7.5 Floor Systems: Framing, Sheathing, Bearing & Crawl Spaces

1. What the Plans Must Show for Floor Construction

Floor construction is its own content area on the B3 outline, and plan review questions focus on spans, bearing, connections, materials protection, sheathing, and the concealed conditions under and above the floor. The reviewer works through five checkpoints on the framing plan and typical details: load path, member size and span, bearing and lateral restraint, sheathing and connections, and moisture/decay protection.

Deflection is the first check. IBC Table 1604.3 limits floor members to L/360 under live load and L/240 under dead plus live load. Long-span wood I-joist and open-web truss layouts often satisfy strength and fail serviceability; the deferred truss or joist submittal must show the governing deflection criterion. A member that spans 24 feet may deflect no more than 24 x 12 / 360 = 0.8 inch under live load.

Live load is the second check. IBC Table 1607.1 sets minimum uniformly distributed live loads — for example, 50 psf for office areas, 100 psf for lobbies and first-floor corridors, 80 psf for corridors above the first floor in office buildings, and 125 psf for light storage. Concentrated loads apply simultaneously to the governing area, and IBC 1607.5 requires a 15-psf partition allowance in office buildings where partitions are subject to relocation and the design floor live load does not exceed 80 psf.

2. Wood Floor Framing (IBC Chapter 23)

Conventional light-frame floor framing is governed by IBC Section 2308.4, and engineered wood framing follows the AWC NDS with manufacturer listings.

Notching and bored holes (IBC 2308.4.2.4) are the single most-cited wood floor defect:

• End notches may not exceed one-fourth the joist depth.
• Top or bottom notches may not exceed one-sixth the depth and are prohibited in the middle third of the span.
• Bored holes must be at least 2 inches from the top and bottom edges and may not exceed one-third the joist depth in diameter.

A 2x10 joist (9.25 inches deep) therefore allows an end notch of 2.31 inches, a top or bottom notch of 1.54 inches outside the middle third, and a maximum bored hole of 3.08 inches diameter. Engineered I-joists and structural composite lumber follow the manufacturer's hole chart instead — the prescriptive rules above do not apply, and cutting an I-joist flange is never permitted.

Bearing and restraint: sawn joists bear on wood or metal supports with the bearing length required by the design; prefabricated I-joists commonly require 1 3/4 inches end bearing and 3 1/2 inches at intermediate supports under the manufacturer's listing. Ends of joists must be restrained against rotation by full-depth blocking, rim board or the wall framing, and squash blocks or full-depth blocking panels are required under bearing walls perpendicular to the joists.

Framed openings for stairs and shafts need doubled headers and trimmers sized for the reaction, with hangers where members frame into the side of a header.

Sheathing: floor sheathing must be identified and rated for the span condition, installed with the strength axis perpendicular to supports, with the fastening schedule from IBC Table 2304.10.2. A glued-and-nailed diaphragm assumption on the structural sheets must be matched by the architectural details.

Protection against decay and termites (IBC 2304.12): wood in contact with concrete or masonry, wood within 18 inches of exposed ground under a floor, girders within 12 inches of the ground, and exposed framing of exterior decks and balconies in heavy-termite areas must be naturally durable or preservative-treated in accordance with AWPA U1 (2304.12.2.7 addresses the termite condition).

3. Steel and Concrete Floor Systems

Composite steel deck is reviewed against the deck manufacturer's listing and the SDI standards referenced by IBC Chapter 22 and AISI S310 for deck diaphragms. Verify gauge, profile depth, span condition, side-lap and support fastening, shear stud size and spacing, and slab thickness above the flutes. Where the deck also serves as a diaphragm, the framing plan must show the diaphragm fastening pattern, chords and collectors.

Precast concrete plank requires the erection drawings, bearing pad detail, minimum bearing length, and the structural topping (usually 2 to 3 inches) where the design relies on composite action or diaphragm continuity. Grouted keyways transfer shear between planks.

Fire-resistance of the floor assembly ties back to Chapter 6: the horizontal assembly rating in Table 601 must be achieved by a listed assembly (UL, GA or a calculated method under Section 722), and the plans must reference the design number. A 1-hour floor/ceiling assembly cannot be assembled from parts of two different listings.

4. Under-Floor Spaces and Crawl Spaces

Where the floor is elevated over an unfinished under-floor space, three code checks apply:

• Ventilation (IBC 1202.4): ventilation openings through foundation walls must provide cross ventilation, with a net area of 1 square foot for each 150 square feet of crawl-space area where the earth floor is uncovered, reduced to 1 square foot for each 1,500 square feet where the ground is covered with a Class I vapor retarder (1202.4.1.1, 1202.4.1.2). Openings must be covered with corrosion-resistant screening or perforated plate with openings no larger than 1/4 inch.
• Access (IBC 1209.1): crawl spaces need at least one access opening of not less than 18 inches by 24 inches.
• Mechanical alternative (IBC 1202.4.3): continuously operated mechanical ventilation at 1 cubic foot per minute for each 50 square feet of floor area, or conditioning the space as part of the building thermal envelope, may replace natural openings where the ground is covered with a Class I vapor retarder.

Under-floor spaces in flood hazard areas must comply with Section 1202.4.4 and Section 1612 — flood openings, not just ventilation openings.

5. Plan Review Scenario: Two-Story Office over Crawl Space

Submittal: a two-story Type VB Group B office building, 60 feet by 90 feet, wood I-joist floors at 16 inches on center spanning 22 feet, a vented crawl space with an uncovered earth floor, and a plumbing riser plan showing 4-inch holes bored through the I-joist flanges.

Examiner findings:

1. Flange penetration prohibited. Boring through an I-joist flange is never permitted; reroute the riser through the web within the manufacturer's hole chart, or provide a header.
2. Deflection documentation. For the 22-foot span, the I-joist submittal must demonstrate live-load deflection not exceeding 22 x 12 / 360 = 0.73 inch (Table 1604.3).
3. Partition allowance. With an office design live load of 50 psf and relocatable partitions, add the 15-psf partition allowance required by Section 1607.5.
4. Crawl-space ventilation. The 5,400-square-foot crawl space with uncovered earth requires 5,400 / 150 = 36 square feet of net free ventilation area; adding a Class I vapor retarder over the ground reduces it to 3.6 square feet (1202.4.1.1, 1202.4.1.2).
5. Access opening. Provide at least one 18-inch by 24-inch crawl-space access (1209.1).
6. Corridor live load. The second-floor corridor must be designed for 80 psf, not the 50-psf office value (Table 1607.1).

6. Common Plan Review and Exam Traps

• Trap 1: Applying sawn-lumber notch rules to engineered joists. I-joists and SCL follow the manufacturer's listing.
• Trap 2: Using the 1/150 crawl-space ratio when a vapor retarder is shown. The correct ratio with a Class I ground cover is 1/1,500.
• Trap 3: Confusing crawl-space access with attic access. Crawl space is 18 by 24 inches; attic is 20 by 30 inches (1209.2).
• Trap 4: Ignoring deflection. A member can satisfy bending and shear and still fail L/360.
• Trap 5: Missing the corridor live load. Corridors above the first floor are not automatically the same as the occupancy served in every case; check Table 1607.1 line by line.