Flat roofs are not ideal - they have a lot of issues with standing water from easily clogged gutters. Ponding water that remains on the roof will eventually find a way in to the substructure. This can easily lead to extensive water damage. Flat roofs require regular maintenance and need to be kept clear of debris as much as possible.

Something is going on with this ceiling. The homeowner noticed a growing stain on the ceiling that seemed to get worse every time it rained. This corner of the ceiling is under a low area of the roof and located directly under a roof valley (junction of two sloping areas of a roof). This stain is a good indication that the roof is leaking.

There was a previous leak in this same area, you can see the square area where a previous repair has been completed. Obviously the leak wasn't repaired adequately or a new leak has formed in the same area.

Common areas of roof leaks are roof valleys, junctions where a vertical surface abuts the roof or penetrations from pipes through the roof. The homeowners had a roofing company inspect the roof and they found an improperly installed valley flashing. The roofers repaired the flashing and made the roof water tight again.

Once the leak has been repaired, it's time to open up the ceiling and assess the damage. The drywall needs to be cut from this section of the ceiling and any wet insulation needs to be removed. Inspect the attic area and try to find all the remaining water on the joists and wooden components. The repair area needs to be left open to dry before completing any repairs.

It seems like an easy solution. Put this zinc strip on your roof one time and you never have to worry about moss accumulation again. Do these zinc strips actually work? If you read the marketing the answer is yes, but reality is a bit different.

Zinc strips do help prevent some moss, but they aren't totally effective. The theory is that you attach the zinc strip to the peak of the roof and use rainwater running over the strip carry particles of zinc down the roof to kill existing moss and prevent future growth.

The picture below shows a zinc strip in real life. It's been in place for a few years and rain run off has been carrying zinc down the roof. As you can see, the top of the roof is fairly clear of moss growth but only for a few feet of roof surface. The zinc can't get all the way down the roof and moss is still growing on the bottom 2/3 of the shingles. 

So are they worth it? Probably not if you only put them at the peak of the roof. They only control moss for a short distance down the roof and don't prevent the need to clean the lower portion.  

You could put a strip at the peak, a second strip about 1/3 down the roof and a third strip 2/3 down. This would probably do a better job of moss control but it is a bit unsightly and putting nails through the shingles to attach the strip can lead to other water damage. 

I wouldn't recommend using zinc strips for moss control. The better option is a yearly cleaning of the roof with a broom and applying a moss control agent to the entire roof surface. 

The picture above shows a common problem in the Northwest; moss growth and piles of pine needles on the roof. Our frequent rains and lack of sunny days lead to a lot of water on our roofs . If the roof is not kept clean of pine needles and other organic debris, water will stick around on top of the roof and eventually be a prime area for moss growth. 

Moss loves these wet areas and likes to grow on top of and in between the roof shingles. The shingles are raised up by the moss which allows water to get underneath which will lead to damage of the roof sheathing. If you trap enough water next to the roof you will eventually get a leak. 

The pictures above are the same area before and after cleaning. This corner of the roof had heavy moss growth from a few years of neglect. The tree branches that hang above this corner aren't helping. They provide shade and an extra source of water from dripping branches. 

It just takes a few minutes of work with a push broom to get all the moss off the shingles. You do have to be a bit careful to avoid taking all the granules off the shingles. Moderate pressure with the broom is all that is needed. Too heavy of a push and you can damage the shingles. 

DON'T USE A PRESSURE WASHER. Seems like it would be a lot easier but if you use a pressure washer you are guaranteed to damage the shingles and shorten the life of your roof. 

You can't see it in the pictures but the last step is to sprinkle a moss preventer on the clean shingles. I like to use the shaker bottle of Moss Out. A 6 pound container will take care of most roofs. This should be repeated every year to maintain a moss free roof.