Buying a house with moss on the roof: cost and risk

Is moss on the roof a problem when you're buying a house?

When you're buying a house with moss on the roof, it's usually a cleaning job, not a roof replacement. The home inspectors' trade group, the International Association of Certified Home Inspectors (InterNACHI), calls moss "mostly a cosmetic issue." Ask for a professional cleaning, priced in a roofer's written bid. Have the same roofer check whether the shingle edges under the moss have lifted, because that is what turns a cleaning into a repair.

Key takeaways

• Never let anyone pressure wash the roof. The shingle makers' trade group says it causes granule loss and very likely early roof failure.
• Removal runs about $560 to $700 on a 2,000-square-foot roof at a cost calculator's basic rate; Seattle-area roofers quote $400 to $1,200 for the same size. Steep pitch and thick moss cost more.
• One insurer says treating moss is the homeowner's job, not the insurance company's.

How much does roof moss removal cost?

Most of the spread below is scope: roof size, pitch, and how thick the moss has grown.

Kept prices every line of an inspection report for $79, usually under 4 hours, and marks the ones that need a professional's quote.

Here is how that works on a 2,000-square-foot roof. At Homewyse's starting rate, 2,000 × $0.28 is $560 and 2,000 × $0.35 is $700. Near Seattle, Johnny Tsunami prices the same roof size at $400 to $1,200. A Western Washington roofer, The Roofing Titan, says a steep roof or tight access can add 30 to 50% to the labor.

None of these figures covers repairing lifted shingles or a damp deck. Only the roofer's bid prices that.

Is moss bad for a roof?

It can be, slowly. InterNACHI says moss "deteriorates shingles by holding moisture against them, but this is a slow process." It also notes there is disagreement in the roofing industry about whether growth like this is destructive or cosmetic.

The shingle makers take the harder line. The Asphalt Roofing Manufacturers Association (ARMA) says moss "can be detrimental to asphalt shingle performance." In its words, moss "can cause the leading edges of the shingles to lift or curl, which increases the risk of shingle blow-off during wind events." In severe cases, ARMA says, moss build-up "can cause lateral water movement resulting in moisture damage to the roof deck or may even cause leaks."

Moss also tells you something about the house. InterNACHI ties almost all growth on shingles to long-term excess moisture. ARMA says moss favors north-facing slopes that stay damp and roofs under overhanging branches. If the report also notes a ceiling stain or wet decking, read roof leak or ceiling stain on an inspection report first.

Don't climb up to look for yourself. InterNACHI says moss is a safety issue for anyone walking the roof.

Who should remove the moss, and how?

A roofer or a roof-cleaning contractor. ARMA recommends a 50:50 mix of liquid chlorine bleach and water, sprayed on, then rinsed with low-pressure water. The moss loosens over time and can come off with a leaf blower. Severe cases may need more than one treatment.

ARMA warns against one method: "Never use a pressure washer to clean an asphalt shingle roof as this will cause granule loss and very likely premature failure of the roof system." It also does not recommend adding zinc or copper strips to an existing roof, since they need exposed nails or a broken sealant bond.

Ask the roofer for a written bid that states three things:

• The cleaning method, and that it uses low pressure.
• Whether shingle edges have lifted or curled under the moss.
• Whether the roof deck shows moisture damage.

To get that bid inside your window, see how to get repair estimates during the inspection period.

What should I ask the seller for?

Ask for a professional cleaning before closing, or a credit equal to the roofer's written bid. Name the scope and the trade. For example: clean moss from the north slope with a low-pressure bleach treatment, no pressure washing, by a roofing contractor, per the attached bid. If the roofer finds lifted shingles, add that repair as its own line with its own figure.

If the seller does the work, ask for the contractor's invoice showing the method used. What to ask for after a home inspection shows how this line fits with the rest of your list. The buyer repair request letter template has the wording around it.

What if the seller says no?

You can pay a roofer to clean it after closing, ask for a smaller credit, or cancel if your inspection contingency is still open. A cleaning is a small, known cost. Lifted shingles or a wet deck are a different decision, so weigh them with the other roof findings on your report. See when to walk away after a home inspection for how to decide. Your contract sets the exact dates and terms, so confirm them with your agent or attorney.

Is moss on a roof an insurance issue?

It can be. Liberty Mutual lists lack of maintenance among the reasons roof damage may not be covered. It says routine upkeep, "and treating moss, is the responsibility of every homeowner," not the insurance company. It also says mold and rot from long-term moisture are usually excluded unless a sudden, covered event caused them. Ask the agent writing your policy how they treat the moss line on your report.

How do I know what the rest of my report is worth?

Your report has more lines than the roof. Upload your inspection report to Kept to put numbers on the rest while the roofer prices the moss.

Sources

• InterNACHI, Mastering Roof Inspections: Asphalt Composition Shingles, Part 42
• InterNACHI, Mastering Roof Inspections: Asphalt Composition Shingles, Part 43
• Asphalt Roofing Manufacturers Association, Algae and Moss Prevention and Cleaning
• Homewyse, Cost to Remove Roof Moss
• The Roofing Titan, Roof Cleaning and Moss Treatment Cost in Washington
• Johnny Tsunami, 6 Factors That Influence Roof Moss Removal Costs
• Liberty Mutual, Roofs and Home Insurance