Clean it yourself, or call: the ten square foot line

Measure it, don't guess it

People guess low on ceilings and high on walls. Take a tape to it. Ten square feet can be a square about three feet on a side, or a strip one foot tall running ten feet along the bottom of a basement wall — the second is the more common shape in Jefferson County, where so much of the growth people find is below grade.

Add up separate patches in the same room. Three spots of three square feet on one wall are one problem, not three small ones, because they share a cause.

EPA's wording, and the part people skip

EPA's page on cleanup says that if the moldy area is less than about 10 square feet, "in most cases, you can handle the job yourself." The same paragraph goes on: if there has been a lot of water damage, and/or growth covers more than 10 square feet, consult its guide for schools and commercial buildings, which it says applies to other building types too.

So there are two triggers, not one. A small visible patch in a basement that took two inches of water from a failed sump is already over the line, because of the water.

EPA also lists cases where size does not matter at all:

• Sewage or other contaminated water. EPA's advice is to bring in a professional experienced with buildings damaged by contaminated water.
• The heating and cooling system. If you suspect the system is contaminated, EPA says not to run it, since it could spread mold through the building.
• Health concerns. EPA's advice is to consult a health professional before starting cleanup.

What a small job looks like

On hard surfaces — tile, sealed concrete, metal — EPA's method is plain: scrub with detergent and water, then dry completely. Absorbent materials like ceiling tile and carpet may have to be thrown away, because growth fills the gaps in porous material and may be impossible to remove completely. EPA also says not to paint or caulk over mold: clean and dry first.

Fix the water first. A bathroom ceiling that grows back every winter has a ventilation problem; scrubbing it is maintenance, not a fix.

What changes above the line

Removal at scale is a different job, and the difference is mostly about not spreading it. The work area is closed off with plastic, the air is filtered so it moves into the work area rather than out of it, wet or colonized drywall and insulation are cut out and bagged, and what remains is cleaned and dried before anything is closed up. The details are in EPA's schools and commercial buildings guide; in a house the same steps happen at smaller scale.

That is the part worth paying for. Anyone can spray a wall. Keeping the rest of the house clean while a wall comes apart takes equipment and practice.

Before you call

Have the measured size, a photo with something for scale, the room or level, and what you think wet it. If the patch is on finished drywall, note whether the wall backs onto a bathroom, kitchen or exterior — that tells a contractor where to look for the source before anything gets cut.

If you rent, the call is the landlord's to make; this page explains how to report it.