Pressure Washing Damage to Wooden Cedar Roofs

When Not to Pressure Wash

• If your shingles are severely cracked - if the cracks are very long and extend through the entire shingle, for example - do not power wash your roof. When you pass the power washer wand over these cracked tiles, you risk splitting them and ripping them from the roof. Additionally, if any shingles show signs of rotting, do not pressure wash. In most cases, the wood rot will have already extended deep into the shingles, and you will only exacerbate things by pressure washing.

Proper PSI

• Pressure washers are rated by pounds per square inch, or psi. When selecting a pressure washer, remember that stronger doesn't necessarily mean the job is going to go faster. In fact, if you get a pressure washer that is too strong, you will damage your roof. The ideal range for most wood surfaces is about 1,500 to 2,000 psi. This is strong enough to clean and strip the wood, but not so strong that you'll be blasting the shingles off your roof.

Proper Handling

• Before you get on your roof and start cleaning the shingles, take the pressure washer on a practice run. If you have some scrap wood lying around, preferably scrap cedar, lay it on the ground, and make a few passes with the pressure washer wand until you get used to handling it. Avoid holding the nozzle too close to the wood -- keep the nozzle about six to 12 inches from the wood. Never allow the nozzle to linger on any one spot -- keep it in constant motion to avoid damaging the wood. Start with a 25-degree nozzle, then switch to a 15-degree nozzle if deeper cleaning is needed. The 15-degree nozzle is quite a bit more aggressive than the 25-degree nozzle, so practice with both nozzles before you use the machine on your roof.

Further Considerations

• Even if you take every precaution, you might see some light splintering or uneven surfaces on your roof after you power wash. If this happens, you can repair the damage by lightly sanding the shingles with medium grade sandpaper. Depending on the saturation of weathering, you might need to chemically strip the shingles after you get them down to bare wood. If there are dark spots, which are typically water stains, strip the wood with oxalic acid -- this will get the shingles back to their original appearance.