Chapter 34

Floor Systems and Foundation Support

Introduction• Common types of floor systems used in

residential construction: – Floor systems built at grade level – Floor systems with a crawl space or basement

below the floor system

On-Grade Foundations• Concrete slab is often used for floor system

– Provides a firm floor system – Little or no maintenance – Generally requires less material and labor– Poured as extension of foundation wall and

footing• Monolithic construction

– 3 ½" is the minimum thickness allowed by the IRC for residential floor slabs

On-Grade Foundations (cont’d.)• Slab joints

– Used to prevent cracking

On-Grade Foundations (cont’d.)• Slab placement

– May be placed above, below, or at grade level• Residential slabs are often placed above grade for

hillside construction• Slabs built below grade are most commonly used in

basements• When used at grade, the slab is usually placed just

above grade level

On-Grade Foundations (cont’d.)• Slab preparation

– Slab is built at grade• About 8" to 12" of topsoil and vegetation is removed

to provide a stable, level building site• Excavation extends about 5' beyond building size• Most codes require the slab to be placed on a 4"

minimum base of compacted sand or gravel fill

On-Grade Foundations (cont’d.)• Slab reinforcement

– If slab is placed on 4“ or more of uncompacted fill, welded wire fabric should be specified

• Spacing and sizes are identified by style and designations

• Refer to the text for ACI placement guidelines

On-Grade Foundations (cont’d.)• Post-tensioned concrete reinforcement

– Allows concrete slabs to be poured over unstable soil

• Flat or ribbed slab

On-Grade Foundations (cont’d.)• Moisture protection

– Slab must be placed over 6-mil polyethylene sheet plastic

• Slab insulation– Depends on location– Insulation can be placed under the slab or on

outside of stem wall

On-Grade Foundations (cont’d.)• Plumbing and heating requirements

– Ducts must be placed under the slab before concrete is poured

– For residential plans, plumbing is usually not shown on foundation plan

– Drawings for commercial construction include a plumbing plan and a mechanical plan

On-Grade Foundations (cont’d.)• Changes in floor elevation

– Floor level may be required to step down to meet design needs

On-Grade Foundations (cont’d.)• Slab foundation

– Refer to the text for:• Common components shown on slab foundation• Common components specified by note only on a

slab foundation

Crawl Space Floor Systems• Crawl space

– Area between floor system and ground• Building codes require a minimum of 18" from the

bottom of floor to ground • 12" from the bottom of beams to ground

Crawl Space Floor Systems (cont’d.)

• Joist floor framing– Floor joists span between foundation walls– Plywood floor sheathing is installed to provide

a base for the finish floor– When distance between the walls is too great

for floor joists, a girder is used for support– Concrete pier is placed under intermediate

girder supports to resist settling

Crawl Space Floor Systems (cont’d.)

• Joist floor framing (cont’d.)– Refer to the text for:

• Common components shown for a floor joist system• Common components specified by note only for a

floor joist system

Crawl Space Floor Systems (cont’d.)

• Post-and-beam floor systems– Built using a standard foundation system– Series of beams support subfloor– Once the mudsill is bolted to foundation wall,

the beams are placed flush with the mudsill top– Beams are supported by wooden posts– Each post is supported by a concrete pier

Crawl Space Floor Systems (cont’d.)

• Post-and-beam floor systems (cont’d.)– Refer to the text for:

• Common components shown for a post-and-beam system

• Common components specified by note only for a post-and-beam system

Crawl Space Floor Systems (cont’d.)

• Combined floor methods– Floor and foundation methods may be

combined depending on the building site• Typically done on partially sloping lots when part of

a structure is constructed with a slab and part with a joist floor system