Mysterious Dark Spot in Hardwood Flooring

We recently received a call from a repeat customer located in Carrollwood. She was asking us to fix a mysterious dark spot in their hardwood flooring. Her wood flooring was installed more than ten years ago by one of our competitors, a reputable company. (We are seeing an increase in these types of calls, especially during the summer months with more rain.) Before a correction, we needed to discuss the possible cause so that they did not suffer another black spot.

Moisture - Most Likely Cause

Usually, the homeowner is seeing a moisture discoloration. Moisture damage can originate from above (e.g., water leak from a potted plant or pet accidents) or from below. If the problem is near an exterior wall, then the moisture may be migrating from under the exterior wall or down from a window.

If the discoloration is near a water pipe or other building structure which penetrates below the slab, the moisture may be seeping up between the slab and the added structure. But, what causes a discoloration which is toward the middle of a room when there has not be a chance of topical water damage?

How Slabs Are Poured

Insight may be gained by how slabs are poured. The earth, especially above the Florida aquifer, is constantly releasing moisture. Ground water, by capillary transfer, migrates upward. To stop the moisture vapor, polyethylene sheeting is used to cover the dirt. Then, the concrete is poured. However, in some cases, the polyethylene sheeting or the tape used over the joints between the polyethylene sheets may degrade and allow moisture transfer. Naturally, the risk is greater for older homes.

Hardwood Floors Magazine published an article in their June-July 2016 issue specifically addressing this problem, an article wherein the author, who was an inspector, noticed an increasing occurrence of these random dark spots, especially in homes built in the 1980s and prior.  In fact, the slabs for some older buildings and homes do not have any type of moisture barrier to retard vapor.

Our Advice - New Flooring Type

Our homeowner acknowledged that her stain was away from exterior walls where there was no risk of topical moisture and that their home was built in the late 1970s. We explained the possible cause and suggested that, instead of paying us to provide another major repair, they should consider a ceramic wood or a waterproof wood luxury vinyl plank. Her slab was not an ideal candidate for new hardwood and could not be warrantied by traditional methods.