Cedar Roof Maintenance - Pressure wash services

It is about this time every year that my blood starts to boil, not because it is so hot (like that is going to happen this year), but because I see the vast deluge of contractors out offering pressure washing services to unsuspecting home owners who want a better 'looking' home.

In the last week alone, just in my neighborhood, I have seen two contractors pressure washing roofs (one shake and one asphalt shingle). If you talk to any knowledgeable building envelope professional, they will cringe when you discuss this subject because this cleaning method often leaves behind a trail of destruction long after the contractor has left.

Lets look at each of these in detail. Bare in mind that most pressure washer contractors are like Tim Taylor from the popular TV show Home Improvement, it always makes sense to buy the biggest & baddest monster machine you can get your hands on. Translation - Lots of Pressure!

First the wood shake roof. Wood shakes are usually split. This results in the wood pieces separating from each other along the cell wall boundaries. The wood cell walls are much harder than the cell interior so this process leaves each shake with a hardened surface. This surface provides better water shedding capabilities and stands up better to UV degradation. Most pressure washers have more than enough pressure to strip away the cell wall leaving the softer cell interior. This cell interior absorbs water much easier and breaks down quicker under the sun's rays. The process also saturates the shake (often on both sides of the shake). The shake would not get this wet under normal rain conditions. As this work is normally done in sunny seasons, the top exposed side of the shake often dries quite quickly in the sun and wind, usually much faster than the bottom-hidden side of the shake. This results in stresses to the shake which often lead to cupping and eventually cracking. Now you need to call a roofer to flash all of these new cracks so that you have a properly lapped system again (do not have a cracked shake lining up over a shake seam in the row below). Now you also have a shake surface that is not as durable as the surface that was blasted off with the pressure washer.

The wood shingles fair even worse. These are almost always cut (not split) so the soft cell interiors are already exposed. It takes very little pressure to etch the wood material away. I have often seen roofs during inspections that have up to 1/4" blasted from their surface. They are also much more susceptible to cupping and cracking after a pressure washing incident.

There is also a secondary source of damage created by these contractors. Most wear some form of cleats while on the roof. Think of these cleats as two meat tenderizers on the bottom of their feet as they are producing the exact same effect. In both wood shingle and shakes, the contractor is peppering the surface full of holes that now lets the rain water penetrate to a much deeper depth which in turn accelerates the rate of deterioration of the roof.

On asphalt roofs, the damage created by pressure washers is obvious. A big pile of granules in the gutters at the bottom of the roof. Granules that are needed to protect the waterproofing asphalt substrate from being damaged by the Sun's UV rays. By removing these granules (and even a low pressure pressure-washer will), you are accelerating the deterioration of the roofing system.

Many of these contractors advise their unsuspecting clients that there services will extend the roof and some even offer a 'guarantee' (good luck trying to collect on it). In all of my experiences, the act of pressure washing a roof has significantly diminished the lifespan of the roof not extended it. I consider these contractors as incompetent at best and fraud artists at worst. It is time for the public to demand better. One step would be to require any contractor, who works on any part of your roof system, to be trained and certified by the Roofing Contractors Association of BC. That way, they would be required to follow the RCABC best practices guide even when it comes to maintaining a roof's surface. Homeowners can check out the RCABC Consumer Guide for instructions on how to properly maintain a cedar roof (pg 117).

Do not be duped by these contractors. Do your research and get informed. Say no thanks to any contractor who offers to pressure wash any part of your homes exterior.