1. Floor Systems and Foundation Support
Chapter 34
Floor Systems and Foundation Support

2. 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

3. 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

5. On-Grade Foundations (cont’d.)
Slab joints
Used to prevent cracking

6. 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

7. 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

8. 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

10. On-Grade Foundations (cont’d.)
Post-tensioned concrete reinforcement
Allows concrete slabs to be poured over unstable soil
Flat or ribbed slab

11. 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

13. 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

14. On-Grade Foundations (cont’d.)
Changes in floor elevation
Floor level may be required to step down to meet design needs

15. 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

16. Crawl Space Floor Systems
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

17. 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

18. 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

19. 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

20. 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

22. 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