Guide to Wood Floor Types, Damage, Diagnosis, Repair
     

• Catalog of types of damage to different wood floor materials
• Description of repair approaches for damaged wood floors
• Buckled wood flooring due to leaks or high moisture
• Cupped wood floorboards, convex & concave floor cupping
• Gaps between floor boards or parquet flooring - cause/cure
• Rotted wood flooring;
• Scratched wood flooring
• Stained wood flooring including pet stains, water stains, and mold stains or mold damaged wood floors
• Cause of Outdoor Deck Board & Stair Tread Cupping & Shelling - Bark Side Down Advice for Decks & Steps
• Questions & Answers about the diagnosis & repair of wood floors
• References

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Our photo at page top shows cupped wooden strip flooring and an easy way to detect this condition..

Also see Wood Floor Types, and FLOOR TYPES & DEFECTS. If examining resilient flooring that may contain asbestos, also see FLOOR TILE HISTORY & INGREDIENTS and ASBESTOS FLOOR TILE IDENTIFICATION. If you are concerned about mold-contaminated wood flooring, see MOLD CLEANUP - WOOD FLOORING. Damage to other types of flooring is discussed at FLOOR DAMAGE DIAGNOSIS.

This 1940's wood floor buckled severely because of wet conditions in a gymnasium after the roof leaked.

But even if a building is not subjected to flooding or wet floors, if the wood floor was installed without adequate clearance around its perimeter buckling can occur when moisture increases in the building.

And as we discuss at FLOOR, WOOD MOISTURE, a successful wood floor installation requires that the wood flooring be acclimitized and at proper moisture level in the building before it is installed.

Very large wood floors (such as in this gym) may need an inch or even more of side clearance at the building walls - a detail accomplished by building out floor/wall baseboard trim if necessary, or in the case of newer products, by the combination of clearance gaps and a floating floor design.

Compare our photo at left of concave (curved down) cupped wooden floorboards with the convex (curved up) cupped floor boards in the photo at the top of this page.

At left you can easily see the concave cupping of the floor at left by looking at our flashlight beam across boards in the upper portion of the photo. (The center of concave cupping is down and edges of concave-cupped floor boards are raised - think of "cave" as "caving in" if you want to remember this distinction.)

Diagnosing why a wooden floor has become cupped is an important step in correcting the problem, since we need to identify and correct the moisture source.

There are exceptions, but in general a board swells on its side that is most moist. So on an interior floor like the one shown at left, the fact that the center of the floorboards is cupped "down" we infer that it is the down-side of these boards that is more moist than the finish side facing the room. We find this condition in homes with finished wood flooring installed over a damp or wet basement or crawl space.

At our page top photo the finish floor boards seem to be cupped "up" - suggesting that this floor has been wet from above.

If the floor board cupping is minor and hard to see, you might decide to just address the moisture problem and leave the floor alone. It won't recover, but it the floor should not get worse. How would we repair a badly cupped wooden floor? Sanding the floor flat is an option unless the boards have not only cupped but come loose from the subfloor. In that case floor replacement may be necessary.

Indoors we find that floor boards tend to cup so that the concave side of the board is towards the floor's more dry side, and the convex side of the cupped or curved boards tend to be facing the floor's more wet side.

Apparently when a wooden board has uneven moisture content across its thickness it will tend to expand more on its more wet side, causing that side to "arch" or become convex.

You might think that outdoors where despite temporary wetting from rain, deck boards and wood step boards open to air on both sides ultimately have a uniform moisture content across the board's thickness. But because the upper board surface is exposed to sun and in some cases more wind or air movement, the under-side of boards may have a higher moisture content. In some installations, significantly higher.

For an explanation of wood board cupping and which side of deck, ramp, and exterior stair boards should face "up" see BARK SIDE UP on DECKS & STEPS.

Cupped ramp boards (or deck and platform boards) with the "cup" facing up will hold water and form algae or ice more quickly than boards that drain properly, increasing the risk of a slip, trip, or fall hazard. Upwards-cuped boards sometimes rot faster too .

Look at the end-grain of any deck, ramp, or wooden walkway board and notice the curved lines that mark the winter wood layers of the tree from which the board was cut. If these lines arch "upwards" when the board is placed, most boards will also be curved upwards (convex) and will drain better. But before nailing a deck or ramp board in place, look at the board surface itself - sometimes the boards don't follow these "cupping rules", or the bark side of the board may be badly damaged - so when building a wood structure you may have a reason to place a few walking-surface boards "upside down".

(Sketch adapted from (flipped over) an original provided courtesy S. Bliss, Best Practices in Residential Construction.)

When building a deck we like to look at the end grain of the wood and install deck boards with the "bark side" up so that cupped boards won't form puddle-collecting dips on our deck.

But look at your actual individual boards or stair treads when you are building an outdoor structure. If we are installing a deck board that is already rather cupped, we install it with the convex or "outward curve" side facing up for best drainage. Also see DECK & PORCH CONSTRUCTION.

Gaps between wood floorboards may occur for several reasons, but the common underlying cause is a moisture problem.

Installing wood flooring before it has had an opportunity to acclimate to the indoor moisture level of its new home often means that too-moist flooring was nailed in place, later shrinking as building heat further dries the wood.

The gap between the flooring shown at left is viewed from the under-side of the floor of an older building. This is a ship-lap joint whose boards have separated about 3/16".

Gaps can also appear between sections of a wood parquet floor, as shown in our photo (left).

Do not rush to fill in gaps like this in wooden floors. Particularly in the case of wideboard flooring the width of individual boards increases and decreases seasonally as building indoor moisture levels increase (typically in the summer) or decrease (typically in winter).

If you fill the gap between floor boards in the dry (wide gap) season, when the building moisture increases and boards swell the swelling floor boards will push against the gap filling material, actually crushing the edges of the floor board.

Then when the building dries out again a new gap will appear. Or in extreme cases the floor may buckle in damp weather. In either case, it's better to just vacuum out the dust and debris and leave these wideboard floor gaps alone.

At Wood Floor Types where we illustrate a restored and re-finished wide-board floor in an 1860's home, if you click and enlarge the photo you'll see that we left the flooring gaps open. In fact during restoration we sometimes have to pick out putty or other stupid crack fillers that someone has applied earlier.

A common complaint we've investigated is shrinkage gaps in wood flooring installed over radiant heat tubing.

The underlying cause may be flooring that was too moist at installation (see above), or operating the heat at a higher than recommended temperature.

In the gapped wood floor over radiant tubing shown here, the owner, attempting to eliminate a smell associated with the radiant heat tubing, ran the heating system at high temperature, perhaps contributing to this floor gap problem.

See MOLD CLEANUP - WOOD FLOORING where we describe removing mold from wood flooring surfaces.

Cleaning mold from wood framing, or wood building sheathing and similar wood surfaces is discussed at MOLD CLEANUP - WOOD FRAMING & PLYWOOD.

Readers should also see the general advice at MOLD CLEANUP GUIDE- HOW TO GET RID OF MOLD and finally, see Wood Floor Damage

Also see Hidden Mold in Flooring & Subflooring. Also see TRAPPED MOLD BETWEEN WOOD SURFACES for a discussion of the immobility of mold between wood framing and other surfaces, and see MOLD GROWTH on SURFACES for an index of what mold genera/species are frequently found on various building surfaces and materials.

Pet urine stains on wood floors - pet stains can ruin wood flooring. Stains are deep enough into the wood that almost no amount of sanding will remove them.

We have had some wood floor stain removal success by careful, by-hand bleaching combined with sanding, followed by careful blending of hand-mixed wood stain products to return the color of the wood floor section to that of its neighbors. See Black Flooring Stains from Animals for details.

Nail & moisture stains on wood floors - Where small black stains developed in a wood floor in a 1920's bungalow that we restored (our lab and office) we had great success by combining sanding with spot bleach using a Q-tip to lighten the black spots where in prior years moisture had caused bleed and corrosion staining around those fasteners.

Any exposed nails were set into the flooring before sanding. After sanding we sat with a q-tip and a damp cloth, dabbing and bleaching black spots, wiping and washing off the bleach when the stain was light enough - thus avoiding over-bleaching the floor. We mixed wood stain products to bring the color of the stained-bleached spots back to the surrounding flooring.

Here is a long strip solid oak wood floor that's shows ugly but fairly superficial scratches. To make this 1970's strip oak wood floor look perfect, sanding and re-finishing may be necessary.

But an alternative to try first is thorough cleaning, perhaps buffing with steel wool, followed by application of a sealant and finish coating. Often superficial scratches in a wood floor disappear from view when the scratched finish coating is restored.

It's usually a better practice to repair a floor with the least aggressive means possible, preserving the material for a longer life.

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