Foundation types: every question answered
68 questions with short answers. Each links to the full, sourced explanation.
How are manufactured homes set on piers in Oklahoma?
They rest on a steel chassis carried by concrete-block piers on small pads, held by anchors. Shallow pads ride up and down as Oklahoma clay swells and shrinks.
- Manufactured home piers: How does the support system work?
- Chassis: two or more steel I-beams running the length of each section; Piers: stacks of 8 x 8 x 16 inch concrete blocks under the I-beams, with hollow cells set vertically; Pads (footings): precast or poured concrete, or solid blocks, under each pier.
- Manufactured home piers: Who sets the installation rules in Oklahoma?
- Installation of new homes, including site preparation, must follow HUD's Model Manufactured Home Installation Standards (24 CFR Part 3285) or manufacturer instructions approved by a design approval agency.
- Manufactured home piers: What are the key numbers in the rules?
- Under both standards, organic material such as grass and loose topsoil comes out from under each support, and HUD requires a registered professional for peat, organic clays or uncompacted fill.
- Why do piers move on Oklahoma clay?
- Under the generic standard, pads rest on firm undisturbed soil (or fill compacted to at least 90 percent) once grass and loose topsoil are removed, so they bear on the soil layer that changes moisture most with the seasons.
- How are manufactured home piers repaired?
- Re-leveling with shims resets the home on the same shallow pads. Supports that reach below the moving layer, such as helical piers, steel adjustable piers on new footings, and site drainage are covered under crawl space repair, drainage solutions and crawl space encapsulation.
- What rules govern setting a manufactured home in Oklahoma?
- New homes must be installed to HUD's Model Manufactured Home Installation Standards (24 CFR Part 3285) or DAPIA-approved manufacturer instructions. Used homes may follow the manufacturer's manual, the state's generic standard, or an engineer's or architect's plan, and the installer must be licensed by the state commission.
- How far apart can piers be under a used manufactured home?
- Under Oklahoma's generic standard for previously occupied homes, piers go under each main frame member no more than 6 feet on center, with end piers within 2 feet of the frame ends.
- What soil bearing value is assumed for manufactured home footings?
- Oklahoma's generic standard assumes at least 1,500 pounds per square foot unless testing shows otherwise. HUD's table rates high-plasticity clays at 1,000 pounds per square foot.
- How much can shims be used to level a manufactured home?
- Both HUD and Oklahoma rules call for nominal 4 by 6 by 1 inch shims used in pairs and driven tight, taking up no more than 1 inch of height.
- Why does a manufactured home go out of level on clay?
- The pier footings sit near the surface, inside the zone where clay shrinks and swells with the seasons, so individual piers can rise or drop at different times.
How do slab-on-grade foundations move on Oklahoma clay?
The slab moves as clay under its edges or center gains or loses moisture, and the shape of that movement decides which repair, if any, fits.
- How is a slab-on-grade foundation built?
- Slab: the code minimum thickness for a concrete floor on the ground is 3.5 in; Base and vapor retarder: a 4-in base of gravel or crushed stone and a 6-mil polyethylene vapor retarder under the concrete; Perimeter beam: the edge turns down into a thickened footing, also called a grade beam or turn-down.
- Why do slabs move on clay?
- Foundations on expansive soil fall under the building code's expansive-soil design section, and a soil counts as expansive if, for example, its plasticity index is 15 or more. Swelling soils lift and crack lightly loaded footings and often distress floor slabs.
- Slab-on-grade foundations: What are the common movement patterns?
- Engineering guidance for swelling soils notes that some cracking of floating slabs should be expected. Whether a crack matters structurally is decided by floor elevations over time, which is covered in elevation surveys and heave vs settlement.
- Slab-on-grade foundations: Which repairs fit each pattern?
- Edge drop or perimeter settlement: perimeter underpinning such as steel push piers, pressed concrete piles, helical piers or drilled piers; Interior settlement: interior piers by tunneling or coring, or slab lifting by injection; Moisture-driven movement: drainage, root barriers and moisture control, and plumbing leak detection.
- How thick is a typical house slab?
- The residential code minimum for a concrete floor slab on the ground is 3.5 inches, placed over a base course with a 6-mil polyethylene vapor retarder. The outside edge is thickened into a footing that turns down into the soil.
- Are hairline cracks in my slab a foundation problem?
- Not by themselves. Concrete shrinks as it dries, and extra water in the mix increases that shrinkage. Floor elevations, not the crack alone, show whether the slab has moved.
- Why is the middle of my slab higher than the edges?
- Clay under the center of a slab can gain moisture over time, from a long-term build-up or a plumbing leak, while the edges dry. Research models of slabs on expansive soil show this center-lift pattern as well as edge lift and edge drop.
- What changed for slabs in Oklahoma's 2024 code adoption?
- Oklahoma's adoption requires fill under slabs to be compacted in 8 to 12 inch lifts, raises the allowed depth of clean sand or gravel fill from 24 to 48 inches while keeping earth fill at 8 inches, and requires two No. 4 bars in all footings.
How does a pier and beam crawl space foundation work?
The house sits on a raised wood floor over a crawl space, with a perimeter footing and wall and interior piers carrying beams and joists.
- What are the parts of a pier and beam system?
- The sill plate is held by anchor bolts at least 1/2 in in diameter, no more than 6 ft apart, embedded at least 7 in, with a bolt within 12 in of each plate end.
- Pier and beam foundations: What are the code rules for the crawl space?
- Vented crawl spaces: at least 1 sq ft of net vent area per 150 sq ft of crawl, with one opening within 3 ft of each outside corner; 1 per 1,500 sq ft when a Class I vapor retarder covers the ground; Unvented crawl spaces: a continuous Class I vapor retarder over all exposed earth, seams lapped 6 in, plus mechanical exhaust, conditioned air or dehumidification; Access: floor hatches at least 18 x 24 in; wall openings at least 16 x 24 in.
- How do pier and beam foundations fail?
- Seasonal moisture change in clay can reach from 3 to 40 ft deep, so shallow interior pads sit well inside the moving layer.
- Why does pier and beam suit clay country?
- Engineering guidance for swelling soils notes that structural above-grade flooring is available for consideration under certain conditions.
- What is a pier and beam foundation?
- It is a raised wood floor carried by beams on interior piers, with a perimeter footing and wall around the outside, leaving a crawl space under the house.
- Pier and beam foundations: How much ventilation does a vented crawl space need?
- The residential code calls for at least 1 square foot of net vent area for each 150 square feet of crawl space, reduced to 1 per 1,500 where a Class I vapor retarder covers the ground, with an opening within 3 feet of each corner.
- Why do floors sag in the middle of a pier and beam house?
- Interior piers rest on shallow pads in clay that shrinks and swells with the seasons, and wood shims and framing can crush or decay. Either can let the beams and joists drop between supports.
- Can a pier and beam house be re-leveled?
- Yes. Because the floor sits on piers and shims that can be reached from the crawl space, supports can be adjusted after the soil moves, which is not possible under a concrete slab.
What are stem wall and grade beam foundations?
A stem wall is a short wall on a continuous footing. A grade beam spans drilled piers set below swelling clay, with a gap under it so clay cannot lift it.
- What is a stem wall on a footing?
- A footing is the wide concrete base that spreads the house's weight onto the soil. A stem wall is the vertical wall of concrete or masonry that rises from it.
- What is a grade beam on drilled piers?
- A drilled pier is a concrete column cast in an augered hole. A grade beam is a reinforced concrete beam that spans from pier to pier at the ground surface and carries the walls.
- Stem wall and grade beam: How do these foundations fail?
- When piers move after construction or repair, the mechanism is covered in pier uplift or movement after repair. Underpinning brackets bear on the stem wall or grade beam itself; spacing and load questions are covered in loads, pier spacing and capacity.
- Stem wall and grade beam: How are these foundations repaired?
- Deeper or belled supports: drilled bell-bottom piers; Steel underpinning on the stem wall: steel push piers or helical piers; Moisture control around the perimeter: drainage and root barriers.
- What is a stem wall foundation?
- A stem wall is a short concrete or masonry wall built on a continuous footing. It raises the house above grade and either carries a wood floor over a crawl space or surrounds a slab poured inside it.
- What is a grade beam on drilled piers?
- It is a reinforced concrete beam that spans between drilled concrete piers. The piers carry the load down below the zone of seasonal soil moisture change, and on swelling soils a gap is left under the beam so the clay cannot push it up.
- Why would a grade beam foundation heave?
- Common causes named in engineering guidance include a missing or inadequate void space under the beam, piers that are too short or the wrong diameter, and mushroom-topped piers.
- Stem wall and grade beam: What reinforcement does Oklahoma require in footings?
- Oklahoma's adoption of the 2024 International Residential Code adds two No. 4 bars to all footings and requires cold joints between footings and stem walls to be tied with No. 4 bars at each corner and no more than 6 feet apart.
What is a post-tensioned slab and does it move on Oklahoma clay?
A PT slab is reinforced with steel tendons pulled tight after pouring; used on expansive clay, it still moves when clay swells or shrinks unevenly.
- What makes a slab post-tensioned?
- Trade terms used on this page: - Tendon: a high-strength steel strand inside a plastic sheath, laid in the slab before the pour. - Stressing: pulling the tendons tight with a hydraulic jack after the concrete has gained strength, then locking them at the slab edge. - Anchor pocket: the recess at the slab edge where a tendon is locked off, later filled with grout or patching.
- Why are PT slabs used on clay?
- Clay-rich soils can swell 10% or more by volume when wetted, and volume increases of 3% or more are generally considered damaging enough that engineers call for specially designed foundations.
- Post-tensioned slabs: How do you identify a PT slab in Oklahoma?
- Panel label: Oklahoma's adoption of the 2024 International Residential Code, effective September 14, 2026, requires post-tensioned slabs on grade to carry a label on the electrical panel door; Anchor pockets: patched circles or squares spaced along the slab edge or garage stem; Paperwork: builder plans, engineering letters or permit records.
- How do PT slabs still move?
- The tendons do not stop the soil from moving.
- Post-tensioned slabs: What should be considered when repairing a PT slab?
- PT slabs use the same repair families as other slabs: perimeter piers, drainage, moisture control and leak repair. Work that cuts or cores through the slab, such as interior piers, passes through the zone where tendons run.
- What is a post-tensioned slab?
- It is a concrete slab foundation reinforced with steel tendons that are tightened after the concrete is placed, rather than relying only on rebar or wire mesh.
- How can I tell if my Oklahoma house has a post-tensioned slab?
- Under Oklahoma's adoption of the 2024 International Residential Code, effective September 14, 2026, post-tensioned slabs on grade must be identified by a label on the electrical panel door. Older homes may show patched tendon anchor pockets along the slab edge or have builder plans that say so.
- Can a post-tensioned slab still move on clay?
- Yes. Expansive clay can still lift the edges or center of any slab when moisture changes, and research models show leaks under a slab changing how it bends.
- Why are post-tensioned slabs used on expansive soil?
- Building codes require foundations on expansive soil to be designed for it, and engineering guidance lists post-tensioned slabs among the options for swelling soils under certain conditions.
Why do driveways, patios and sidewalks move in Oklahoma?
Flatwork is thin, lightly loaded and sits on the soil, so swelling clay lifts it and drying or washed-out soil lets it drop more readily than the house.
- What counts as flatwork?
- Driveways and garage aprons; Sidewalks and front walks; Patios and back slabs.
- Why does flatwork move so much?
- Swelling soils lift and crack lightly loaded strip footings and frequently cause distress in floor slabs. Clay-rich soils can swell 10% or more by volume when wetted, and cracked garage slabs, driveways, sidewalks and patios are among the most common signs of swelling soil.
- Driveways, patios and flatwork: Which concrete details matter?
- Adding 2 gallons of water per cubic yard makes concrete about 10% weaker and increases shrinkage about 20%; Exterior exposed slabs carry a minimum strength of 3,000 psi in the residential code, compared with 2,500 psi for interior footings and basement slabs; Oklahoma's 2024 code adoption exempts interior slabs on grade and enclosed garage slabs from the air-entrainment requirement.
- How does flatwork affect foundation drainage?
- The residential code requires the ground to fall at least 6 in within the first 10 ft from the foundation, and requires impervious surfaces such as concrete within 10 ft of the building to slope at least 2% away from it.
- What are the repair options for flatwork?
- Lifting settled slabs by injection: slabjacking and polyurethane; Filling voids from washout: void and compaction grouting; Re-sloping or redirecting water: drainage solutions.
- Driveways, patios and flatwork: Why does my driveway or patio crack and lift?
- Flatwork is thin and lightly loaded, so swelling clay underneath can lift it easily, and shrinking or washed-out soil can let it drop. Concrete also shrinks as it cures, which causes cracks that are not caused by soil movement.
- Driveways, patios and flatwork: Should a patio slope toward or away from the house?
- The residential code requires impervious surfaces within 10 feet of the foundation to slope at least 2 percent away from the building, so water drains away instead of toward the foundation.
- Driveways, patios and flatwork: Is a cracked sidewalk a sign of foundation trouble?
- Not necessarily. Cracked sidewalks, driveways and patios are common on expansive soil, and flatwork usually rests on the soil without deep footings, so it moves more than the house does.
- Driveways, patios and flatwork: Can exterior concrete be damaged by freezing in Oklahoma?
- Yes. Water that freezes inside concrete expands about 9 percent, which can crack and spall the surface, especially where water ponds on the slab.
Why do Oklahoma basement walls crack and bow?
Most trouble comes from sideways pressure: clay that swells when wet and water that builds up in the backfill behind the wall.
- Why are basements less common in Oklahoma?
- A University of Oklahoma geology professor interviewed by NPR described how Oklahoma's red clay expands and contracts with soil moisture, cracking basement walls so they leak.
- Oklahoma basements: What types of basement walls are there?
- Foundation walls can be built from concrete, masonry, wood or insulated concrete forms. The types most often seen are:
- Oklahoma basements: What does the code require for water?
- Footing drains (IRC R405): required around concrete or masonry walls that retain earth and enclose habitable or usable space below grade, placed at or below the top of the footing and discharged by gravity or a pump; Gravel and pipe: a gravel drain extends at least 1 ft beyond the footing and 6 in above its top; perforated pipe sits on at least 2 in of washed gravel with 6 in of cover; Exception: drains are not required on well-drained Group I sand and gravel soils; clay soils are not part of that group.
- Oklahoma basements: What are the signs of basement trouble?
- Water at the joint between wall and floor slab, or seepage through cracks; the NPR report notes that cracked basement walls leak; White crystalline residue (efflorescence), which forms when water moves through masonry, dissolves salts and evaporates at the surface; Cracked basement floors, a common sign of swelling soil.
- Oklahoma basements: How are basement walls repaired?
- Wall movement is handled under basement wall repair; water is handled under drainage solutions. Perched water behind walls in wet seasons is explained in groundwater and perched water, and the mechanism is described in lateral earth, swell and hydrostatic pressure on walls.
- Why are basements uncommon in Oklahoma?
- Oklahoma's red clay expands and contracts with moisture, which can crack basement walls and let them leak. Southern building codes also do not require deep frost footings, so a basement means extra excavation instead of using a hole that has to be dug anyway.
- Oklahoma basements: What pushes a basement wall inward?
- The soil behind the wall presses sideways, and the code assigns clay soils the highest design pressure of the common soil groups. Swelling clay and water pressure from saturated backfill add to that load.
- Oklahoma basements: Does my basement need waterproofing or just dampproofing?
- The residential code calls for dampproofing on below-grade walls in most cases and requires waterproofing where a high water table or other severe soil-water conditions are known to exist.
- Oklahoma basements: What does white powder on a basement wall mean?
- White crystalline residue is efflorescence: salts left behind when water moves through concrete or masonry and evaporates. It shows that moisture is getting through the wall.