How are manufactured homes set on piers in Oklahoma?
In short: Most manufactured homes rest on a steel chassis carried by stacked concrete-block piers on small footing pads, held down by ground anchors and hidden by skirting. New homes in Oklahoma are installed to the federal HUD standard, and used homes to the manufacturer's manual, the state's generic standard or an engineer's plan. Because the pads sit near the surface, they ride up and down with Oklahoma clay as it swells and shrinks.
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 1.
- Pads (footings): precast or poured concrete, or solid blocks, under each pier 13.
- Shims: wedges that fill the last gap between pier cap and beam 1.
- Anchors: steel ground anchors and straps that resist wind uplift and sliding 1.
- Skirting: a perimeter screen that encloses the crawl space.
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 2. Used (previously occupied) homes may be set using the manufacturer's manual, the generic standard in state rules, or a plan approved by a professional engineer or registered architect 2. Using an installer not licensed by the Oklahoma Used Motor Vehicle, Dismantler, and Manufactured Housing Commission is an unacceptable procedure for new or used homes 2.
What are the key numbers in the rules?
| Item | Oklahoma generic standard (used homes) | HUD 24 CFR 3285 |
|---|---|---|
| Assumed soil bearing | At least 1,500 psf unless tested 3 | 1,500 psf default; high-plasticity clays (CH, MH) rated 1,000 psf 1 |
| Footing pad | Three 4x8x16 solid blocks or one 4x16x16, at least 384 sq in 3 | Precast pads 4 in nominal; poured pads at least 6 in thick 1 |
| Single-block piers | Under 36 in tall 3 | Frame piers under 36 in 1 |
| Double interlocked piers | 36 to 67 in 3 | 36 to 67 in; over 67 in needs an engineer or architect 1 |
| Pier spacing | No more than 6 ft on center; end piers within 2 ft of frame ends 3 | Per the HUD support tables, or an engineer's or architect's design 1 |
| Shims | 4x6x1 in, in pairs, no more than 1 in total 3 | Same 1 |
| Clearance to grade | At least 12 in under the lowest frame member 3 | At least 12 in 1 |
| Site drainage | Not addressed in the support-system section 3 | Ground slopes at least 1/2 in per ft for the first 10 ft 1 |
| Ground cover | Not addressed 3 | 6-mil polyethylene vapor retarder when skirted, seams lapped 12 in 1 |
| Frost | Not addressed 3 | Conventional footings below the local frost line 1 |
Under both standards, organic material such as grass and loose topsoil comes out from under each support 3, and HUD requires a registered professional for peat, organic clays or uncompacted fill 1.
Anchors
HUD requires ground anchors certified by a registered engineer or architect, with a working load equal to the ultimate load divided by 1.5 1. Anchors listed before November 10, 2014 had to meet a 3,150 lb working load and 4,725 lb ultimate load 1. HUD's soil table pairs each soil class with a torque-probe reading, and high-plasticity clays without test data must use anchors rated for the weakest clay class, 4B 1.
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 3, so they bear on the soil layer that changes moisture most with the seasons. Seasonal moisture change in clay can extend from 3 to 40 ft deep, and lightly loaded supports in that zone can be lifted in wet seasons and drop in dry ones 5. Oklahoma State University testing of western Oklahoma benchmark soils found montmorillonite-rich clays, with Kirkland rated severe and Bethany and Norge rated moderate as sites for low buildings because of shrink-swell 4.
The results show up as:
- Perimeter piers and center frame piers moving by different amounts, racking the floor 5 (interior support and framing failure).
- Shims loosening in dry months or the home being pushed up in wet months, the seasonal lift-and-drop cycle described for lightly loaded supports 5.
- Water ponding under skirting where the site was not graded to drain 1.
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. Statewide figures are collected in OK Data - Manufactured Homes.
Related
- OK Data - Manufactured Homes
- Crawl Space Repair
- Helical Piers
- Surface Drainage and Grading
- Shrink-Swell and Active Zone
Common questions
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.
Sources
- 24 CFR Part 3285 – Model Manufactured Home Installation Standards (Code of Federal Regulations, 2020) U.S. Government Publishing Office
- Section 765:37-7-1 – Acceptable procedure (Oklahoma Administrative Code) Justia
- Okla. Admin. Code § 765:37-7-3 – Support Systems Legal Information Institute, Cornell Law School
- Shrink-Swell and Other Characteristics of Five Benchmark Soils of Western Oklahoma (Technical Bulletin T-150) Oklahoma State University Agricultural Experiment Station
- Damage to Foundations from Expansive Soils Missouri University of Science and Technology (J. David Rogers, Robert Olshansky, Robert B. Rogers)
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