1. Unit 42
Floor Framing
Floor Unit Resting on Sill Plates • Floor Unit Resting on Cripple Walls • Posts and Beams • Floor Joists • Subfloor • Floor Underlayment • Floor Trusses • Wood I-joists

2. Basic parts of a floor unit include posts, beams, joists, bridging or blocking, and a subfloor. Floor framing begins after foundation work is completed.
Floor framing begins after the foundation work has been completed. In platform construction, the floor unit is framed directly over the foundation walls or short stud walls used as cripple walls. Most floor units include posts, beams, joists, bridging or blocking, and a subfloor. See Figure Posts and beams support the lapped or butted ends of the joists, or they may provide central support for long joist spans. Bridging or blocking keeps the joists aligned and helps to distribute the load carried by the floor unit. The subfloor is the wood deck that rests on top of the joists.

3. When constructing a floor unit resting on sill plates with a crawl-space foundation underneath, an 18″ minimum clearance should be maintained between the bottoms of floor joists and the soil, and 12″ between the bottom of the beam and the soil.
The floor unit may be framed directly on the sill plates of a building with a crawl-space foundation and low foundation walls. See Figure For a crawl-space foundation, one end of the joists rests on the outside foundation walls. The lapped ends rest on top of an interior foundation wall running down the center of the building. Posts and beams provide midspan support to the long span of the joists. There should be at least 18″ clearance between the bottoms of the floor joists and the soil, and at least 12″ between the bottom of the beam and the soil.

4. Cripple walls, including sill plates and studs, are used to extend the height of low foundation walls.
Cripple walls are used to extend the height of low foundation walls, usually for a full-basement house. See Figure Under some conditions, cripple walls are less costly than the additional concrete and formwork required for higher foundation walls. Cripple wall studs are toe-nailed to the sill plates and are commonly spaced 16″ OC. Cripple wall studs should be no smaller in cross section than wall studs above the floor. Floor joists placed on top of the wall should be directly above the cripple wall stud (stacked).

5. Posts are fastened at the top of the pier and at the bottom of the beam.
The bottom of each post may be nailed to a pier block that is secured to the top of a concrete pier. Another method is to place a 1/2″ steel dowel in the concrete pier at the time concrete is placed. The dowel fits into a hole drilled at the bottom of the post and holds the post in position. See Figure The dowel should extend at least 3″ into the concrete and 3″ into the post.

6. No bolts or other types of fasteners are necessary to secure the fixed post bases to concrete; the lower section of the base is set into a concrete pier when concrete is placed. An adjustable post base should have 1″ to 2 1/2″ standoff height above the concrete. An adjustable base is secured in position by drilling a hole into hardened concrete and fixing a threaded rod into place with epoxy, or by setting a J-bolt into fresh concrete.
Fixed metal post bases are placed when concrete is placed. An adjustable post base is placed after the concrete has set. A J-bolt may be set into the concrete as it is placed, or a hole can be drilled and a threaded rod fixed into place with epoxy. Figure 42-5 shows two types of metal bases that are set into a concrete pier when concrete is placed. A wood post fits into the base and is secured to the base with lag bolts. No bolts or other fasteners are required to attach the base to the concrete. These bases must be positioned in the concrete very accurately.

7. Metal post caps tie together posts and beams.
Figure 42-6 shows two types of metal post caps used to tie together wood posts and beams. Post caps are nailed or bolted to the posts and beams. The twin-design post cap can be installed after the beam has been placed on top of the post since it is available in two pieces.

8. Joints on a built-up beam are staggered
Joints on a built-up beam are staggered. Plank joints occur directly over a post.
Wood beams, also referred to as girders, may consist of a solid timber, built-up lumber, or engineered products such as glulam or laminated veneer lumber. A built-up beam, for example, may be fabricated with two or three or more planks. Joints between the planks are staggered. When assembling a built-up beam, the joints of the planks should fall directly over a post. See Figure Three 16d nails are driven at the ends of the planks, and other nails are staggered 32″ OC.

9. Allowable beam sizes are based on the span of supported flooring, clear opening, lumber grade, and live and dead loads. This table is for a floor beam supporting a 40 psf live load and 10 psf dead load.
The allowable span, also referred to as the clear opening, is the distance between supporting posts permitted for different size beams. Factors such as beam size, grade of lumber, and weight of live and dead loads determine allowable beam spans. Tables are available for determining the allowable beam size for different species of lumber (and steel beams) and different live loads and dead loads. The table in Figure 42-8 is used to determine allowable beam size for Southern Pine beams supporting a 40 pound per square foot (psf) live load and 10 psf dead load. The load duration factor shown at the top of the table is an engineering calculation relating to the capability of the wood to carry maximum loads for longer or shorter durations.

10. Steel pipe columns are frequently used to support steel beams or wood beams.
Steel pipe (Lally) columns are often used as posts in wood-framed buildings, and can be placed beneath either steel or wood beams. See Figure The base of a steel pipe column is bolted to the top of a pier or floor slab. The bolts holding the base must be accurately set into the concrete during concrete placement. The cap at the top of a steel pipe column is secured with machine bolts and nuts or by welding when it is attached to a steel beam. Lag bolts are used when the columns are attached to a wood beam.

11. A hollow steel section column may be used to support the end of an LVL beam.
Figure shows a hollow steel section (HSS) column supporting an LVL beam.

12. Wood plates may be attached to the top of wide-flange steel beams with a powder-actuated tool.
A standard steel beam, called a wide-flange beam, is commonly used with wood framing. Wood joists either rest on top of the beam or butt against the sides of the beam. Figure shows a wood plate attached to the top of a steel beam to provide a nailing base for joists.

13. When placing wood posts and beams, the top of each beam should be aligned with the upper surface of the sill plate.
One procedure for placing posts and beams using a line stretched from opposite walls is shown in Figure 42-12; in this procedure, the upper surface of the beam is aligned with the upper surface of the sill plate. The same procedure can also be performed using a laser level or other surveying equipment.

14. Allowable floor joist spans are based on the lumber species and grade, and joist size and spacing.
Tables are available for finding the allowable joist spans for different species of lumber as well as different live loads and dead loads. Figure shows a table providing allowable joist spans between exterior walls or posts for a variety of framing lumber species supporting a 40 psf live load and 10 psf dead load. Species, lumber grade, and loads are key factors in determining joist spans.

15. Header joists prevent regular joists from rolling or tipping, support the wall above, and fill spaces between regular joists.
Header joists, also known as rim or band joists, run along the outside walls. Three 16d nails are driven through the header joists into the ends of the regular joists. See Figure Header joists prevent the regular joists from rolling or tipping, support the wall above, and fill in the spaces between the regular joists.

16. Joists should be lapped the full width of the beam or wall supporting them.
Often, joists are lapped over a beam running down the center of a building. The lapped ends of the joists may also be supported by an interior foundation or framed wall. It is standard procedure to lap joists the full width of the beam or wall supporting them. See Figure The minimum lap should be 4″. Joists should not extend more than 4″ past the beam. Solid blocking is installed between the lapped ends as the joists are placed or after all the joists have been nailed down.

17. Joists butting over a beam should have the ends scabbed together with a metal or wood tie.
The ends of the joists can also be butted over a beam. The joists should then be scabbed together with a metal or wood tie. See Figure The ties can be omitted if the plywood subfloor panels straddle the butt joints.

18. Joist hangers are available in a variety of standard configurations
Joist hangers are available in a variety of standard configurations. Face-mount and top-flange joist hangers are the most common types.
Face-mount and top-flange hangers are the most common types of joist hangers. See Figure The flanges of face-mount hangers are fastened to the face of a wood beam or ledger board using galvanized nails. Heavy-duty face-mount joist hangers may be fastened to the beams or ledger boards with nails or screws. The flanges of top-flange hangers are fastened to the top of wood beams using galvanized nails. Top-flange hangers are faster and easier to install and support heavier loads than face-mount hangers. Joist hangers are available for solid lumber or engineered lumber for skewed (angled) and adjustable applications.

19. Double-shear joist hangers provide greater strength than standard joist hangers.
Double-shear joist hangers provide greater strength than standard joist hangers of the same size and are easier and more economical to install. Joist fastener holes are capped with a dome or tab to guide the nails through the joist and supporting member at a 45° angle, resulting in a strong connection. See Figure

20. Always refer to manufacturer installation information regarding the proper nail size and required number of nails in the header and the joist.
When installing joist hangers, select the deepest hanger that fits the joist being installed. Galvanized nails should be used to install the joist hangers per the manufacturer’s instructions. In some cases, nails may have a mark on the head for ease of identification for inspection after installation. Tables that provide general load and installation information are provided by manufacturers. See Figure Improperly installed joists may not be able to support anticipated loads and can result in floor squeaks.

21. A sill plate can be bolted to the top of a steel beam
A sill plate can be bolted to the top of a steel beam. Joists are toenailed to the plate. The sill plate may also be attached to the steel beam with a powder-actuated tool.
Often, wood floor joists are supported by a steel beam instead of a wood beam. The joists may rest on top of the steel beam and sill plate as shown in Figure A plate is fastened to the beam and the joists are toenailed into the plate.

22. Joists butted against a steel beam may be supported with joist hangers or a steel plate welded to the bottom of the beam.
Joists can be butted to a steel beam by welding joist hangers to the upper surface of the beam. Hot-dipped galvanized nails are driven through the hangers to secure joists in position. See Figure

23. Blocks separate doubled joists to allow drain pipes, water supply pipes, and heating ducts to extend into a wall cavity above.
Joists should be doubled under partitions that run in the same direction as the joists. Some walls have water pipes, vent stacks, or heating ducts coming up from the basement or the crawl space below. Blocks are placed between the doubled joists to allow space for these utilities. See Figure

24. Cantilevered joists provide support for a floor or balcony that projects past the wall below it.
Cantilevered joists are used when a floor or balcony of a building projects past the wall below it. See Figure The permissible amount of cantilever is based on the intended application. For exterior load-bearing walls and roofs, 1′ of cantilever is allowed per each 3′ of backspan (amount of joist on opposite side of load-bearing wall). For exterior balconies, 1′ of cantilever is allowed for each 2′ of backspan. A header joist is nailed to the ends of the joists.

25. When regular floor joists run parallel to the intended overhang of the floor, the inside ends of the cantilevered joists are fastened to doubled joists.
When regular floor joists run parallel to the intended overhang of the floor, the inside ends of the cantilevered joists are fastened to doubled joists. See Figure Nailing should be through the first regular joist into the ends of the cantilevered joists. Metal connectors are strongly recommended and often required by the building code. A header joist is also nailed to the outside ends of the cantilevered joists.

26. Cross bridging is precut to fit between the joist spans.
Also known as herringbone bridging, cross bridging usually consists of 1″ × 3″ or 2″ × 2″ wood members installed as shown in Figure

27. Wood cross bridging is installed by toenailing each end with 6d or 8d nails. Allow 1/2″ space between adjacent bridging members to prevent them from rubbing together and producing squeaks.
Cross bridging is toenailed at each end with 6d or 8d nails. The pieces are usually precut on a power miter box or radial arm saw. The nails are started at each end before the cross bridging is placed between the joists. The upper ends of the cross bridging are nailed into the joists and the nails at the lower end are not driven in until the subfloor has been placed. If both ends are nailed in place before the subfloor is placed, the joist could be pushed out of line when the bridging is nailed in. An efficient method for placing wood cross bridging is shown in Figure

28. Metal cross bridging does not require nails for installation.
Another approved system of cross bridging uses metal pieces instead of wood and requires no nails. See Figure Metal cross bridging is available for 12″, 16″, and 24″ joist spacing.

29. Solid pieces are often used for bridging between joists.
Solid bridging, also known as solid blocking, serves the same purpose as cross bridging. Solid bridging is commonly used in conjunction with cross bridging where non-standard joist spacing occurs. Solid bridging members are cut from lumber that is the same width as the joist material and are installed in a straight line or staggered. Straight-line bridging may be required every 4′ OC to provide a nailing base for the panel subfloor. If solid bridging is staggered, the members can be spiked from both ends, resulting in a faster nailing operation. See Figure

30. Floor openings are required for stairways, fireplaces, and chimneys.
A floor opening must be framed where stairs rise to the floor. Fireplaces and chimneys also require specially framed floor openings. When joists are cut for floor openings, there is a loss of strength in the area of the opening. The opening must be properly framed to restore the lost strength as shown in Figure

31. When framing a typical floor opening, headers are nailed to trimmers and tail joists are nailed to headers and header joists.
Figure shows the procedure for framing a floor opening. A pair of joists called trimmers are placed at each side of the opening to support the headers. The headers should be doubled if the span is more than 4′. Nails supporting the ends of the headers are driven through the trimmers into the ends of the headers. Tail joists run from the header to a supporting wall or beam. Nails are driven through the header into the ends of the tail joist.

32. Various metal connectors are used to strengthen framed floor openings.
Various metal connectors may be used to strengthen a framed floor opening. See Figure

33. When laying out joists, an “X” mark indicates the side where the joists are to be nailed.
Before floor joists are placed, the sill plates and beams must be marked to show where the joists are to be nailed. Floor joists are usually placed 16″ OC. Most carpenters use a 20′ or longer steel tape for layout purposes, and these tapes have special markings every 16″ and 19.2″ to aid in joist layout. Some framing systems permit 24″ OC joist spacing. Figure shows on-center layout for joists spaced 16″ OC.

34. In this application, floor joists are placed 16″ OC and layout has been started from the left side. The first joist is marked at 15 1/4″ to ensure that the 4' or 8' edge of a panel will fall on the center of a joist. The layout for the following joists is then 16″ OC. A doubled joist is laid out at the right.
Joists should be laid out so that the edges of standard subfloor panels fall directly over the centers of the joists. This layout eliminates additional cutting of panels when they are being fitted and nailed into place. One method of laying out joists is to mark the first joist 15 1⁄4″ OC from the edge of the building with subsequent joist spacing of 16″ OC. See Figure

35. When laying out a floor unit, joist placement is marked on the sill plates.
A procedure for laying out the entire floor is shown in Figure

36. A floor unit is framed by setting the header and end joists, placing the floor joists, and installing blocking and bridging. The floor opening is framed as described in Figure Most framing members should be precut before construction begins.
Most framing members should be precut before construction begins. The joists should all be trimmed to their proper lengths. Cross bridging and solid blocks should be cut to fit between the joists that have a common spacing. The distance between joists is usually 14 1/2″ for joists spaced 16″ OC. Blocking for the odd spaces is cut afterward. A typical procedure for framing is shown in Figure

37. Joist hangers can be installed prior to installing the joists
Joist hangers can be installed prior to installing the joists. Joist hangers may be installed using a hammer, palm nailer, or pneumatic nailer specifically designed for metal connectors.
Two methods may be used to install joist hangers. One method involves toenailing the joist into position and then installing the joist hangers. The other method involves preinstalling the joist hangers and then installing the joists as follows:
1. Insert a small block of joist scrap material into the joist hanger. See Figure
2. Carefully position the joist hanger on the structural member along the layout line. In some cases, the joist hanger may be flush with the top of the structural member while in other cases it may be placed lower along the line.
3. Drive two nails through each side of the hanger and into the structural member. Some joist hangers have nailing tabs that can be driven into the structural member to hold it in position before nailing.
4. Remove the joist block and finish nailing the hanger on each side.
5. Cut all joists to length and place the ends in the hangers.
6. Drive the nails through the joist hangers and into the joist on each side.

38. Subfloor panels are placed over joists so the long sides run at a right angle to the joists. Note that the panel joints are staggered.
The subfloor, also known as rough flooring, is nailed to the top of the floor frame. See Figure The subfloor strengthens the entire floor unit and serves as a base for the finish floor material. The walls of a building are laid out, framed, and raised into place on top of the subfloor. Panel products are typically used for the subfloor.

39. Deformed-shank nails or deck screws fasten the subfloor to the joists.
Subfloor panels are applied with the longer edge at a right angle to the joists. Blocking should be nailed under the long edges of panels if tongue-and-groove panels are not used. A 1/8″ space should be provided between the end joints, with a 1/8″ space between the edge joints for square and T&G edges, except where otherwise indicated by the manufacturer. See Figure

40. Subfloor panels are attached to joists using a heavy-duty pneumatic nailer or self-feeding screwgun.
When installing subfloor panels, carpenters initially use just enough fasteners to secure the panels in position. When all the panels are placed, chalk lines are snapped to locate the centers of the joists below. The entire subfloor is then nailed or screwed down in one operation. Heavy-duty pneumatic nailers are used to drive nails. See Figure

41. Aligning the first row of panels with a chalk line 4′-0″ from the edge of the building ensures that the first row of panels will be perfectly straight even if the edge of the building is not straight. When placing the remaining panels, drive a few fasteners to secure the panels in place. Then, snap chalk lines across the panels in line with the joist centers below and complete nailing or screwing down the panels.
Self-feeding electric screwguns are used to drive screws. A procedure for placing subfloor panels is shown in Figure

42. In the glue-nailed panel method, a construction adhesive is applied to the top of the joists before subfloor panels are placed.
For some construction projects, a construction adhesive is used in addition to nails or screws for fastening the panels to the joists. Although this system is commonly referred to as glue-nailing, a mastic adhesive rather than a glue is used. The mastic is applied to the upper edges of the joists, and the panel is then set in place and fastened before the mastic sets. See Figure The combination of an adhesive with nails or screws helps to further stiffen the entire floor system. In addition, there is less squeak to the floor, and the nails or screws are less likely to pop over a period of time. The nailing schedule for a glue-nailed panel requires nails or screws only 12″ OC along the panel edges and along the joists.

43. In a post-and-beam subfloor, the floor unit receives its main support from beams rather than from floor joists.
In a post-and-beam subfloor system, the floor unit receives its main support from floor beams rather than from floor joists. See Figure The beams are usually spaced 4′ OC and are supported by wood posts resting on concrete piers. The subfloor panels are 1 1/8″ or 1 1/4″ thick and have tongue-and-groove edges. Nails are 10d common, spaced 6″ apart.

44. Underlayment is placed over the subfloor to provide a smooth and even surface for finish floor materials.
Floor underlayment consists of thin panels that are placed directly over the subfloor. See Figure Underlayment provides a smooth, even surface for finish floor materials such as tile, linoleum, and carpet. Underlayment panels may be oriented strand board, plywood, particleboard, or hardboard. Underlayment panels must be a high grade of material that has good resistance to dents and punctures from concentrated loads.

45. The top and bottom chords of floor trusses are tied together by webs
The top and bottom chords of floor trusses are tied together by webs. The chords and webs are typically 2 × 4s and are joined together with metal connector plates.
Floor truss systems are frequently used in residential and commercial wood-framed construction. A typical floor truss is composed of top and bottom chords tied together by webs. See Figure The webs are fastened to the top and bottom chords with metal connector plates. Webs may also be connected to the chords by finger-jointing and gluing.

46. Floor trusses may be constructed with 2 × 4 wood top and bottom chords and tubular steel webs. The 2 × 4 shown along the bottom chord toward the left side stabilizes and ties the trusses together.
Another type of truss design uses tubular steel webs. See Figure Although the top and bottom chords in this type of truss are 2 × 4s, the webs are tubular steel. A 2 × 8 is installed below the top chords at standard intervals to provide lateral support for the truss.

47. Wood I-joists provide a wide nailing surface for a subfloor.
Wood I-joists consist of top and bottom flanges and webs that fit into grooves machined in the flanges. Flanges are dimension lumber or LVL and webs are plywood, LVL, or OSB. See Figure The webs are bonded to the flanges with exterior-type adhesives.

48. Trademarks provide information regarding recommended clear spans and spacing of wood I-joists.
APA Performance Rated I-joists (PRI) are wood I-joists designed for use in residential floor construction. APA PRIs are manufactured per APA Standard PRI-400, Performance Standard for APA EWS I-Joists. Wood I-joists are identified by their depth (overall measurement from top to bottom of I-joist) and a joist designation ranging from PRI-20 to PRI-80. A typical I-joist trademark is shown in Figure

49. Span tables specify on-center spacing of various sizes of wood I-joists.
In general, wood I-joists do not require intermediate blocking between spans if the I-joists conform to specifications in wood I-joist span tables. See Figure Blocking is required, however, if header joists or rim boards are not installed between wood I-joists supporting load-bearing floors, and between cantilevered wood I-joists.

50. Spacing and layout of wood I-joists is the same as for solid dimension lumber. Engineered wood rim boards are used to support the ends of I-joists.
The spacing and layout of wood I-joists is the same as for solid dimension lumber. APA Performance Rated rim boards are used as header joists for wood I-joists. See Figure A rim board ties together the wood I-joists, and also transfers lateral and vertical bearing forces. Rim boards manufactured from plywood, OSB, glulam timber, or LVL are used to tie together wood I-joists since they shrink less than dimension lumber and correspond to the depth of the wood I-joist and other engineered wood products.

51. Joist hangers for wood I-joists should be deep enough to support the top flange.
Wood I-joists are fastened to rim boards or other flat surfaces using top-flange or face-mount joist hangers. Hangers should be deep and wide enough to allow the bottom flange of the I-joist to fully seat in the hanger and support the top flange. See Figure If joist hangers are not deep enough, bearing stiffeners must be used to provide proper support. Some joist hangers have a tab that is bent over the upper surface of the bottom flange and fastened with a 10d nail.

52. Backer blocks provide a flat, flush surface for attachment of top-flange or face-mount joist hangers or other structural elements.
Backer blocks should fit tight against the top flange of an I-joist and are fastened into position with three 10d nails, which are clinched if possible. See Figure For top-flange hangers, a backer block is required only on the nailing side. For face-mount hangers, backer blocks are required on both sides of the adjoining I-joist to provide good anchorage for joist hangers.

53. Web stiffeners reinforce I-joist webs
Web stiffeners reinforce I-joist webs. The direction of an applied load determines the location of the gap between the stiffener and top or bottom flange.
Bearing stiffeners and load stiffeners are two types of web stiffeners and are differentiated by the applied load and the location of the gap between the stiffener and top or bottom flange. See Figure Bearing stiffeners are installed when increased load-bearing capacity is required. Load stiffeners are installed between supports when significant load points are anticipated. The depth of web stiffeners is 1/8″ to 1/4″ less than the distance between flanges. Bearing stiffeners are fitted tightly against the upper surface of the bottom flange; load stiffeners are fitted tightly against the lower surface of the top flange. Most web stiffeners are fastened into position with four staggered 8d nails, which are clinched if possible. Web stiffeners for wood I-joists with 3 1/2″ flanges are fastened into position with four staggered 10d nails.

54. Filler blocks permit a vertical load to be shared between two I-joists.
Filler blocks are installed the full length of the double I-joists, either as a single member or as shorter pieces. The depth of filler blocks is 1/8″ less than the distance between the flanges. Filler block thickness must also be appropriate for the size of wood I-joist being used. Filler blocks slightly thicker than the recommended size shown in Figure can be used without affecting performance of the structural unit. Thinner filler blocks leave a void between the webs and filler blocks, permitting twisting or rotation of the webs during nailing.

55. Squash blocks carry a point load that would otherwise bear directly on a wood I-joist.
Squash blocks carry a point load that would otherwise bear directly on a wood I-joist, such as when a post is installed directly over another post on a lower level. Squash blocks are pieces of dimension lumber or rim board, and are typically installed in pairs to allow even distribution of a load. If pieces of dimension lumber, such as 2 × 4s or 2 × 6s, are used as squash blocks, they should be cut 1/16″ longer than I-joist depth to ensure the squash blocks provide support for the vertical loads. See Figure If rim board is used as squash blocks, select rim board of compatible depth with the I-joists being installed.

56. Blocking provides lateral support for floor I-joists, transfers shear loads from walls above to floor or foundation below, and transfers vertical loads from the walls above to the floor or foundation below.
Blocking is recommended for the following situations:
• at each end of floor joists that are not restrained by a header joist or rim board. See Figure
• between floor joists supporting load-bearing walls extending perpendicular to joists
• between cantilevered wood I-joists over an adjacent supporting wall