Pressure washing problems

Roofs. Asphalt shingles are protected by their granules. ARMA states plainly: never use a pressure washer to clean an asphalt shingle roof, because it will cause granule loss and very likely premature failure. Malarkey Roofing gives the same warning, noting pressure washing can dislodge a shingle's granules, which are their primary defense against damage and UV aging, and that excessive walking on shingles does the same. There is no safe PSI for this. Fixr's house washing guidance likewise says most roofing materials should never be pressure washed because it can remove protective layers and lead to future leaks.

Siding. The damage here is often invisible: water driven up behind the panels. Polymeric Exteriors Association guidance says to hold the washer straight at eye level and not to aim the stream upward, because that can force water behind the siding panels, and to be cautious around windows, doors, electrical wiring and plumbing, noting many manufacturers caution against pressure washing those areas. Water inside a wall does not announce itself for months.

Masonry. NPS guidance on cleaning historic masonry warns that water at too high a pressure — "power washing" or "water blasting" — is very abrasive and can easily etch marble and other soft stones as well as some types of brick, and that nozzle distance and flow rate matter as much as pressure. Its practical test is worth knowing: if you see grains of stone or sand in the runoff on the ground, the pressure is too high. It also notes that eroded mortar joints let more water penetrate the wall, which then requires repointing.

Wood. Today's Homeowner warns that pressure above 2,000 PSI is excessive for wood and can cause splintering, etching and stripped wood fibers, recommending 1,200 to 1,500 PSI for cedar with a wide-angle nozzle about two feet away at a 45-degree angle. Fixr's deck guidance recommends 1,500 PSI for wood and composite and notes that too much pressure leaves grooves in the boards and can chip, peel and crack them.

Chemical damage. Cleaning solutions cause their own problems. ARMA advises protecting landscaping from the bleach solution. Polymeric Exteriors guidance lists cleaners to avoid on siding entirely, including organic solvents, undiluted chlorine bleach, liquid grease remover, nail polish remover and furniture polish.

How to choose

Never let anyone pressure wash asphalt shingles

Two independent manufacturer-side sources say it causes granule loss and premature failure. If a contractor offers, hire someone else.

Watch the spray angle on siding

Straight on, at eye level, never angled upward. Water forced behind panels is the damage you won't find until it's expensive.

Check the runoff on masonry

NPS's test: stone grains or sand in the water running off means the pressure is too high. Stop and reduce it.

Insist on a test patch on wood

Somewhere hidden, at the low end of the range, before anyone touches the visible boards. Grooves in decking do not buff out.

Confirm the wall is sound before washing masonry

NPS advises checking that mortar joints are sound and the building is watertight first — otherwise water seeps inside and damages interior finishes.