Building a Porch
Building a
Southern Pine Porch
Are you looking at building a porch? An icon of American architecture, Southern Pine porches have withstood the test of time, adding outdoor comfort, distinction, and value to a home.
Today’s home designs incorporate porches as a natural extension of the family’s living space, matching it to the home’s architectural style rather than something tacked on as an afterthought.
Porch Flooring Selection
Unlike decks, porches typically have 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.
Many building codes require pressure-treated wood with specific preservation retention levels when used close to the ground. This requirement applies to flooring and floor joists within 18 inches of exposed soil and structural materials such as columns, posts, girders, and beams within 12 inches of exposed soil.
When building a porch, make sure the wood is treated for its intended exposure by checking plastic end tags or ink stamps affixed to the lumber for “above ground” or “ground contact.” SFPA’s Pressure-Treated Southern Pine provides full specifications.
Southern Pine Porch Flooring Selection Guide
| Grade | Characteristics |
|---|---|
| C&Better | Combination for B&B and C grades; satisfies requirements for high-quality finish. |
| D | This grade requires a face as good as D Finish grade except scant width of face not permitted and only medium warp allowed. Economical, serviceable grade for natural or painted finish. |
| No. 1 | No. 1 Flooring is not provided under SPIB Grading Rules as a separate grade but, if specified, will be designated and graded as D flooring. |
| No. 2 | This grade requires a face as good as No.2 boards, which is suitable for high-quality sheathing and provides high-utility value where appearance is not a factor. |
Moisture Content and Acclimation
Before pressure treatment, Southern Pine T&G porch flooring has a typical moisture content around 12% if marked as kiln-dried (KD). Pressure treatment increases wood’s moisture content, often to 50% and higher.
For porch flooring, it’s recommended to specify material that is kiln-dried-after-treatment (KDAT) as it can enhance dimensional stability and reduce the lumber’s tendencies to warp, twist, and cup. Re-drying will return each piece of lumber to a workable moisture content, generally to 19% or less.
Allow all pressure-treated porch components, framing, and flooring time to acclimate, as flooring installed too soon on wet framing risks buckling or separation. A good rule of thumb is one to two weeks after the frame is built 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 begins with wood’s 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, loosely cover the wood to protect it from moisture (4-mil polyethylene is commonly used) but allow adequate air circulation within the package. Avoid storing the flooring in an area that receives direct sunlight most of the day or in an enclosed, heated space so it doesn’t over-dry below EMC.
Planning and Preparation
When building a porch, attention to proper 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 impact the porch’s long-term appearance.
Four key elements to ensure the proper design and construction of a fully covered porch are:
- Slope soil underneath the deck so water drains away from the structure and prevents ponding.
- Cover the soil below the porch with a moisture barrier (4-mil polyethylene is acceptable) to reduce the upward migration of moisture. Leave two feet of exposed soil inside the perimeter of the porch, and anchor edges of this barrier with gravel.
- Use ornamental vents or lattice skirting to encourage air flow beneath the porch. Also, vent columns and newel posts at top and bottom.
- Check with your local building official to ensure your porch design complies with all code requirements.
Framing and 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 T&G porch floor is a solid surface, the joist system must be sloped away from the house ¼-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 of a half-inch expansion space between the flooring area and house (or wall) to allow for dimensional change. This space can be concealed with decorative exterior trim.
Don’t forget to extend flooring beyond the porch front band joist to allow a 1-inch overhang.
Priming and Finish
Proper porch flooring installation involves applying certain finishes before installation.
Many builders will first coat the top of all floor joists with a water-repellent sealer to protect against joist expansion because of moisture collection. For the porch flooring, apply a coat of high-quality, mildew-resistant oil-based primer on all four sides and ends. Next, consider applying a coat of the final oil-based porch enamel to the T&G (and 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 ensures an effective seal against moisture penetration and provides a bond between floorboards.
After installation, apply two coats of the oil-based porch enamel to the surface and exposed ends.
For an unpainted porch, it’s still recommended to apply a coat of water-repellent sealer to all four sides and ends before installation. If using a stain, select a penetrating oil-based semi-transparent product, but avoid opaque stains because they can wear prematurely.
Maintenance
Expect to refinish a Southern Pine porch every three to five years, depending upon weather conditions and the amount of exposure to direct sunlight.
Conduct regular maintenance and inspections to maintain optimum performance and quality. Inspect the porch for water accumulation, integrity of the soil barrier, and any fastener problems such as raised heads or corrosion. Take any necessary corrective actions as soon as possible.
Additional Resources
Check out our Southern Pine Decks and Porches publication, a comprehensive guide to deck and porch specification and construction using pressure-treated Southern Pine, to learn more about proper planning, building, finishing, and maintenance.