “ground contact.” Do not use lumber marked for above-ground contact for ground contact applications. If using Southern Pine, refer to SFPA's publication Pressure-Treated Southern Pine for full specifications.
Southern Pine Porch Flooring Selection Guide
Moisture Content & Acclimation
Before pressure treatment, Southern Pine T&G porch flooring has a typical moisture content of around 12% if marked as kiln-dried (KD). Pressure-treatment greatly increases the moisture content of the wood, often to 50% and higher. For porch flooring, specification of material that is kiln-dried-after-treatment (KDAT) is highly recommended. Re-drying will return each piece to a workable moisture content, generally to 19% or less. The advantages of KDAT flooring material include enhanced dimensional stability, plus reduced tendencies to warp, twist and cup.
A period of acclimation should be given to all pressure-treated porch components, framing and flooring. Flooring installed too soon on wet framing risks buckling or separation. Generally, after the porch framing is up, one to two weeks is considered an adequate acclimation period for KDAT porch flooring to reach equilibrium moisture content (EMC). After installation, wood used in exterior applications will reach an average 12% moisture content in most areas of the U.S.
Proper acclimation of the flooring begins with its delivery to the job site. The material should be unloaded in a dry place and stacked on stringers to permit adequate air circulation. If the temporary storage area is not fully protected from precipitation, the flooring must be loosely covered for protection from moisture (4-mil polyethylene is commonly used), yet allowing adequate circulation of air within the package. Avoid storing the flooring in an area that receives direct sunlight most of the day, or in an enclosed, heated space as this could over-dry the flooring below EMC.
Planning & Preparation
Attention to proper porch design and site preparation is as important to the longevity of the structure as are the details of porch flooring specification and installation. Inadequate air circulation beneath the porch and trapped moisture between framing components will reduce serviceability and hurt long-term appearance of the porch.
The following recommendations are key elements to the proper design and construction of a fully-covered porch:
- To eliminate ponding of water under the porch, slope soil underneath so runoff exits away from the structure.
- To reduce the upward migration of moisture from the exposed soil beneath the porch, cover with a moisture barrier (4-mil polyethylene is acceptable), leaving two feet of exposed soil inside the perimeter of the porch. Anchor edges of this barrier with gravel.
- Encourage air flow beneath the porch by using ornamental vents or lattice skirting. Also, vent columns and newel posts at top and bottom.
Check with your local building official to be sure all code requirements are satisfied within your porch design.
Framing & Installation
The recommended maximum joist spacing for installing Southern Pine porch flooring is 16 inches on center. For a more solid feel, some builders will opt for 12 inches on center. Since a tongue-and-groove porch floor is a solid surface, the joist system must be sloped away from the house 1/4 inch per foot to permit adequate water runoff.
Unlike interior flooring, T&G porch flooring is fastened directly to the floor joists. No additional nailing base is needed. Each piece of flooring is blind-nailed at every joist, using hot-dip galvanized 8d ring-shank nails.
Maintain a minimum ½" expansion space between the flooring area and house (or wall) to allow for dimensional change. This space can be concealed with decorative exterior trim (also acclimate). Extend flooring beyond the porch front band joist to allow a 1" overhang.
Priming & Finish
Reducing the opportunities for dimensional changes to the porch flooring due to moisture fluctuations is key to long-term performance. Proper porch flooring installation actually involves applying certain finishes prior to installation.
Many builders will first coat the top of all floor joists with a water-repellent sealer to protect against joist expansion due to moisture collection. For the porch flooring, apply to all four sides and ends a coat of high-quality, mildew-resistant oil-based primer. Next, consider applying a coat of the final oil-based porch enamel to the tongue and groove (also to the end of any piece that will be adjacent to the house) and install as you go while the paint is still wet. This procedure not only assures an effective seal against moisture penetration, but also provides a good bond between floor boards. After installation, apply two coats of the oil-based porch enamel to the surface and exposed ends.
For those who prefer an unpainted porch, applying a coat of water-repellent sealer to all four sides and ends before installation is still recommended. If a stain is desired, select a penetrating oil-based semi-transparent product. Opaque stains should be avoided because they will wear prematurely.
To maintain optimum performance, regular maintenance and inspection should be performed. Inspect the porch for water accumulation, integrity of the soil barrier and any raised fastener problems such as raised heads or corrosion. Take corrective action as soon as possible. Refinishing porch flooring can be expected every 3 to 5 years, depending upon weather conditions and the amount of exposure to direct sunlight.