R403.1.3.2 Slabs-on-ground with turned-down footings.
This common construction detail relates to wood framed residential structures with an adjacent exterior: concrete patio, concrete walk or concrete driveway.
Why would you use a turned-down footing over other alternatives?
Turned-down footings are a convenient and expedient construction alternative to traditional footings, concrete stem walls or concrete masonry unit (CMU) stem walls for a variety of reasons.
From a cost perspective, the integral style of concrete slab and turned-down footing can effectively be formed by the "outside" edges of a slab. The trench is dug, the slab area is compacted and leveled, aggregate base course (ABC) is added, a vapor barrier is laid down and then the slab and turned-down footing is ready to be poured. After the preparations, this style of footing can be formed in one day.
With a simple rectangular slab this means (4) concrete forms are needed. Whereas, with a traditional footing and stemwall, a footing must be dug, then poured in the trench and allowed to dry. Later that day, or the following day, a crew returns to form the stemwalls with (2) forms each side of stemwall—for a grand total of (8) forms.
WIth twice the number of forms, the material costs of forming doubles. It doesn't stop there. Now, a crew needs to be there for another day to pour the stemwalls—which adds a day of labor costs. These extra steps add to the construction timeline.
Depending on weather conditions, the forms can be stripped later that day. It is better to let the concrete cure one day prior to stripping, but this is a judgment call on the builder.
After stripping, water proofing, insulation or other items may be added. The slab area can be backfilled carefully on both sides of stemwall to prevent pressure from being applied to the new wall and cracking it. With compacted subgrade, ABC and vapor barrier in place the slab may be poured.
If the slab stops at the inner edge of stemwalls no more forms are required. If the slab "caps" the stemwall, for a turned-down slab then the outer concrete form needs to be move up to accommodate the new slab edge which rests on top of the stemwall.
The cost is decreased and construction timeline is shortened with a slab-on-ground with turned-down footing. The field conditions may make this style preferred, especially when pouring against an existing wall or structure. In that case, the wall or structure acts as the form.
Despite these advantages, the slab-on-ground with turned-down footing does not work well with retaining walls, steps in grade, or with heavier structures (e.g. 2 stories or more) where a spread footing is required to distribute larger loads to bearing soil.
Chapter 4 FOUNDATIONS
- R401.4 Soil tests.
- R401.4.1 Geotechnical evaluation.
- TABLE R401.4.1 PRESUMPTIVE LOAD-BEARING VALUES OF FOUNDATION MATERIALS
- R403.1 General.
- TABLE R402.2
- FIGURE R403.1(1) CONCRETE AND MASONRY FOUNDATION DETAILS
- TABLE R403.3(1) MINIMUM FOOTING DEPTH AND INSULATION REQUIREMENTS FOR FROST-PROTECTED FOOTINGS IN HEATED BUILDINGS
- R403.1.4 Minimum depth.
- R403.1.6 Foundation anchorage.
Chapter 5 FLOORS
- R506.1 General.
- R506.2 Site preparation.
- R506.2.1 Fill.
- R506.2.2 Base.
- R506.2.3 Vapor retarder.
- R506.2.4 Reinforcement support.
Chapter 6 WALL CONSTRUCTION
- R602.1 Identification.
- R602.3 Design and construction.
- TABLE R602.3.(1) FASTENER SCHEDULE FOR STRUCTURAL MEMBERS
- TABLE R602.3(3) REQUIREMENTS FOR WOOD STRUCTURAL PANEL WALL SHEATHING USED TO RESIST WIND PRESSURES
- TABLE R602.3(4) ALLOWABLE SPANS FOR PARTICLEBOARD WALL SHEATHING
- TABLE R602.3(5) SIZE, HEIGHT, AND SPACING OF WOOD STUDS
- FIGURE R602.3(1) TYPICAL WALL, FLOOR AND ROOF FRAMING
- FIGURE R602.3(2) FRAMING DETAILS
- TABLE R602.3.1 MAXIMUM ALLOWABLE LENGTH OF WOOD WALL STUDS EXPOSED TO WIND SPEEDS OF 100MPH OR LESS IN SEISMIC DESIGN CATEGORIES A, B, C, D o, D1, D2
Chapter 6 WALL COVERING
- R703.7.5 Flashing.