What Is a Flood Cut and When Is It Needed?

A flood cut is the removal of the lower section of drywall along a straight horizontal line, a set distance above where the water reached. Restorers flood cut drywall when the board, the insulation behind it or the framing cannot be dried properly with the wall closed, or when the water was contaminated. It opens the cavity, gets wet material out and leaves a clean edge for the repair.

Why cut the wall when only the bottom got wet?

Water on a floor does not stop at the baseboard. It runs under the wall, soaks the bottom plate and climbs. Drywall wicks water upward like a paper towel, and it climbs higher on the back side, inside the dark cavity, than on the painted face you can see. A wall that sat in two centimetres of water can be wet 30 centimetres up or more by the next day.

Behind the drywall there may be insulation holding water against the studs, and on exterior walls a sheet of poly vapour barrier keeping it there. None of that dries through a painted wall in any useful amount of time. Cutting the bottom of the wall open solves several problems at once:

• Wet insulation can be pulled out.
• The bottom plate and the base of the studs are exposed to moving air.
• Hidden mold, staining or old damage becomes visible.
• Drywall that has gone soft or was touched by dirty water leaves the house.

How high does a flood cut go?

The cut goes above the highest point where the wall is wet, not the highest point where water stood. That point is found with a moisture meter, working up the wall until the readings return to normal. The line is then set a safe margin above it, commonly 30 to 60 centimetres.

In practice most flood cuts land at one of two heights: about two feet or about four feet off the floor. Drywall sheets are four feet wide, so those heights let the repair be done with half sheets or full sheets and one straight seam. A neat, level cut at a standard height is quicker and cheaper to rebuild than a ragged line that follows the water.

If the wall is wet higher than four feet, or the water came from above, the whole sheet or the whole wall usually comes off.

When flood cut drywall is needed

• The water was dirty. Sewer backups, floodwater from outside and water that sat for days carry contamination that soaks into drywall and cannot be washed out.
• The drywall is soft, swollen, crumbling or sagging.
• There is insulation in the cavity that got wet, which is the case for most exterior walls and many finished basements.
• There is visible mold on the wall or a musty smell from the cavity.
• The wall sat wet for more than a couple of days before drying began.
• Readings in the bottom plate will not come down with the wall closed.

When a flood cut can be avoided

A clean-water leak caught the same day, on an interior wall with no insulation, is the usual case where the drywall stays. The baseboards come off, small holes are made low on the wall where the baseboard will cover them and air is pushed into the cavity while dehumidifiers run. Whether a given wall qualifies is a judgment made from readings, and it is a topic of its own. The short version is that clean, firm and recently wet drywall can often be dried in place.

What happens during and after the cut

1. Furniture and contents are moved clear and the floor is protected.
2. Baseboards and trim are removed. Sound trim can sometimes be set aside for reuse.
3. If there is mold or dirty water, the area is sealed off with plastic and put under negative air through HEPA filtration before any cutting, so dust and spores stay in the work area.
4. A level line is marked and the drywall is cut along it with a shallow blade, to stay clear of wiring and pipes. Outlets in the wet zone are dealt with safely first.
5. The cut section and wet insulation are bagged and taken out.
6. The exposed framing is cleaned as needed and dried with air movers and dehumidifiers until it reads under 16 percent moisture content.
7. The wall stays open until the final readings are logged.

One caution for older houses. Plaster, and drywall joint compound and some textured finishes used into the 1980s, can contain asbestos. In a home of that age the material should be tested before it is cut, and London has many such homes in Old North, Wortley Village and Old East. Plaster walls also behave differently from drywall and are assessed case by case.

We do the cut, the removal and the drying. We do not rebuild. Once the readings pass you get a clearance report, and a drywall contractor can hang the new strip, tape one horizontal seam, and prime and paint. Done at a standard height, the repair disappears once it is painted.

Before anyone cuts, ask to see the moisture readings that set the height of the line. The cut should follow the meter.

Common questions