Here in the Great Northwest moss build-up on roofs is a huge problem. It is very unsightly and will shorten the life-span of your roof. Here is some good advice you can look into at a reasonable cost, without causing more damage to the roofing shingles than the moss would cause; like the damage pressure-washers will do to all types of roofing.

If you are not able to do this work yourself, and you happen to live in the Portland Oregon Metro Area you can call me to help you.

(a) Most moss on roofs are seen on the North facing sides of the roof and where there are trees shading and dripping sap and other debris over the roof, but moss spores are so small they will travel great distances through the air, so there's no way to avoid them here in the Great Northwest.

The North facing sides of the roof and heavily shaded areas grow more moss because the lack of direct sun light exposure does not dry it out like it will on the other areas of the roof. Moss is a fungus and grows differently than how plants do. It likes darker areas, but that is not the main reason. The heat of direct mid-day sunlight will dry it out to the point it cannot survive without any moisture. Thick layers of moss will trap moisture like a sponge for a long time, but on the other sides it does not get a chance to build up thick like it will on the North side, unless there are some shade trees near by.

Moss feeds on the decomposing dust and tree debris that gets trapped on roofs, which is another reason you will see a lot more moss on the roof under an overhanging tree that has littered the roof with it's fall-out and sap over the roof.

I think we've all seen these sort of quaint looking roofs covered in thick layers of moss. this one was so bad off it had ferns growing in the heavy blankets of moss near the bottom edge.

It is true that the Sun is one of the main causes of aging any roofing material, and some people think it's actually good to have a heavy layer of moss to save a roofing from exposure to the elements. Some even jokingly say the moss is the only thing holding the roof together. This of course is not true. Moss growth is clearly more detrimental than it could help on most any surface. It is one of nature’s ways to help recycle life. It works to eat away and decompose what it resides on.

It may look charming to walk through a forest with huge draping coverage of moss on tree trunks and branches, but that is not a healthy sign. Moss will attack sick trees. In more extreme cases you will even see ferns growing in the thick moss growth up in the trees and even on roofs that were left too long this way. The Forestry department now recognizes how decades of forest fire prevention has actually hurt many areas of Oregon and Washington forests by not allowing that natural sterilization and thinning of these areas.

Some feel that moss is a great way to shade your roof from the harsh rays of the Sun, but Mother Nature works hard to recycle anything it can in several different ways. Not only through breaking it down with the Sun's UV rays, but also with corrosive oxidization, and bacterial decomposition.  Moss is a fungus and the root fibers will dig into the roofing shingle surface and loosen the sand off the petroleum material under it. They also work up under the overlapping shingles, splitting them apart. Moss will also hold moisture against the shingles and flashing, causing the bacteria growth to rage and decompose your roofing the best it can. Asphalt shingles are very resistant to this sort of decay, but it has limits and eventually will causing leaks. Heavy moss growth can eat up 5 to 10 years life span from your roof if left unattended.

It is common to see roof valleys built-up with debris for several years. Moisture held in suspension, causing that debris to decompose into mud under the surface debris you can see. To the point you will have plants begin to grow out of that mud. Roof valleys and other metal flashing areas are the #1 source of roof failure and leaks.

If the roof is not too old, these areas can be repaired in many cases without replacing the whole roof, but few roofing contractors will admit this to you. Their motivation is to sell you a whole new re-roofing job.

A Pressure Washer
Make no mistake, a pressure washer is death to a roof or siding! It is true that most contractors will use a pressure washer to quickly and easily blast off the dirt, mildew, moss, and larger debris that may have collected on your roof.  It is true that a pressure washer is the fastest and easiest way to clean off a roof riddled with debris and moss, but take a moment to ponder how this is a very harmful way to clean a roof:

Normal house-hold water pressure is only about 150 psi (pounds per square inch pressure). The problem with any pressure washer is how they are designed to deliver 800 to 4,000 psi that will blast a good 30% or more sand off your shingles, which is the shade cover from the Sun attached over the petroleum material those shingles are made of. Any roofing material is not designed to withstand more than around 100 mph winds, and this high pressured water would certainly simulate a great deal more than that and will be forced into places that can cause a good deal of water damage to your house, as well as your roofing shingles.

If you let them remove much of the sand off your roofing shingles, it will leave them exposed to the UV rays of the Sun. The Sun will rip the shingles to shreds in a matter of only 5 years or so. This is a good example of how the medicine can be worse than the disease. You are far better off to leave the moss there to grow and slowly eat away at your shingles.

Bleach
Chlorine is helpful to kill moss and every thing else it comes in contact with, but it is very harmful poison to you and the environment, let alone to the person applying it, so we do not recommend using bleach on roofs. Chlorine was originally designed to be one of the deadliest chemical warfare weapons of that era. It may kill the moss and temporarily stunt it's growth, but it will not remove the moss or make it much easier to remove. It may also discolor your roofing. It is very costly as well.

Detergent
Soap may help a bit to loosen it up, but it would also require a great deal of elbow grease. It is not going to solve the issue with just a simple application. It is also not that good for the environment. We have a great deal of it soaked into our soil already over the last century of our normal cleaning process, making it hard to acquire good clean drinking water.

Zinc Metal Strips
Poking a lot of holes in the roof to attach zinc strips have been used to help keep the moss down. It is helpful, but has proven to be not effective enough our area. It is only effective about 4 to 6 feet down the roof. This one narrow gray metal strip is unsightly enough, so few are willing to have several mid-strips strung across their roof to keep it free of moss. This would also meant there would need to be hundreds of nail holes punched through the roofing shingles to hold the zinc strips in place. Zinc is a very soft metal that's barely stronger than lead. These strips would commonly become dislodged in high winds.

The worst part is how these strips would curl up over time like a tape measure blade and troth the rainwater, dipping in a few places. There would be clean streaks at those dip points were it would be moss and mildew free, but that made the roof look even more noticeably messy with such an uneven cure.

Here's a couple shots I took of a house where they did attach the top and mid strip of zinc on this black shingled roof. If you click on these pictures to see them larger you can see better how they were still not effective enough to keep the moss from forming on the roof. Perhaps it looks even worse now. Besides the look of that mid strip; they now have these irregular streaks you can see from where the roof is clear of moss.

Also, this zinc is not able to remove the moss that is there. See below for how to safely remove the moss. Being such a soft metal zinc is not good as a ridge cap, but a zinc galvanized steel sheet metal can be used effectively for a temporary solution, but you will need to make sure it gets replaced before it starts to rust, or you will have unsightly rust stains on your roofing shingles.

Powder Moss Treatment
One good and inexpensive way to control the moss growth is to get several canisters of moss killing powder. Several different companies make it. I don't think it much matters which brand you use. It will not remove the moss build up and depending on how much tree coverage you have near your house it has to be re-applied each year or two. Long term this could be more costly than the one time expense of a copper ridge cap, than to hire a licenses pesticide contractors to apply this to your roof over the next decade.

Many people think it is the mild detergent in this powder that kills the moss, so they think they can just sprinkle laundry detergent on their roof, but it is actually the zinc phosphate that does the trick. Zinc is an inexpensive metal compared compared to copper, which has shown to have an effective anti-fungal property in the same way. It should only take a half hour to treat a low slope roof on most houses, but a steep roof may be too dangerous for a Homeowner to attempt.

It is best to wet the roof just before sprinkling on the powder. This help keep the powder from getting blown off the roof from the wind before it has a chance to work it's magic. You do not want to wet the roof right after applying it, or you will just be rinsing the powder off the roof.

Another concern is not to use that type of powder on a roof that has copper gutters. I have personally seen evidence of a chemical reaction that dissolved holes through the floor of the copper gutters. The only viable option for roofs with copper gutters is to have copper ridge and hip caps installed if you want to prevent moss growth. I have not seen a powder like product made with copper in it in stead of zinc.

Copper Hip and Ridge Caps and Other Roof Flashing

Copper roof flashing: T-edge metal, W-valley, and a deluxe ridge cap
(designed to have a hidden ridge vent under it) ready for delivery

This is a lot more costly for the initial investment, but it is a long lasting trouble free solution that can be re-used over and over again when re-roofing is required. It is an excellent option for several reasons:
(a) it has enough copper surface to actually do some good against moss growth
(b) it tarnishes to a \n unobtrusive dark brown within a few months
(c) and has a much better lasting seal of the top ridge of the roof than ordinary shingles bent over the top
(d) it is the only product made to be re-used after a re-roofing again decades later, since it is attached with stainless steel screws with metal and rubber washers every 2' on each side.

The 10' long section should overlaps about 4" to 6", so this needs to be factored in when ordering these. It should come down at least 7" or more on each side. The more bends it has, especially near the bottom edge, the stronger and straighter it will be after it is mounted. There are several ways and designs to do this that can add a decorative touch to your roof. This is also a good way to incorporate a hidden ridge vent as well. There are numerous different companies making ridge vent material that is then covered by roofing, but it can be covered with copper just as well. Here is better screened roof vent design for you to consider:

Contact us for more details and pricing.

Custom copper ridge caps made to order

Scraping:
This powder will kill the moss and mildew and keep it from returning, but it will not remove it from your roof. To remove it without too much damage to the shingles takes a lot of patient scraping with a plastic or metal scraper. I have found that a narrow masons tuck point trowel to work best on composite roofs. The abrasive sand has ground the end of the steel squared tip down to a point.

To remove only about 90% of the bulky moss; a garden hoe can work to scrap it off, but it is really tedious since it is not very wide. A better tool is to use a stiff-tooth rake and attach a scrapper on it, so not to use the teeth of the rake. A stiff 3" to 4" tall strip of aluminum sheet-metal that is taller than the teeth of the rake is attached on the inside edge of the rake. It is best to have the bottom edge of the sheet metal bent at a 45 degree angle about 1" up, in towards the handle along the bottom edge to scrape the moss downwards. This bend can allow the scraper to be a few inches wider than the rake. A few small holes are drilled through the sheet-metal to run a few nylon ties through it to fasten it temporarily to the rake teeth.

A wire brush may be too aggressive to the sand on the shingles. A garden hose can be used to sweep down the loose moss and debris,

Clean-up:
A plastic brush and broom w/dustpan and a bucket will be needed to clean up the mess, so you can see what your doing again. It is best not to use anything stronger than just garden hose pressure on the roof to help clear away the mess. Remember to only spray downwards over the roofing (keeping in mind that water and mildew make a very slippery combination). As stated above, roofs are not designed to handle more than a 100 mph storm at best. Most household water pressure is around 150 psi pressure, where a light duty pressure washer has 10 times that much pressure and will cause more damage than the moss could.

Disclaimer:
It is of course a lot safer to hire a licensed professional to do this type of work, who has plenty of experience and is trained to apply these sort of pesticides. If you attempt to follow our instructions here, we cannot accept any liability for your personal safety or damage that could occur. Most homeowners are not willing to get up on their roof to do this sort of work, but if you are looking to do this work yourself, bare in mind that OCA Safety Standards say you should be wearing an approved and anchored safety harness if you are working higher than 10' off the ground.

If you are not fortunate to be living here in the Portland Oregon Metro Area, this info should help you find the right contractor to do this as I specified. Feel free to insist they read this web site and follow it to the letter. Or just print it out for them to read, and insist they read it there in front of you. If they balk at it or try to debate the validity of this web page, that should be very telling, and you should not hire them! The damages will be your loss, not theirs. You are not likely to get them to volunteer to repair those damages, and it will be very hard to prove their responsibility years later.

If you are fortunate to be living here in the Portland Oregon Metro Area, I will be happy to assist you with your needs, doing it the right way, but I am not licensed to apply that powder treatment, as it is considered a dangerous pesticide and not covered by my contractor's license. I would need to get the licensing and insurance to cover insecticide applicators, but you are free to apply that to your own roof.

As to cost: roofing is calculated in 100 square foot sections, or a 10' x 10' area called a square of roofing. These example prices below are based on a small low-slop roofed house (6/12 pitch or less) composite roof, with around 10 squares of roofing (or 1,000 square feet, which is different than the floor space in a house). A medium size house typically has around 25 squares of roofing. The cost varies due to the complexities and safety issues to access all the areas of your roof.  Go to our Contact web page and send us an e-mail telling us what you need or may want along with some digital photos of your house.

(a) $300 to do a simple scrape and clean up; removing the bulk of the moss, which is around 70 to 80%including t a gutter cleaning.

(b) $800 to carefully scrape the moss off, removing around 95% of the moss build-up, along with clean up including the gutters.

(c) $1,350 to do a simple scrape and clean up; removing around 70 to 80% of the debris, and install an 18” wide custom 20oz copper roof ridge cap 30' long. Keep in mind that this should be the last time this needs to be done, and the copper ridge cap is removable to be reused after a new roofing is installed.

(d) $1,850 for a 95% scrape and install an 18” wide custom copper ridge cap 30' long.

Moss and mildew on metal roofs is more a problem for anyone who needs to access your roof than the deterioration of the sheet-metal. They are very dangerous to access for servicing, since they mildew in short order and become very slick when damp!  Even for a low slop roof.  Wood shakes also moss-up like this and become very slick, but at least workers can wear special cleated boots.  There are no special shoes made to walk safely on metal roofs.  There are numerous problems with all types of metal roofs, as described on our Roofing web page.

The moss will also hold moisture against the painted metal, causing the bacteria rage and buckle the paint off.  Then resulting in quickly rusting through that thin steel sheet-metal.  Once that steel begins to rust it is too late to just have it re-painted.  The paint may stick to the rust, but the rust will flake off the rest of the steel.  It will have to be carefully sanded down and primed, which is very costly and may cause a penetration to the steel sheet-metal surface.  It is usually necessary to replace the roof at that point.  Replacing only part of the metal roofing could be done, but would odd to see the oxidized paint of the older roof with the new replaced section.

Also, most any roofing contractors will not even give you that kind of option.  It would not be something they would be willing to do or warranty, even thought the CCB only requires a 1 year warranty on construction. There incentive is to sell you a complete new roofing job.

Cedar wood roofing shingles are far more vulnerable to this kind of damage. Regular treatment of these 40 year cedar shake roofing is so costly that few home owners will have their roof treated as they should.  Because of this it is rare to get a 20 year life span from these 40 year roofs.  If they had gotten it treated as they should, in 40 years they would have spent more money than the cost of 2 new roofs, as well as the head-ache of finding competent workers to do this without causing more damage than the moss. That dry-rot of the expose wood may not be your primary concern, since the painted steel valley metal and other flashing, that is most always used, will rarely last more than 20 years anyway.

It is just a shame that these owners were fooled into paying so much more for that short lived roof.  Often more than the cost of a good 40 year composite roof with copper valleys and flashing.

Tile roofs are resistant to wear and tear of the elements, depending on what it is made of.  There is a huge difference between the different roof tiles sold.  Tiles tend to grow moss and mildew more than most any other roofing, clogging up the gutter each year.  It looking quite unsightly. It can grow up under the overlapping shingle and crack them, causing substantial leaking that may be hard to spot and fix.  Clay tiles with a glazing on them are much less porous and are much more resistant to damage of this kind, but rare to see on a house.

The steel sheet-metal flashing and valley trays under the tiles are the biggest concern for this issue, and harder to access to clean out right.

Note: It is highly recommended to check this and hire a contractor to replace all the flashing with aluminum, lead, copper, or stainless steel. We recommend copper flashing.  A simple test can be preformed: magnet should not be able to stick to any of the rust free metals. If it is painted or zinc galvanized, it is not stainless steel.

There is nothing much to say on this subject, since you would be hard pressed to find a thatch here in the States, but the damage potential should be pretty obvious.

First off; a contractor's license does not allow for applying a moss killing agent chemical or powder. That has to be done by a pesticide contractor, although most pesticide contractors will not work on roofs.

Most contractors do not carry the liability insurance and workman's comp that covers roofing work.  This liability insurance is about 4 to 5 times as expensive as all the other types of construction work. This has nothing to do with the dangerous nature of roofing work. That is what Workman's Comp covers, so if a worker falls off and gets hurt, they will not need to sue the homeowner. 

The liability insurance is to protect you incase the contractor or his workers screw up the roofing project and damage your house.  It seems that the roofers in this area have been doing such crappy work that they have accounted for 70 to 90% of all the claims processed by the CCB in Oregon.  This is one of the worst statistics you are likely to run across.  And that is even considering how most homeowner are not able to act on their complaint within the first year, and in many cases it is a moot point, since the CCB only requires a 1 year warranty by all licensed contractors.  Few homeowners will really check into the contractor's background for reported complaints to check and see that they carry the proper insurance to cover roofing work.  Take the time, or you will be sorry.  Ask for a written warranty that is longer than one year.  There should be no reason that a reputable contractor would not be proud to write you a 20-year installation warranty if they truly believe in their work.

To recap, insist on:

(a) No pressure washers to be used on your roof! Nothing stronger than a garden hose spraying downwards only. They need to use a hand scraper, or a wider hoe like scraper as described above, and carefully scrape the moss off without damaging your roofing.

(b) To fully clean up their mess and haul that debris away (no illegal dumps).

(c) A new custom copper roof ridge and hip caps are best to prevent further moss build-up.

(d) A moss control powder can be applied to kill the moss and mildew that was missed as well as the moss spores left behind, but not to be used on roof with copper gutters.

(e) Get some leaf protection for your gutters (if you have them) to help minimize the traffic on your roof in the future.

E-mail DMR Gutters at (503) 351-7082
for a free no-obligation estimate for the
finest in rain management.
(as long as you are local to the Portland Oregon metro area)

Below is a photo of our
Better Business Bureau's
NW Business Integrity Award
for the year 1998

We were also a 1997 finalist for this same award. See our referral web page to see how we managed to be honored with this special award