On-Grade, Below-Grade and Above-Grade Floors

Why this matters

Every flooring maker's instructions sort substrates by grade level before they say anything else, because grade level is a stand-in for how much moisture the floor will live with. Put a product on a level its maker excludes and the failure is not a maybe: it is a warranty that never existed and a floor that cups, releases or grows mold on the schedule the ground sets. The grade call takes two minutes at the site visit and decides the product list.

The three levels, defined by where the ground is

• Above grade. A suspended floor with space under it: wood joists or a concrete deck over a basement, a crawlspace, a garage or another living space. The floor never touches soil.
• On grade. A concrete slab poured on the ground with its top at or above the surrounding exterior grade. Typical of slab houses, garages converted to rooms, and additions.
• Below grade. Any floor below the surrounding exterior ground level. Most makers treat a floor as below grade if the ground outside is above it on any side, which puts most walkout basements and split-level lower floors in this group even though one wall opens to daylight. Read the definition in the instructions for the product you are quoting; some state it in exactly those terms and some do not define it at all.

       +----------------------------------+
       | UPPER FLOOR: wood on joists      |
       | ABOVE GRADE, living space below  |
       +-----------------+----------------+
       | MAIN FLOOR      | MAIN FLOOR     |
       | wood on joists  | wood on joists |
       | ABOVE GRADE     | ABOVE GRADE    |  ADDITION:
 grade +=================+================+=slab ON
       | CRAWLSPACE      | BASEMENT       |  GRADE
       | soil, ground    | slab BELOW     |
       | cover film      | GRADE          |
       +-----------------+----------------+

Note what the drawing shows about the two above-grade main floors. They are the same construction, and their grade level is the same, but the half over the crawlspace sits over soil and the half over the basement sits over a slab inside a conditioned space. The grade label says both are above grade. Their moisture risk is not the same, and the crawlspace is the reason.

Why the ground sets the risk

Soil is always wet a short way down. Water moves from it toward a drier building by several paths at once: liquid water wicking up through concrete that has no capillary break, vapor diffusing up through the slab, water pushed through cracks and joints by hydrostatic pressure when the water table rises against a basement, and humid crawlspace air touching the underside of the floor above. How each path works is covered in How Moisture Moves Through a Building; this card is about which path each grade level exposes.

• Below grade gets all of them. The slab sits in soil on its underside and has soil against the walls around it, and it is usually the coolest surface in the house, so the air right at its surface runs at a higher RH than the room air.
• On grade gets the slab paths but not the walls, and the surrounding grade usually drains away from it.
• Above grade over a crawlspace gets only what the crawlspace air delivers. With a ground cover and a dry crawl, that is very little. With bare soil, it can be more than an on-grade slab.
• Above grade over conditioned space is the driest case a flooring installer meets.

What the level decides

Solid hardwood is generally not allowed below grade. Solid boards move the most of any wood floor with moisture, and below grade is where moisture is least predictable. Engineered wood, laminate and the vinyl-core products are the usual answers there, each within its maker's limits. See related: Nail-Down, Glue-Down or Floating: Choosing the Method.

Slabs need vapor control, and you cannot see whether a slab has it. The IRC, in section R506 as adopted by your jurisdiction, calls in substance for a vapor retarder under slabs of the building's living spaces, with exceptions for spaces like unheated garages; older slabs often predate it, and a sheet that was punctured during the pour does nothing. The only way to know what a slab will do to a floor is to test it, by ASTM F2170 in-situ RH or ASTM F1869 calcium chloride, against the flooring and adhesive maker's limits. See related: Slab Moisture Testing for Flooring. Where the maker calls for a film under a floating floor on concrete, it goes down on any slab, at any grade, including an elevated concrete deck.

A crawlspace without a ground cover feeds the floor above. The IRC addresses crawlspace moisture in section R408, as adopted by your jurisdiction; in substance, a vented crawlspace needs net vent area of 1 square foot per 150 square feet of crawl, reducible to 1 per 1,500 where the soil is covered with a Class I vapor retarder, and an unvented crawlspace needs a continuous ground cover plus conditioning, exhaust or (in later editions) dehumidification. Check the current edition. The flooring installer is not usually the one who fixes a crawlspace, but is the one whose floor fails if nobody does, so the condition of the crawl belongs in the site assessment for any wood floor over one.

Worked example: a split-level in three readings

A split-level house getting new floors on three levels. The estimator classifies each one before quoting.

Upper level, bedrooms. Wood subfloor on joists over the main level's living space. Above grade over conditioned space. Subfloor readings average 8.9 percent on a pin meter with the panel setting. Every product on the customer's list is allowed. No moisture concern.

Main level, living room and kitchen. Wood subfloor on joists over a crawlspace. Above grade. The customer wants 3/4 inch solid oak, which is allowed above grade. Before saying yes, the estimator goes into the crawl. Entry first: a look through the access for standing water near wiring or equipment, because water with energized equipment is a reason not to enter until the circuit is shut off and proven dead by someone qualified to do it; there is none here, and the crawl is dry underfoot. What he finds is bare soil, no ground cover, vents blocked by landscaping on two sides, and a hygrometer reading of 85 percent RH at 65 F. On the Wood Handbook's EMC table, air that humid holds wood in the high teens. The underside of the subfloor reads 15 percent under the kitchen. That is above the 12 percent subfloor ceiling NWFA guidance commonly states, before the flooring even arrives.

The level is above grade and the product is allowed, and the floor is still not ready. The quote says so: a ground cover and vent repair, by the shop or the trade it subcontracts, before any wood floor goes down, then a re-read of the subfloor once it has dried with the house running.

Lower level, family room. A slab with the ground 4 feet up the outside of the front wall and a walkout door at grade in the back. The customer calls it "the ground floor". The maker's instructions for the oak the customer chose define below grade as any floor below exterior grade on any side, so this is below grade, and solid oak is excluded there. The estimator offers an engineered oak in the same species and stain, glued down or floated as its maker allows below grade, subject to an ASTM F2170 test of the slab.

What the customer hears. One product upstairs, a matched engineered product downstairs, and a crawlspace repair the main floor depends on. Three different answers in one house, each one set by where the ground is.

Above grade is not automatically dry

Three above-grade cases behave worse than their label:

• Over an unheated garage. The underside of the floor runs cold in winter and the room above runs warm and dry, so a wood floor there sees a larger seasonal swing than the rest of the house. Expect wider winter gaps, and check that the insulation under it is in contact with the subfloor rather than sagging away from it.
• Cantilevers and bump-outs. Bay floors that hang past the foundation are exposed on three sides. A cupped board line that stops exactly at the foundation wall below is the tell.
• Converted porches. Often framed low to the ground, sometimes with no access under them at all. If you cannot see under a floor, you cannot call its moisture risk, and the quote should say so.

What moves a floor from one category to another

• Regrading. A new patio poured against the house, or soil built up in a flower bed, can raise exterior grade above a floor that used to be on grade. Classify by where the ground is now.
• Encapsulating a crawlspace. Sealing and conditioning a crawl brings the floor above it close to the over-conditioned-space case. Verify by reading the subfloor afterward, not by the contract that says it was done.
• A slab on grade with a known water history. The grade label is a starting point. A slab that has flooded or shows efflorescence behaves like a below-grade slab and gets tested like one.

Old floors on below-grade slabs

Basements are where old resilient tile and black cutback adhesive are most often still in place, under carpet or a later floor. Any resilient floor or black adhesive of unknown age is presumed to contain asbestos until tested, and it is not sanded, ground or scraped before the result is back. See related: Asbestos in Old Resilient Floors and Adhesive, and Nine-Inch Floor Tile and Black Mastic.

References

• IRC section R506 (slab vapor retarder) and section R408 (under-floor space), as adopted by your jurisdiction, current edition
• ASTM F2170 and ASTM F1869 (concrete slab moisture tests)
• USDA Forest Products Laboratory, Wood Handbook (equilibrium moisture content)
• See related: How Moisture Moves Through a Building
• See related: Slab Moisture Testing for Flooring