Though many people find moss on a roof attractive, it can actually cause significant damage and should be removed quickly and prevented if possible. Moss grows in moist, shady areas. While it’s beautiful under trees and between rocks in a woodland path, you don’t want it on your roof. Since we experience all seasons of weather here on the Philadelphia Main Line, there are many opportunities for moss to grow.  

Moss can grow on many surfaces, including asphalt shingles, tiles, wood, and cedar shakes. A spongy plant that soaks up large volumes of water, it can harm your roof in multiple ways. First, the retained water adds weight to your roof, potentially causing sagging or excessive pressure on substructures. The retention of moisture against the shingles will also hasten the deterioration of your roof, wearing off the granules of an asphalt roof and working its roots into any kind of shingles, causing them to begin to fall apart and become fragile, brittle, or cracked. 

The constant moisture will eventually make its way into your underlayment, soaking through to the wood sheathing below. This creates a perfect environment for the growth of mold and bacteria, which could make their way into your walls and ceilings. Damage to the shingles and substructures can create leaks, letting in rainwater, as well as rodents and bugs that are attracted to the wet, enclosed environment.

 If moss is not addressed as quickly as possible, it can become a very expensive problem. 

Cleaning off Moss

Power washing or brushing off moss with a broom is not recommended, because it can cause already fragile shingles to shatter, creating serious leaks into the substructure and possibly into your home. 

If your moss growth is fairly recent and not very extensive, and if your roof is fairly new, you may be able to gently clean off the moss using a soft brush or a flexible leaf rake gently pulled across the moss to loosen it. When you have loosened moss, gently rinse your roof with a garden hose, not a power washer whose pressure can be too strong for your roof. Try to avoid rinsing the debris into your gutters, which would cause additional problems. Put a cover over your gutters or rinse debris off the side of your house that is gutterless.

There are chemical products on the market that kill moss. If you use one, make sure that it won’t damage your kind of roof, and keep in mind that runoff can damage your landscaping or local wildlife. Once the moss dies, you may still have a problem with clogged gutters as it is washed off by the rain, so put screens or other guards over your gutters during the period of time in which the chemical is active. 

Removing moss can be dangerous since it makes roofing slippery. If your moss growth is extensive, you may need to engage a professional, both for safety purposes and for the expertise that a professional can provide. Professionals have the right equipment to protect themselves and your roof while removing moss. 

If the damage to your roof is extensive, it may be best to replace the roof. Your roof expert can evaluate if it is possible to save the roof or if it would be better for your home value if it were replaced. After all, many new home-buyers consider a new roof an asset and are wary of a house with an existing moss problem. 

Preventing Moss

Many new roofing materials come pre-treated with a moss-resistant chemical. Metal flashings also have an inhibitory effect on the growth of moss on the roof areas that are below them, as rainwater washes the chemicals in the metal onto the roof below. Installing metal strips, especially zinc strips, on the ridge of an existing roof may help deter the growth of moss and lichen. 

If possible, cut back tree limbs that are shading the roof. This will decrease the likelihood of moss growth and may also protect your home from damage, in the event that limbs break off in a storm and land on your roof.

As long as any part of your roof is moist and shaded, you are likely to eventually deal with moss. Keep ever vigilant and remove it as soon as it begins to grow. Leave the moss on the ground where it belongs. If you have a problem with moss on your roof, call us at (610) 353-2895 for a free consultation.