Pressure Washing Your Roof


  • Roofing damage in progress:

    I was at an inspection, talking to my client about his plan to add a bathroom to a 2nd floor area, when out of the corner of my eye... "I am sorry Mark, but would you mind if I ran downstairs to get my camera?" I didn't wait for an answer. I needed a photo of this pressure washer damage "in action".

    class="caption">Roof Damage in Progress!

    Well, Mark was a nice guy and I needed a picture of this crime. The roof in question (see above) had been in fine shape (just a few minutes earlier). It had a little moss on it, some debris, but nothing that couldn't be solved with a garden blower or a soft broom and a moss control agent.

    Enter the pressure washer, and what a pressure washer it was. Just take a close look at the stream of 2400 PSI water being directed at the roof! Mark was also looking at this crime and spotted the fact that some of the roof tabs were being visibly lifted by the force of the water! (a video clip would work here). This fine roof was being destroyed! Granules were being washed off the shingles by the pound! Roofing tabs were being loosened and the self sealing adhesive broken! The roof was loosing years of life by the minute! (ok, ok, I will calm down.)

    So, what's wrong with pressure washing composition and most other types of roofs?

    • Composition roofing is a relatively soft material. Any abrasive action removes the protective granular surface, breaks the bond between the shingles and damages the fabric of the material.
    • At best pressure washing only does some damage to the roofing, at worst it destroys perfectly good roofing.
    • When pressure washing removes moss and debris from the roof, it also removes some of the roofing material.
    • If there is a lot of moss on the roof, the moss has already done some damage to the roofing material, the roofing material has already lost some of its integrity. Killing the moss with a moss control agent reduces further damage but leaves the roofing in place. Pressure washing removes the moss and the partially damaged roofing.
    • The bottom line is: pressure washing a roof will shorten the life of the roofing. Pressure washing removes moss, but also part of the roofing.

    Do pressure washers have any value? Yes! for deck maintenance, preparing siding for paint, and to clean concrete driveways. But even in such applications, keep in mind that pressure washers are very strong tools and can damage wood and other surfaces. They are used in logging operations to remove bark from trees!

  • ...and in the interest of presenting a different opinion on the subject, here is a note from Nathan (I am withholding last and company name):

    Hello George Guttmann,

    I recently read an article you wrote about not pressure washing roofs. You have some very valid points. However, I believe you should have done some more research before making the statement, "pressure washing only does some damage to the roofing, at worst it destroys perfectly good roofing."

    The fact is, a professional that knows how to use his equipment can pressure wash a roof with as little as 200 p.s.i. and still be rinsing at 5 g.p.m. With roofs its not pressure washers that damage the roof, it's HIGH pressure. I suggest you make amendments to your article just to be on the safe side. There are MANY roof cleaning professional that use pressure washers without any granule loss. Using the same pressure as a hose only with much more flow will clean a roof in a quarter of the time a homeowner could do so. Many people make their living cleaning roofs and at this time many are being introduced to your article.

    Please do some more research, it may be that the picture you took is of a person using to much pressure but I would question how you know he is using 2700 p.s.i. Just because a machine can put out say 3000 p.s.i. doesn't mean that's what is being used. The nozzles, unloader, and machine can be changed to get different pressure. I use a 3000 p.s.i. hot water washer. However, when washing roofs I use different nozzles and cold water only. I get the pressure so low I can put my hand directly in front of the nozzle while spraying and it feels like a shower. If your interested in learning all the correct techniques please let me know.

    Thanks, Nathan


    Thanks for taking the time to write and to present your side of the issue. I am sure that you make every effort to reduce the amount of damage to the roofing when you use a pressure washer, however, moss and lichen are very strong plants and some roofing granules are dislodged even when one removes clumps of moss by hand - let alone a tool such as a pressure washer.

    In my opinion, you would do a much better job on behalf of your clients by treating their roofs with moss control agents and allowing the dead moss to release itself from the roofing. This usually takes two or three applications but it does work and it does not damage the roofing. -George

    (I better go now before my wife reminds me that our roof needs some moss control work.)

  • May 2007

    And here is another exchange of ideas regarding the pressure washing of roofs:


    I just happened to run into your website today after typing in pressure washing on the search engine. I saw your tirade about pressure washing a roof. I do commercial pressure washing with a 3000 pound 10 gallon minute, hot water machine. I don't do roofs however, I do commercial sidewalks. However I've been cleaning my own roof with my giant machine for 22 years, every year. This is a composition roof and yes I get a handful of granules come off but no more than what I get after a rainy season accumulating in the gutter. I started doing this after I re-roofed 22 years ago and I had to re-roof because moss had curled and destroyed the previous roof.

    I see you are in the Northwest and should be familiar with moss growing on roofs. Moss will cause considerably more damage than properly performed pressure washing. Certainly any job can be performed badly and cause damage. There can also be damage done by giving bad advice. I think this is an example of a jack of all trades master of none.

    Allen Cox


    Thanks for the note and your opinions, but I beg to differ.

    I would have you consider one more reason why pressure washing is the wrong way to deal with moss. As you point out, moss kills many roofs. But what does most of the damage to the roofing material is not the stuff at the top of the roofing, but the roots etc. that have found there way into the roofing tabs and under the edges of the roofing material and eventually into the material itself. And its that part of the moss that is largely inaccessible to the pressure washer. The pressure washer 'prunes' of most of the visible parts of the moss thus allowing the moss to grow back very quickly. And even if you were so skillful and cause minimum damage to the roof during this process, it is my opinion that you only 'mowed the lawn'.

    A much better alternative would have been to make sure that trees limbs don't overhang your roof and to have installed a zinc strip under the roofing caps, see the topic page on Roof Moss. This would have prevented moss growth for at least 5 years and possibly much longer. Any debris on the roof (leaves etc.) could have been removed with a soft broom or garden blower and walking on the roof should have been kept to an absolute minimum.

    If moss did start to grow on the roof, it would have started in the form of a stain that is usually visible from the ground or a ladder. Remember, don't walk on the roof unless you must! (its not good for the roofing and can be dangerous to you). Once the moss staining starts, you could have used a low toxicity spray-on 'soap' to kill the early stages of the moss. Usually this can be done from a ladder and a garden sprayer. Again, no roof damage. If some of the moss starts to grow into small lumps, spray them with the low toxicity material, allow the moss to die and leave it in place. The dead moss will wither and the remainder will flow into the gutter during the next rain.

    And what do you do with the roof that has a heavy moss cover? Nothing! The moss has already done so much damage that none of the processes will do any good and the roofing material is so weak by this time from the moss damage that any walking on the roof can only cause more damage. It's too late!

    Pressure washers are great for all sorts of things. But not on roofing! - George

    Well George,

    I guess I have to agree to disagree on this one. Though certainly I do accept your points on the thought of using any oxide of metal to deter the growth of moss this on its own is not the entire answer to moss in the Northwest. I also agree that cutting the trees away from one's house is a good idea in order to save the roof but many of us like our trees and are willing to undergo the expense of having them.

    But I don't know what you're entire background is. I am a third-generation contractor and have benefited from my father and grandfather's experience as well and that experience tells me your blanket statement of not using a pressure washer indicates at least on this subject you have incomplete information. We can't all live in California with no trees around our house and no moisture that will grow moss. In the Northwest moss is just a reality. Usually by time the homeowner recognizes they have a problem pressure washing is by far the best and most cost effective solution.

    I believe you're advice in the last e-mail is particularly destructive when you suggest the moss be left on the roof even when dead. Dead moss even if sterilized chemically still holds moisture and speeds the growth of the next-generation. The remaining dead moss even gives a foothold to that next-generation insofar as growing underneath the tabs. And yes a skilled pressure washer can clean up three tab roof without breaking the seal tab bond. As I told you in my previous e-mail my owner roof gets cleaned at least once a year.

    You also mention that walking on the roof will increase its deterioration and there's some truth to that particularly when the roof is only supported by half inch plywood because that thin thickness of plywood allows more flexing of the roof and as the roof ages it does become more brittle, however a brittle roof needs to be cleaned even more so in order to extend what life it has left.

    In any event this is an area of home maintenance I am particularly knowledgeable and experienced in, area that I revisit almost daily in my professional life. By this I mean I actually work at this particular part of home maintenance instead of just talk about everything to do at home maintenance.

    So again were back to the phrase jack of all trades master of none, you may know a little bit on every subject and that little bit particularly shows when you talked on this subject.

    Allen Cox


    A brittle roof can't withstand pressure washing, walking or any other kind of impact. So why do that when you could try to prolong the life of the roofing by spraying the roofing with a low toxicity agent and in the process save a lot of water and energy.

    Would it be best if the dead moss were removed from the roofing? Yes. But it would be even better if one prevented the moss growth in the first place. But if you want to remove the moss, spray it first with a low toxicity solution and then use a garden blower and not a pressure washer.

    I thank you for your ideas, but you have not budged mine. Having inspected many thousands of roofs and seen a lot of moss and pressure washer damage I remain convinced that pressure washers should not be used on composition, tile or wood roofing and is not much help and potentially dangerous with metal roofing. - George

    OK Allen, I will give you the last word:

    Well George, I'm impressed you were willing to post my entire e-mail to you on this subject. It was not my intent to admonish you publicly but to try to educate you for when you give information in the future. I can see you intend to stand by your convictions which that part is honorable even if wrong.

    I think there may be an additional piece of information why I am so successful at roof cleaning where most others aren't and that is I do use hot water. Very hot water around 200 degrees. Perhaps this heat momentarily softens the brittle roof as well as does more to kill the remaining moss even that which is under the tap. That however is just a guess on my part.

    Allen Cox

    See also Topics: Decks, Topics: Exterior Siding and Paint