Stop moss growth from dam­ag­ing shin­gles

Winnipeg Free Press - Section G - - HOMES - ARI MARANTZ

QUES­TION: Ten years ago we re­placed the roof with 25-year shin­gles and I have now dis­cov­ered moss growth on the shady side of this roof. What can we do about clean­ing it up and what are some pre­ven­tive mea­sures to keep it from com­ing back? I ap­pre­ci­ate your ad­vice. — Jack van Dam

AN­SWER: While this prob­lem may be a nui­sance in our area, my cur­rent view of this is­sue from a win­dow on the up­per floor of a build­ing over­look­ing houses in St. John’s Nfld., puts it into a unique per­spec­tive. Many build­ings here have this prob­lem and my ex­pla­na­tion of the cause and sug­ges­tions for re­moval will bring this into fo­cus.

Moss on roofs is caused pri­mar­ily by ex­ces­sive mois­ture. This may be due to the lack of proper slope on the roof or other fac­tors. Low-slope and flat roofs are par­tic­u­larly prone to this mal­ady. If a roof has dif­fi­culty drain­ing af­ter heavy rains or snowmelt, it is much more sus­cep­ti­ble to moss growth.

In many of those cases, re­me­di­a­tion to pre­vent re­oc­cur­rence of moss af­ter re­moval may be dif­fi­cult. With­out chang­ing the slope of the roof or rais­ing low ar­eas to pro­vide ad­e­quate drainage, the moss is likely to re­turn af­ter clean­ing. On built-up roof­ing sys­tems with rock cov­ers, good main­te­nance of the sur­face may pre­vent this from hap­pen­ing, but on shin­gled roofs like yours, main­te­nance is dif­fer­ent.

To get rid of the moss on your roof, which is cov­ered with as­phalt shin­gles, clean­ing the roof sur­face with wa­ter and a brush or broom may be all that is re­quired to re­move the growth. A hose with a spray noz­zle will of­ten do the trick, but be sure that the wa­ter is sprayed from the top of the roof down to en­sure no shin­gles are lifted or dam­aged. This method will also en­sure no wa­ter is forced un­der loose or curled shin­gles, which can cause a leak.

Depend­ing on the thick­ness of the moss cov­er­ing, a broom or brush may also be re­quired to de­tach stub­born clumps of the green veg­e­ta­tion from the as­phalt sur­face. Care must be taken to pre­vent re­mov­ing too many sur­face gran­ules from the shin­gles along with the moss.

You have touched upon one of the other root causes of moss growth in your ques­tion. As you have stated, the moss may only grow on the “shady” side of the roof. This is a typ­i­cal sit­u­a­tion and il­lus­trates my point about roofs that re­main wet be­ing the only ones on which this green stuff will grow.

One thing you may be able to rem­edy in that sit­u­a­tion is to reg­u­larly trim trees that may over­hang one side of the roof or an­other. This is more of a con­cern on the north and east side of roofs, as they re­ceive less sun­shine or sun when it is not at its peak strength. Even on the south and west sides, moss can still flour­ish if large tree branches con­stantly block the roof from the sun. Reg­u­lar trim­ming of branches in this sit­u­a­tion is crit­i­cal to pre­vent re-growth af­ter clean­ing.

An­other ben­e­fit of good tree man­age­ment is less de­bris will sit on the roof if over­hang­ing limbs are elim­i­nated. Moss can grow on many wet sur­faces, but of­ten needs a medium to pro­vide a sub­strate. Leaves, twigs, sap and seeds from over­hang­ing tree branches of­fer an ex­cel­lent source of this ma­te­rial. You may no­tice even roofs with a low slope and a fair amount of shade re­main free of this nui­sance, as long as they are reg­u­larly cleared of de­bris. Most of this ma­te­rial will land on your roof from veg­e­ta­tion that is di­rectly ad­ja­cent to, or over top of, the sur­face.

The fre­quency of clean­ing de­bris and moss from your roof will de­pend on the sever­ity of the growth. A small amount of moss at the bot­tom of the in­di­vid­ual shin­gle tabs af­ter a decade of ser­vice may not be a big is­sue. Un­for­tu­nately, thicker coat­ings of the fuzzy green stuff may cause pre­ma­ture de­te­ri­o­ra­tion of the shin­gles as they hold ex­ces­sive mois­ture on the shin­gle sur­face.

If the shin­gles re­main con­stantly wet, the mois­ture will even­tu­ally wear away at the gran­u­lar sur­face and the mat be­neath will quickly erode. For this rea­son, re­moval of the moss and sur­face de­bris is crit­i­cal to max­i­mize the life of the shin­gles and get close to the 25-year mark be­fore re­place­ment is re­quired.

Left unchecked, moss can trap mois­ture on shin­gles which erodes the gran­u­lar sur­face and causes the

sub­strate to de­grade.

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