Floor Systems

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Basement floors should be concrete having a minimum compressive strength of 2500 psi., 3½ to 4 inches thick, and be pitched to a previously installed drain or sump system. The floor slab should be placed over a vapor barrier of heavy ( 6 mil.) polyethelene sheet, or other suitable vapor-resistant material which will withstand construction traffic without puncturing. The vapor barrier is laid over a 4 inch layer of compacted gravel or crushed stone. The purpose of the vapor barrier is to prevent upward migration of moisture from the soil into the slab, resulting in dampness or wetness of the floor. Any joints or seams in the vapor barrier should be taped together with water resistant tape. The gravel layer should be clean, washed stone, free of soil and other fines which would negate its purpose of further preventing upward migration of moisture to the floor. The gravel must be well tamped to stabilize and compact it. See (ill. 19).

Basement floors should be separated from the surrounding foundation walls by a continuous pre-formed expansion joint material, a minimum of 1/2 inch thick.

If it’s determined that a significant ground water problem exists, a waterproof membrane similar to the membrane discussed in the section on foundation walls, may be required under the floor slab. If installed, it’s placed on top of the gravel layer in lieu of the vapor barrier, extend across the top of the footings, and be flashed (made watertight) to the membrane on the outside face of the foundation wall. Any penetrations thru the membrane such as floor drains, pipes, re-bar, etc., are potential sources of leaks, and must be very carefully flashed and sealed tight.

To prevent a build-up of hydrostatic pressure which can actually lift up floor slabs, a drain system beneath the floor and a foundation drain system as discussed earlier should be installed to collect the water and lead it to a method of disposal away from the building. In a location where ground water is truly a serious factor, it may be prudent to simply avoid the basement.

ill. 20: Membrane Waterproofing At Basement Floor


In warm regions of the country, grade level concrete floors can be 3½ to 4 inches thick, of 2500 psi. concrete placed over a 4 inch gravel, stone or A.B.C (aggregate base course) layer which is placed on the prepared and compacted soil base. Floors for garages or carports should never be less than 4 inches thick, and preferably be reinforced with 6” x 6”, #10/10 welded wire steel fabric. These slabs may or may not be poured integrally with the stem, or foundation, wall, depending on the stem wall material and type of above-grade exterior wall it’s supporting.

In cold regions, grade level concrete floors should be isolated from the exterior foundation wall and be insulated to prevent signi1k heat loss at the floor perimeter. Such floors should also have the vapor barrier discussed previously for BASEMENT FLOORS.

Partitions which rest on concrete floor slabs and support floor and /or roof framing, require an equivalent footing under them, to uniformly transmit those loads to the soil. See ill. below for acceptable alternatives of constructing the footing.

ill. 21: Footing Poured Integrally With Slab; Footings For Interior Bearing Walls

Ideally, concrete floors should be installed before significant amounts of wood framing proceeds; especially before finish carpentry and cabinet work, so that the floor has a chance to set and dry out without its moisture being absorbed by the dry finish lumber, causing the wood to swell and warp.

The same concerns regarding curing of concrete as discussed earlier apply here, with even greater emphasis, because floors are relatively thin members with very large surface areas, and so are very subject to premature dry-out or to freezing.

There are available some very good chemical hardeners which when applied (usually by spray) within specified times of the finishing of the concrete, will densify and harden the top surface much more than un-treated concrete would achieve. This can be a very desirable treatment for floors which won’t receive tile, carpet or other covering, such as a garage or the basement floor. The hardener makes the floor more resistant to wear and surface damage from foot or vehicular traffic.

Large concrete floor surfaces will crack, due to shrinkage of the concrete as it sets and loses moisture. Certain locations of probable cracking can be anticipated, at which deliberate joints or breaks in the continuity of the concrete should be provided. These are called control joints. Control joints should be provided at: 1) Approx. 20 to 25 ft. intervals in large floor areas; 2) locations of major changes in the direction of the slabs; 3) Any point of significant narrowing or constriction in the shape of the slab. Expansion joints should be provided in concrete floor slabs at their intersections with different materials, such as where basement slabs meet foundation walls, and around columns or piers which penetrate thru ground-supported slabs.


This is the rough lumber used to construct the structural skeleton, or frame, of a building. It’s almost exclusively made from so-called “softwood” which is cut from evergreen trees such as fir, hemlock, spruce, etc. The lumber is sawn into dimensional sizes (2”x4”, 2”x6”, 2”x8”, 4”x4”, etc.), and either left with rough sawn faces, or machined smooth. It’s then dried to lower the moisture content. Drying is done by either a forced hot air process in a kiln, or by simply being allowed to air-dry. Drying is important, because as the moisture content of lumber is lowered, the wood shrinks; and , if incorporated into a building in the non- dry, or green, state, would ultimately dry and shrink by itself, with all the unfortunate consequences of poor fits, loosened connections, etc. “Nail popping” is one common evidence of this, especially nails used to attach gypsum drywall.

Lumber is graded according to rules of several Agencies and Manufacturers Associations into varying grades of structural quality and appearance. Building Codes will stipulate the grades which can be used for framing purposes, and these will vary depending on the application; i.e., load bearing or non-load bearing, studs, joists, beams, etc. Generally, construction grade or better should be used for load carrying studs; No.2 or better for floor or roof joists, and No.1 or better for heavy beams, posts, and timbers. All of these grades allow certain size knots and imperfections such as splits, checks, etc.

The designated sizes of dimensional lumber are always nominal (theoretical), not actual, since the finishing and drying processes always reduces the cut member to its actual size. For example, a smooth-faced 2”x4” will actually measure 1½”x3½”, a 2”x8 will measure 1½”x7½”, and so on. Lengths, however, will be the actual specified dimensions (8ft., 10ft., 12ft., etc.).


ill. 22: Platform Framing, Showing Diagonal Bracing; Note: Dotted lines indicate alternate locations of diagonal bracing.

The most typical wood frame configuration in use today for residential buildings, is called the platform frame. This simply means that each story is an entirely separate entity or platform the essence of which is shown in. Other configurations such as post and beam, braced frame and balloon frame do exist. However, it’s not pertinent to the purposes of this guide to discuss the particulars of each of those systems, since the basic points covered here will apply.

The repetitive floor support members in a wood frame building are called joists, selected for size based on how much distance they must span, their spacing, and the amount of load they must carry. Joists are normally 2” or 3” in thickness x the required depth such as 6”, 8”, 10” or 12”. If the joists are undersized, or of a less acceptable grade of lumber than should have been used, the floor may feel bouncy or springy when walked on. This is a sign of excess deflection under load, and can be at the minimum very annoying, at the worst structurally dangerous. Joists are usually spaced at 16” or 24” on centers, because these dimensions fit properly into the usual 4ft. or 8ft. modules of most materials (plywood gypsum board, etc.)

Wood sills, or plates, which are bolted directly to the foundation as the base for the joists, as well as any other wood in direct contact with concrete or masonry, should be of one or two 2” members, preservative treated to protect against damage by moisture and insects.

Joists should be supported by resting on a horizontal girder, plate, sill, or metal support connector at least 2 inches deep, preferably more. Joists should not be supported merely by end-nailing to the side of a member, with no bottom support.

All members in contact with each other must be nailed or spiked together, the quantity of nails being dictated by codes, size of members, loads, and good practice. A single nail per connection, even though it may be adequate in some instances to carry the particular load, is never acceptable. Nails driven into the sides or edges of lumber are much stronger and resistant to withdrawal than those driven into the ends. See ( ill. 25).

ill. 23

ill. 24

FIG 25: Nail Withdrawal Resistance vs. Faces Of Wood


All structural components of a building must be securely connected together to resist lateral and up-lift forces of wind, and to hold parts together under seismic (earthquake) action where this is a possible occurrence. Anchorage to the foundation system is covered our article on ANCHORAGE. Connections are commonly achieved by mechanical fasteners which include nails, screws, and bolts. A requirement of some local Codes—and a good precaution in any building—is the use of metal tie-down connectors, which are pre-formed shapes of galvanized steel, pre punched or drilled for nails and other fasteners. These connectors are made in many gauges, shapes and sizes to suit a great variety of typical connection conditions. Local Codes will stipulate the frequency and locations for such tie-down connectors. If no Code requirement exists, use one at every other joist, stud, truss, etc. These connectors are nailed into the sides of members, and are always in addition to the normal nailing requirements for connection of frame members. See ( ill. 26).

(CAUTION) Sub-flooring and /or underlayment material such as plywood, wafer board, or tongue-and-groove boards which are installed over the floor joists as a base for the finished flooring material, should be fastened to the joists with nails which have been specially coated with resin or are of other special types made for that purpose, to develop exceptionally high withdrawal resistance This is to prevent noisy and squeaking floors which can occur due to nails working loose.

WOOD FLOOR systems should only be constructed above ground levels with an air space below the wood floor. See ( Figs 9, 12 and 27). If the space is a crawl space rather than a basement the space must be ventilated. The most common means is to install fixed grills, louvers, or vents in the foundation walls at several locations around the building perimeter below the floor level, to allow air to move in and out of the space and thus prevent moisture build-up or dry rot from occurring. For example, the UNIFORM BUILDING CODE requires one square foot of net vent area for each 150 square feet of underfloor area, located as close to corners as possible, and arranged to provide cross ventilation. If there are signs, or concerns, about the dirt floor of the crawl space being wet or moist, it should be covered completely with a good heavy-duty vapor barrier. See ( ill. 28).

ill. 26 Examples Of Metal Framing/Tie-Down Connectors

ill. 27: Wood Frame Floor Construction Over Crawl Space

ill. 28: Of Crawl Space Depending Conditions Optional Treatments On Moisture: 4-inch gravel fill, the smallest granule not less than 1/8-inch in diameter.

A possible precaution against termite damage for buildings with above-grade wood floors is the inclusion of termite shields at the top of masonry walls on which wood floors, walls and girders rest. These are continuous metal sheetings which cover the tops of the foundation wall, with their outer edges extending out beyond the wall and bent down. The metal must be aluminum, copper, or protected steel to prevent corroding or rusting out. Joints and penetrations such as anchor bolts must be made tight. See ( ill. 29).

ill. 29: Termite Shield Details: At Wood Sills; At Posts in Cellars Or Basements; At Girders

ill. 30: Types Of Joist Bridging: Cross Bridging; Solid Bridging; Metal Bridging

All joists must be braced from deflecting sideways or twisting by installation of bridging, which is wood or metal cross-bracing installed in at least one continuous row at mid-span of the joists, or at the third points for unusually long joists. See ( ill. 30).

If a veneer of masonry is being installed on the exterior walls of the house, a ledge must be provided on the foundation wall sill to support the masonry.

(CAUTION) Flashing must be installed at this sill to prevent water which will seep thru the masonry veneer and run down its backside, from damaging an adjacent wood framed floor. The flashing can be one of several materials made for the purpose such as metal, fabric, plastic, metal-coated fabric, etc., and must be installed continuously along the sill, with all joints and penetrations sealed up. Also, weep holes must be provided at the base of the masonry veneer. These are simply small openings at periodic intervals to allow any accumulated water to seep (weep) out the face of the wall. See ( ill. 31).

ill. 31: Flashing and Weep Holes At Sill Of Veneer

Double floor joists should be provided under overhead partitions which run the same direction as the joists, if the partitions are non-bearing, or are only for the enclosure or division of spaces, not to support any floor or roof loads. Partitions which support other floor or roof framing should have load bearing partitions directly beneath them, or be located over main support beams or girders which have been designed to carry the extra load. See ( ill. 32).

ill. 32: Non-Bearing Partition On Wood Floor Framing; Load Bearing Partitions On Wood Floor Framing

ill. 33: Isometric View; Plan View Of Framing For Stair Opening.

Openings in wood framed floors for stairs or other purposes should have double joists all around the opening. See ( ill. 33).

Wood members must not penetrate into a masonry chimney or fireplace, and should be placed at least 2 inches away from all such masonry surfaces. Similar, or greater, clearances are .required around pre-fabricated metal fireplaces and flue assemblies. Be sure to check the fireplace manufacturers’ written instructions for exact required clearances, since they can vary.

Upper floors of wood are framed the same as the first, except that the ends of the joists rest on the tops of wood stud partitions or on beams of wood or steel, instead of on the foundation sill. See ( ill. 22).

(CAUTION) Large notches or holes cut in joists for the passage of large pipes or ducts should not be allowed, for such practices can significantly weaken or completely destroy their structural integrity. Small drilled holes for the passage of electrical cable are usually not detrimental, however, unless large quantities of them are drilled at the same locations.

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