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10

Chapter

Floor Framing

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Objectives

Explain the difference between platform, balloon, and post-and-beam framing.
Identify the main parts of a platform frame.
Calculate the load on girders and beams used in residential construction.
Lay out and install sills on a foundation wall.

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Objectives (Cont.)

Describe how layouts are made on a header joist.
Explain the correct procedure to follow when correcting problems with floor frames.
Identify the parts of a floor truss.
Describe materials used for subflooring.
Estimate materials (sizes and amounts) required to construct a specific floor frame.

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Types of Framing

Three types used in residential construction

Platform framing, or western framing, is popular
Balloon framing is no longer used in new construction
Post-and-beam framing, also called plank-and-beam framing, uses heavy structural members

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Framing Example

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Platform Framing

First floor built on top of foundation wall as platform
Wall sections are one story high
First-floor platform rests on sill or underpinning

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Architectural Detail Drawings

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Balloon Framing

Studs are continuous from sill to rafter plate
Second floor joists rest on ribbon set into studs
Shrinkage is reduced because amount of cross-sectional lumber is low

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Girders and Beams

Joists are horizontal members of floor frame
Girders, also called beams, support joists at midpoint
Built-up girders can be made of pieces of 2″ lumber nailed together

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Steel Beams

May be used instead of wood girders
Vary in depth, width of flange, and weight
Size needed depends on load

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Posts and Columns

Steel posts are popular for girder and beam support
Adequate footings must be provided for girder posts and columns
Post anchors used to securely hold wood posts

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Pro Tip

Be sure tops of posts and columns and pockets in foundation walls are flat
This ensures girder or beam is well supported with its sides plumb

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Framing over Girders and Beams

Joists are supported on top of steel beam
Top of beam is set flush with top of wall
Ledgers and hangers or stirrups may be used

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Sill Construction

Sill plate (mudsill) rests on foundation
Supports building frame
Sill location depends on building’s exterior covering
Sill sealer typically used

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Code Note

IRC requires anchor bolts be

Embedded at least 7″ into concrete foundation
Minimum diameter of 1/2″
Spaced no further than 6′ O.C.

Anchor bolt required within 12″ of end of sill
Each board in sill must contain at least two anchor bolts

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Termite Shields

Wood sill should be at least 8″ above ground
Protective metal shield should extend out over foundation wall
Chemically-treated lumber may be used for framing
Soil around and under structure can be treated

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Installing Sills

Two sill anchor types

Anchor straps
Anchor bolts

Each section is laid out and holes bored
Sills are positioned over bolts to check for accuracy

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Joists

Carry weight of floor between sills and girders

I-joists
Nominal 2″ lumber placed on edge
Open-web truss joists

Steel bar joists and reinforced concrete joists
Building codes usually specify allowed deflection of joists

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Laying Out Joists

Floor joists can be laid out on sill or band joists
Sills and band joists may need to be set back from edge
Joists are doubled where extra loads must be supported and around openings

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I-Joists

Engineered joists with flanges made from Douglas fir in solid lumber or LVL
Have web glued into grooves cut in flanges
Often attached to joist headers and girders with steel hangers

Weyerhaeuser

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Nailing I-Joists

I-joists have specific requirements for nailing
Nail joists at bearing points with two 8d nails
Nail rim joist 1 3/4″ or thinner to wood I-beam
Attach 2 × 4 or wider “squash” plates to flanges

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Rim Boards

Framing members between sill and bottom of wall framing
Engineered rim boards designed for use with �I-joists
May be made with LVL, LSL, or OSB

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Safety Note

Never allow workers to walk on joists until braced
Do not stack building materials on unsheathed joists
Install and nail all blocking, hangers, and rim boards at ends of joists
Keep flanges straight with tolerance of no more than 1/2″ of true alignment

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Thinking Green

I-joists use 50% less wood than sawn 2 × joists
LVL, used for girders and rim boards, also conserves wood
LSL can be made from small-diameter trees, conserving resources
Engineered wood products come in any length
This reduces waste at construction sites

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Framing Openings

Trimmers are full-length joists or studs that reinforce rough openings
Length of headers can be found from layout on band joist
Metal joist hangers are used to assemble headers, trimmers, and tail joists

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Bridging

Keeps joists in vertical position and transfers load to next joist

Herringbone, or cross
Solid, or blocking

After bridging installed, frame is ready to receive subflooring

Timber Engineering Co.

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Special Framing Problems

Buildings may include section of floor that overhangs lower floor
If joists are parallel to supporting wall, use cantilevered joists
Smaller, doubled joists are used when concrete base is needed

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Cutting Openings in Wood Floor Joists

Cut holes at or close to vertical middle of joist
Hole limited to 1/3 of total joist width has little effect on joist’s strength
Hole is more likely to reduce strength of joist if near center of span

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Cutting Holes in I-Joists

Most I-joists have perforated knockouts
Special rules apply for cutting through I-joists

Leave 1/8″ of web on top and bottom of hole
Do not cut flanges
Cut holes through cantilever no more than 1 1/2″ in diameter
Provide at least 1 1/2″ between hole and bearing surface

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Open-Web Floor Trusses

Made of lumber chords and galvanized steel webbing
Factory-built to specs for their intended use
Provide wide nailing surface because chord is laid flat

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Subfloors

Final step in completing floor frame
Serve three purposes

Add rigidity to structure
Provide base for finish flooring
Furnish work surface for additional framing

Made from panel materials: OSB and plywood

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OSB

Installed with long edges perpendicular to joists
Joints staggered in successive courses
Construction adhesive applied to joists
Subfloor fastened with screws or nails

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Installed OSB Subfloor

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Plywood Subflooring

Applied much like OSB
May have tongue-and-groove edges
Available in panels for joist spacing of 16″, 20″, 24″, �or 48″
Has maximum support spacing stamped on each panel

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Estimating Materials

Check plans and determine lengths of floor joists needed
Allow sufficient length for full bearing on girders and partitions
Numbers of joist headers will be lengths of foundation sections
For subflooring, multiply length by width and subtract major areas not to be covered

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