Raised Floor Systems 10 Advantages of the Raised Floor Classic style. Homebuyers are rediscovering the classic elegance that a raised floor design adds to the look of the American home. Drive through the older neighborhoods of your community. The homes with ageless grace and beauty are usually raised. Visually, the raised foundation functions as a pedestal, enhancing curb appeal regardless of architectural style. Uplifting comfort. Inside the raised floor home, there are special qualities that nurture the body and soul. The feeling is warmer, more intimate. Remember that special place when you were a child, a tree house perhaps, where you felt cozy and safe? Raise the floor above the ground and your perspective changes. You feel more secure. An ordinary view from the window becomes a vista. There is calm and quiet above the din of street noise. The subtle "give" of a wood floor system provides a more comfortable walking surface, putting less stress on your back, legs and feet. Enjoy your porch and deck. If it is not raised, a porch is not really a porch. Porches and decks are natural amenities for a raised floor system, adding usable living area to your home outdoors. Consider the charm and hospitality of a front porch, the pest-free privacy and comfort of a screened back porch, or a spacious deck for outdoor cooking, entertainment and relaxation. On slab, a "front porch" is just a front patio. The special ambiance you come to expect when gathering on a porch or deck can only be experienced with the raised floor system. A lifetime foundation. The raised floor system has withstood the test of time. Some of the oldest, most historic homes in America are still standing proud with Southern Pine floor assemblies. Southern Pine is the strongest of all North American softwoods, so you can expect a lifetime of strength, stability and durability when framing with Southern Pine. And today, modern pressure-treatment adds near-permanent resistance to decay and termite attack. A simple foundation. All foundations will settle, but use of piers with a raised floor system makes leveling or repairs simple. Repairing or leveling a cracked, damaged concrete slab can be very expensive. With the raised floor system, leveling is as easy as jacking up the floor and adding shims. The raised floor is also less susceptible to disruption from tree roots. Easy home improvement. Installation, maintenance and modification of utilities such as water, sewerage and electrical are comparatively simple with a raised floor. Remodeling? Plumbing fixture modifications are easy with the raised floor system. If you want to move the location of a toilet or bathtub on a slab foundation, get the jackhammer! Routing and rerouting of wiring for electrical, telephone, television and computers is relatively easy and less expensive. If the home is raised high enough off the ground, air conditioning ducts can be installed below where floor registers can direct air closer to the actual living area. The ceiling remains uncluttered. Natural insulator. Why live on the cold, damp ground? Wood is a natural insulator. A properly constructed and insulated raised floor system isolates your home from potential moisture problems and can provide a warm and comfortable walking surface. High and dry. Flooding is a potential risk for many homeowners. A raised floor system could be the solution to raising your home's foundation above base flood elevation required by the National Flood Insurance Program. Considering other options - slab atop dirt fill, slab on backfilled perimeter wall, or ring levee - the raised floor may be the most practical and cost effective way to protect your property and meet local building ordinances in flood prone areas. Pest control. Most pests are ground dwellers. With proper barriers to control termites and other pests, your raised floor home will welcome fewer unwanted guests. For optimum protection against termites, pressure-treated and kiln-dried after treatment (KDAT) Southern Pine lumber is available. Flexible landscaping. Landscaping looks best around the foundation of the home. It helps "ground" the structure to the site. However, the installation of built-up flowerbeds and other landscaping against a slab foundation can invite termite infestation and rot. With a raised floor, landscaping can be located near the foundation without inviting this risk. Furthermore, root-severing slab construction often demands the removal of existing trees in close proximity to the structure. Near a raised floor, these valuable, beautiful and energy-saving shade trees can be preserved. Footings of a raised pier and beam structure only penetrate the root system, allowing trees to thrive. For additional raised floor advantages, refer to the SPC publications Raised Floor Advantages and Raised Floor Design and Construction Guide available at www.southernpine.com. Key Elements of the Raised Floor System 
Footings and Foundations A firm foundation, including properly installed footings of adequate size to support the structure and prevent excessive settlement, is essential to the satisfactory performance of buildings including raised floor systems. Foundation systems are often classified as shallow or deep foundations, depending on the depth of the load-transfer member below the super-structure and the type of transfer load mechanism. The required foundation system depends on the strength and compressibility of the site soils, the proposed loading conditions, and the project performance criteria (i.e. total settlement and differential settlement limitations). Foundation designs are based on the assumed bearing capacity of the soil at the building site. In construction sites where settlement is not a problem, shallow foundations provide the most economical foundation systems. Shallow foundation construction is typically utilized for most residential and light commercial raised floor building sites. Where poor soil conditions are found, deep foundations may be needed to provide the required bearing capacity and to limit settlement. Additionally, structures in coastal high-hazard areas are required to be elevated above the base flood elevation (BFE), commonly on piles. Examples of deep foundation systems include driven piles (e.g. pressure-treated timber piles, concrete, or steel), drilled shafts, or micropiles. Types of Footings Footing requirements are generally covered in the building code and sized in accordance with the bearing capacity of the soil and the weight of the building. In areas subject to seasonal frost, the bottom of the footing must be placed below the frost line to prevent damage to the footing and structure due to frost heave. Typical footing types include:
Spot Footings A spot or pad footing is used to support a single point of contact, such as under a pier or post. A spot footing is typically a 2' by 2' square pad, 10" to 12" thick, and made with reinforced concrete rated to 3,000 to 5,000 pounds per square inch (psi) in compression. Continuous Spread Footing A continuous spread footing is commonly used to provide a stable base around the entire perimeter of a structure. Buildings with spread footings often include interior spot footings. A spread footing supports the weight (load) from the exterior or foundation walls. The footing thickness provides the strength needed to support the weight. The wider width of the footing base creates a large area to transfer this weight to the ground and to prevent settlement. The dimensions of a continuous spread footing vary according to the soil conditions under the building, the load placed on the footing, and the construction style of the structure being supported. It is common practice to make the footing thickness equal to the thickness of the foundation wall, and to provide a footing projection on each side of the foundation wall equal to one-half the foundation wall thickness. Spread footings are frequently 16" to 24" wide, 6" to 16" thick, and made with reinforced concrete rated to 2,000 to 5,000 psi in compression. Grade Beam Footing A grade beam footing is a continuous reinforced-concrete member used to support loads with minimal bending. Grade beams are capable of spanning across non-load bearing areas, and are commonly supported by soil or pilings. A continuous grade beam is frequently constructed by digging a trench at least 8" wide to the depth needed to span the distance between supports. Grade beam footings differ from continuous spread footings in how they distribute loads. The depth of a grade beam footing is designed to distribute loads to bearing points, while the width of a continuous spread footing is designed to transfer loads to the ground. Types of Foundations The two most commonly used foundations with raised floor systems are pier-and-beam and stem wall foundations. Regardless of the foundation system used, the foundation and the footings must be of adequate size and strength to support the design loads. Pier-and-Beam-Foundations Pier foundations are commonly constructed of reinforced masonry (brick or concrete block) supported by individual, reinforced-concrete pad footings or by continuous, reinforced-concrete spread footings. For pier-and-beam foundations, pier spacing will also depend upon arrangement of floor framing, particularly the location of bearing walls and partitions. Spacing of piers in the range of 8' to 12' is common practice. The openness of pier foundations creates natural venting of the crawlspace. Continuous Foundation Walls Continuous (stem wall) foundations are frequently constructed of reinforced masonry or poured concrete, supported by a continuous, reinforced-concrete spread footing. Stem wall foundations may include interior spot piers for support of the raised floor system. Moisture control of the crawlspace created by the stem wall foundation is an important issue. Pile Foundations Where poor soil conditions are found, foundations may need to be constructed on preservative-treated timber piles capped with wood or concrete sills. In such buildings, support may be provided by the end-bearing capacities of the piles or by friction between the pile and soil. In pile-supported structures where the building support relies upon friction between the pile and soil, two important soil parameters must be known or determined:
Friction piles may also be required to support standard foundations in unstable soil. In buildings supported by pile foundations, the layout of the horizontal girders and beams should consider that the final plan locations of the tops of the piles may not be precise. Irregularities in the piles and the soil often prevent the piles from being driven perfectly plumb. The use of thick shims or over notching for alignment at bolted pile-girder connections will adversely affect connection capacity. A rule of thumb regarding notching is to notch no more than 50% of the pile's cross-sectional area. Notching more than 50% will require reinforcing the pile with a steel plate or other suitable material. Pile foundations are also used in coastal areas where the foundation may be subject to inundation and possible wave action. Elevated wood pile foundations enable buildings to be constructed above the base flood elevation (BFE) as required by the National Flood Insurance Program. For information on coastal construction, consult the FEMA Coastal Construction Manual.1 For additonal information on footing and foundations for raised floor systems, including numerous construction detail figures, refer to the Raised Floor Design and Construction Guide available at www.southernpine.com. 1 Coastal Construction Manual: Principles and Practices of Planning,
Siting, Designing, Constructing and Maintaining Residential Buildings in Coastal
Areas, Federal Emergency Management Agency, www.fema.gov.
Moisture Control Protecting wood products from moisture is an important factor in preventing fungal decay. Wood framing maintained at a moisture content of less than 20% will not decay. A raised floor foundation separates a structure from one of the biggest sources of moisture – the ground itself. With proper design, construction and maintenance practices, a raised floor system can remain dry and free of moisture-related problems. Moisture control can be accomplished primarily through the application of basic, proven construction practices:
For additional details, see the Design of Wood Structures for Permanence — Wood Construction Data No. 6 from the American Wood Council at www.awc.org. Site and Building Drainage Providing proper site and building drainage is critical to moisture control for any foundation system. Proper drainage is needed to keep the foundation and underfloor areas dry. For a raised floor system, it is especially important that standing water be kept out of the crawlspace. Controlling moisture requires effective control of rainwater and ground water. Managing rainwater drainage from the building's roof helps to keep the foundation and under-floor areas dry. Most important is the use of gutters, downspouts and splash blocks or drainpipes to direct the water runoff away from the foundation. Also, floor areas of adjacent porches or patios should be sloped to drain rainfall away from the structure. Whenever possible, the elevation of the crawlspace floor should be higher than the exterior grade. When that is not possible, perimeter drains should be included. For open pier-and-beam foundations, positive drainage within the crawlspace is important. The ground should be graded to maintain a dry crawlspace and to drain water away from the structure. On a level site, this could involve slight crowning, centered beneath the raised floor. On a sloped site, the ground should be graded so the water exits through and away from the pier system. For continuous wall foundations, drain tiles installed around the entire footing perimeter can greatly reduce moisture within the crawlspace. The drain tiles should lead to a storm drain, a sump with a pump, or other positive drain system that carries the water away from the structure. The finish site grade for any project should slope away from the structure to provide positive drainage away from foundation walls. This is important for keeping any type of foundation dry and trouble free. A minimum slope of 5% away from the structure is recommended. Typical site grading creates a fall of at least 6" over the first 10' away from a foundation. Drains or swales can also be provided to ensure drainage away from the structure. Crawlspace Design and Construction1,2 Designers, contractors and homeowners must understand the connection between the crawlspace of a raised floor system and the living space above. Every effort should be made to keep moisture out of the crawlspace. Provisions should also be made to maximize drying of any moisture that does manage to get into a crawlspace. Local climatic conditions will dictate the specific design and construction details for a particular raised floor system. The subject of moisture control in crawlspaces is an area of ongoing research. Most crawlspaces today are constructed as unconditioned and vented systems, with building codes mandating minimum ventilation requirements. As an alternative, conditioned and unvented crawlspace systems are sometimes used and also recognized in the codes. Unconditioned and Vented Crawlspaces. Design detail considerations for unconditioned and vented crawlspaces are available at www.southernpine.com. Important elements for the best moisture control include:
HVAC systems of many houses built over crawlspaces deliver conditioned air through ducts located in the crawl-spaces. Whenever possible, ductwork should be located within the floor cavity (e.g. between joists, or between or through trusses or I-joists). When that is not possible, adequate clearance between the bottom of the ductwork and the ground should be provided to maintain proper ventilation. All ductwork should be meticulously sealed to avoid unnecessary energy losses from air leaks. Penetrations for plumbing, wiring and air ducts should also be sealed to minimize air exchange between the crawlspace and the living space. Ductwork should also be insulated to prevent condensation on the ducts. In addition, insulation should be carefully installed to the underside of the floor within the floor cavity. Never vent moisture or heat-producing sources (e.g. clothes dryers, kitchen or bath vents) into the crawlspace. Crawlspace Ventilation. Building code requirements for ventilation openings through foundation walls are intended to reduce moisture levels in the crawlspace. Section 1203.3 of the 2003 International Building Code sets forth the underfloor ventilation openings and cross ventilation requirements for enclosed crawlspaces, such as within stem wall foundations. Open pier-and beam foundations, commonly used with raised floor systems, already create a fully vented crawlspace. Generally, building codes mandate that the minimum net area of ventilation openings required are not less than one square foot for each 150 square feet of crawlspace area. When an approved vapor retarder covers the underfloor ground, the minimum vent opening area can be decreased to one square foot for every 1500 square feet of crawlspace area. Vent openings are placed to provide cross ventilation of the underfloor space. These vent openings should be screened to inhibit pest entry into the crawlspace. They also should not allow rain water or runoff to enter into the crawlspace. Conditioned and Unvented Crawlspaces. Conditioned and unvented crawlspaces are only recommended when mechanical systems distribute conditioned air within the underfloor area. A conditioned and unvented crawlspace typically has insulated walls and can be thought of as a short basement. This type of crawlspace is designed to communicate with the living space. It should be dry, temperate, and have good air quality. Conditioned air spaces should not be ventilated with outdoor air. Conditioned and unvented crawlspace systems should have a continuous ground cover sealed to insulated perimeter walls and any supporting piers. Care should be taken with all air-sealing construction details. This is necessary to minimize the unintentional introduction of unconditioned air, reducing the possibility of condensation on cold surfaces. In addition, extra care should be taken to prevent moisture from being trapped in the crawlspace. Any moisture that does get into the crawlspace should be remediated immediately. Ground Cover (Vapor/Gas Retarder). Draining storm water away from the foundation, preventing standing water beneath the crawlspace, and making provisions to remove excess moisture entering the crawlspace, are all-important elements needed to provide a dry, trouble-free raised floor system. Control of ground moisture is also essential. One of the best ways to control this moisture is through the use of a ground-applied vapor retarder. Exposed soil in crawlspaces and under porches and decks should be covered with an approved vapor retarder. A ground cover that retards transmission of water vapor from the soil into the crawlspace provides an effective way to prevent moisture and humidity problems. It should have a permeance of no more than 1.0 perm, complying with ASTM E1745, to resist alkali and other chemicals that can be contained in soils. It should also be rugged enough to withstand foot and knee traffic. The most commonly used ground cover material is a 6-mil (0.006 inch) polyethylene. Before installation of the ground cover, the crawlspace floor should be smooth and free from sharp rocks and construction litter. Exact installation details will vary depending on the primary function of the ground cover (i.e. moisture control or radon gas control). For any crawlspace system, it is important to avoid standing water on top of the ground cover. For unconditioned and vented crawlspaces, the edges of the cover should be overlapped 4" to 6". The cover does not need to extend up the face of the foundation wall, and no sealing is required. If the control of radon or other soil gases is not of primary importance, the ground cover may be cut in several low spots to provide drainage if needed. Conditioned and unvented crawlspaces should have a continuous ground cover over all crawlspace soil. The ground cover should be sealed at the joints, as well as sealed to the perimeter wall and any piers. A thin layer of concrete added over the ground cover provides a better seal and further inhibits the entry of rodents. Inspection and Maintenance As with any structure, long-term performance of a raised floor system requires regular building inspections and maintenance. Building codes require access to all under-floor spaces, with a minimum opening of 18" by 24" through a floor, or 16" by 24" through a perimeter wall. The crawlspace should be inspected periodically to check for excessive moisture conditions from leaky plumbing, HVAC ducts, or standing water. The crawlspace should also be inspected periodically for the presence of damaging pests. Needed corrective actions or repairs should be made promptly to avoid problems related to moisture or pests. For additional information on moisture control, as well as a variety of other important design/build considerations for raised floor systems, refer to the Raised Floor Design and Construction Guide available at www.southernpine.com. Guide to Porch Flooring & Construction The porch has withstood the test of time as an icon of American architecture, adding comfort, distinction and value. Today's home designs incorporate the porch as a natural extension of the family's living space. Southern Pine flooring has enjoyed a long history in porch construction. As with indoor flooring material, the effect of moisture in contact with wood is a top concern when designing and building a porch. Southern Pine, combined with the technology of wood preservation, is a superior porch flooring choice. With its built-in resistance to decay and termites, pressure-treated Southern Pine porch flooring, properly installed, will provide decades of satisfying service. For more detailed information on porch flooring and construction, refer to the Guide to Southern Pine Porch Flooring brochure and Porch Flooring Construction video available at www.southernpine.com. To find a source of supply for Southern Pine porch flooring, use the Product Locator at www.southernpine.com. 1 Handbook of Fundamentals, American Society for Heating, Refrigerating
and Air 
Sizes, Grades and Patterns
The size, grade and pattern of flooring utilized in porches will depend upon the type of protection given to the structure. Porches without complete roof protection are generally constructed in the same manner as outdoor decks, incorporating a surface of either 2"x6" nominal size or 5/4x6 radius-edge pressure-treated Southern Pine. A dimension (2x) lumber grade of No.1 provides optimum appearance. Radius-edge decking is available in Premium or Standard grades. Typically, nominal thicknesses are 1" and 1¼" (¾" and 1" actual) with the tongue-and-groove pattern, available in widths of 4" to 6" nominal (3⅛" to 5⅛" actual). The grade of C&Better is most-often specified for porch flooring applications. Porch Design Considerations Attention to proper porch design 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 greatly reduce the serviceability and long-term appearance of the porch. The following recommendations are key elements to the proper design and construction of a fully-covered porch:
Southern Pine Council 2900 Indiana Ave. Kenner, LA 70065 Tel: (504) 443-4464 Fax: (504) 443-6612 E-mail: help@southernpine.com Web site: http://www.southernpine.com |