What causes sloping floors in an Oklahoma house?
In short: Sloping floors are one of the most common signs of foundation movement on expansive clay, but they have many possible causes, from a drying slab edge to settling interior piers. A slope over about 1 percent is usually noticeable, and the cause is pinned down by measuring the whole floor with an elevation survey rather than by feel.
What does this sign look like?
Floors run downhill toward one wall or corner, doors swing open or shut on their own, and furniture or cabinets sit out of level. Research from Oklahoma State University lists tilting floors, jamming doors and wall cracks among the visible effects of expansive soil movement on houses 2. Inspectors read sloping floors, sticking doors and warped openings as indicators of differential settlement, meaning one part of the house has moved more than another 4.
How much slope is noticeable?
Engineering guidance gives these reference points 1:
| Floor slope | What the guidance says |
|---|---|
| Over 1% (about 1/8 inch per foot) | Usually noticeable to occupants |
| 2% | Considered too large under the Americans with Disabilities Act |
These are comfort and usability benchmarks. Structural significance is judged separately, using deflection (how much the floor curves between two points) and the distress visible in the house 1.
What causes sloping floors?
| Possible cause | How it produces slope |
|---|---|
| Perimeter drying settlement | Clay under the slab edge shrinks as it dries, and the perimeter drops 3 |
| Tree-root desiccation | Trees and large shrubs withdraw water, causing downward foundation movement 5 |
| Differential movement at additions | Settlement cracks form at the joints of bay windows, porches and additions 4 |
| Slope creep | Clay soils on slopes can slowly drift downhill, causing settlement 4 |
| Interior support failure (crawl) | Settling or rotating interior piers drop, and the wood frame sags to follow 4 |
| Karst subsidence | Sinkholes and caverns in soluble rock can cause land settlement or collapse 6 |
Other listed mechanisms, such as fill consolidation and soft soil consolidation, are covered in the causes section of this library.
The Oklahoma clay connection
Most of Oklahoma has expansive soils 3. Seasonal wetting and drying can make a slab perimeter rise when the soil swells and drop when it shrinks 3. Oklahoma State University researchers have used Oklahoma Mesonet climate data to map the long-term soil moisture conditions that drive this movement across the state 2.
Karst areas of Oklahoma
Limestone and dolomite karst is found mainly in the Ozark area of the northeast and the Arbuckle Mountains in the south, while gypsum and salt karst occurs in many parts of western Oklahoma 6. Karst means rock that groundwater can dissolve, leaving voids.
How are sloping floors measured?
- Elevation survey. A relative floor elevation survey, with readings drawn on a floor plan, is the level of evaluation engineering guidance uses beyond a first impression 1. See elevation surveys.
- Short-distance caution. Over distances under about 20 feet, built-in unevenness from construction can dominate the readings 1.
- Deflection ratio. The survey is checked against a deflection of L/360 (span divided by 360). Below it, damaging deflection is unlikely unless visible distress says otherwise; above it with enough symptoms, an engineer is generally justified in calling the deflection excessive 1.
Related
- Elevation Surveys
- Heave vs Settlement
- Tolerances and Standards
- Perimeter Drying Settlement (Edge Drop)
- Symptom Matrix
- Gaps Under Baseboards
Common questions
How much floor slope is too much?
Engineering guidance notes that a floor slope greater than 1 percent is usually noticeable, and the Americans with Disabilities Act treats a 2 percent slope as too large. Whether slope is structurally significant is judged with an elevation survey and visible distress together.
What causes floors to slope in a house?
Common causes include the soil under the outside edge drying and shrinking, trees drawing moisture from the soil, settlement where additions or porches meet the house, soft or poorly compacted soil, sinkholes in soluble rock areas, and settling interior piers in crawl space homes.
Is a sloping floor always a foundation problem?
No. Original construction leaves some built-in unevenness, and over short distances, under about 20 feet, that unevenness can dominate floor readings. A survey across the whole house shows whether there is a consistent pattern of movement.
How are sloping floors measured?
With a relative floor elevation survey, which records the height of the floor at many points and draws them on a plan. Engineering guidance treats this survey as the step beyond a visual first impression.
Sources
- Guidelines for the Evaluation and Repair of Residential Foundations, Version 3 Texas Section, American Society of Civil Engineers
- Residential House Foundations on Expansive Soils in Changing Climates U.S. Department of Housing and Urban Development, Cityscape
- The Mystery of Expansive Soils STRUCTURE magazine
- Inspecting Foundation Walls and Piers International Association of Certified Home Inspectors (InterNACHI)
- Recommended Practice for the Design of Residential Foundations, Version 2 Texas Section, American Society of Civil Engineers
- Mapping of Karst Terranes in Oklahoma to Identify Potential Environmental and Hydrologic Problems AAPG Search and Discovery (Oklahoma Geological Survey author)
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