Call Today (612) 254-8060

Dark Spots or Streaks on Your Roof? 3 Reasons Why

Dark Spots or Streaks on Your Roof? 3 Reasons Why

Dark Spots or Streaks on your Roof

If you are a homeowner and you notice you have roof discoloration, it is most commonly some type of algae growth. It could consist of algae, moss or lichens.  This is not uncommon, especially in wet and humid climates.

 There is also the possibility of asphalt bleed-through or rust stains.  It is nationally recommended that you should inspect your roof twice a year for obvious signs of damage such as a missing shingle. In addition to the homeowners inspection, one should have a professional inspection every 3-5 years. 

Reason #1: Roof Shingle Algae

What would appear as a darkish green stain on a roof is probably going to caused by algae growth. A majority of the time, algae is more common on north facing sides and in the shaded areas. Areas that receive minimal sun maintain the water from humidity or precipitation a little longer than other areas which contributes to the black or green streaks that appear on shingles.

The reason this happens is because a food source rests among the asphalt shingles. If left to grow, algae can completely infest your roof causing moisture to remain trapped and lead to water damage. Eventually, algae can combine with other fungi and form lichens. Algae is much easier to deal with, unlike lichens. 

Roof discoloration will not cause immediate harm your roof but it will take away from your overall curb appeal. This look will lead people to believe your roof is in poor shape with old materials, even if this is not the case.

Reason #2: Shingle Bleed-Through

A grey roof with moss or algae that appears as a darker grey

At the end of a asphalt shingle’s lifespan, it is typical to notice black streaks appear on the slope of your roof. What a shingle faces in life – ultraviolet rays, high winds and thermal shock- is extreme and tough.

The extreme environmental variables will eventually cause a shingle to deteriorate causing the pigments contained in black asphalt to rise to the surface. Such “bleeding” can be referred to as bleed through. Once the pigments are at the surface, you can see them in a runoff pattern after heavy rain or snowfalls. 

If your roof has dark/black streaks, it may mean the end of your roof material’s lifespan. However, a new roof can also experience this with low quality materials.

Reason #3: Rust Stains

With anything metal, rust is inevitable. If your metal roof has said dark red or brown stains, it is most likely rust.

 Even if you just have metal fixtures on a roof, rust can still cause stains. Over time, these stains will become more widespread on metal materials. If your roof only contains metal components such as chimneys, flashings, antenna mounts, and all others, rust discoloration will only appear around these elements. 

How to deal with a discolored roof.

If a discolored look is one you want to avoid, we will provide you with a few tips on how to prevent and get rid of a poor curb appeal.


Shingle manufacturers are now incorporating copper and zinc (elements that prevent the growth of algae) into their products. If your roof does not already contain this system of protection, you could consider a power wash or chemical cleanse of your roof to eliminate algae until you are ready for a reroof.

It’s recommended to use a 1:1 ratio of bleach to water to clean a roof. We advise against using a pressure washer on asphalt shingles, as the strong pressure of the water can damage them. After soaking on the roof surface for up to 20 minutes, solution of bleach should be rinsed away with clean water.

Bleed Through:

This type of problem can be best addressed by roofing professionals. They would know best whether it is the end of a roofing material’s lifespan (calling for a new roof) or if it is result of low quality materials.

Rust Stains:

Initially, you need to check and be sure that flashing and other metal components are not compromising the integrity of a roof with leads to leaking.

If you and a professional are confident that no metal elements need replacing, then  you can fight stains with vinegar and/or oxalic acid. These treatments will prevent metal items from further rust. 

If you have a completely metal roof, a professional roof painter can be an asset. A painted roof can be protected from oxygen and water which are the two main attackers on metal.


There are a couple different reasons you might see dark spots appear on your roof, them being roof shingle algae, shingle bleed-through, and rust stains. Luckily, there are some solutions that don’t require the help of your local roofing company.

 If you need more information, like how to find a roofing company near you, check out this blog.

If your roof has staining or discolorations that you are unsure about, be sure to schedule a FREE inspection with T-10 Construction.