Many po­ten­tial causes for stained shin­gles

Winnipeg Free Press - Section G - - FRONT PAGE - ARI MaRaNTZ

QI was won­der­ing if you can tell me what causes roofs to be­come dis­coloured. A black, wavy line ap­pears on the shin­gles, par­tic­u­larly on the lower rows. I think it has some­thing to do with how the roof breathes. — Lyle Ri­ley AN­SWER: De­te­ri­o­ra­tion to bi­tu­men-based shin­gle roof­ing is a nor­mal process that may dis­play it­self in var­i­ous ways. There are many fac­tors that will con­trib­ute to the way shin­gles look over time. Some are se­ri­ous, while oth­ers are sim­ply a cos­metic is­sue. Be­cause of the com­po­nents of bi­tu­men-based shin­gles, de­grees of de­te­ri­o­ra­tion can nor­mally be seen at var­i­ous stages in their life cy­cle. Most older shin­gles had a pa­per base (of­ten re­ferred to as felt) that was sat­u­rated with tar-like bi­tu­men and topped with coloured gran­ules. Newer ones usu­ally have a fi­bre­glass base that is sup­posed to be stronger and longer last­ing than felt. Oth­er­wise the over­all com­po­si­tion is sim­i­lar. The gran­ules help re­flect ul­tra­vi­o­let (UV) rays, ex­tend­ing the life of the shin­gles. The tops of the shin­gles are not gran­u­lated, to al­low for easy bond­ing to the next row of shin­gles with a small ad­he­sive strip, ac­ti­vated by heat from the sun. Aging of shin­gles can be at­trib­uted to de­te­ri­o­ra­tion in one or more of th­ese com­po­nents. On older roofs, shin­gles are seen to begin aging when gran­u­lar loss is ob­served. This may be ob­served by ex­ces­sive de­bris in the eave­stroughs, dull­ness in the over­all ap­pear­ance of the roof­ing, shrink­ing of the in­di­vid­ual shin­gles or curl­ing at the bot­tom edges. As the bi­tu­men erodes af­ter years of ex­po­sure to UV light, rain, snow and other en­vi­ron­men­tal fac­tors, the un­der­ly­ing felt or fi­bre­glass mats ap­pear to shrink. The keys at the bot­tom of three-tab shin­gles get wider and of­ten curl up­ward, ex­pos­ing more of the un­der­ly­ing layer be­low. The dis­coloura­tion of your shin­gles may be be­cause of this gran­u­lar loss, an un­der­ly­ing de­fect in the shin­gle or en­vi­ron­men­tal con­di­tions. As shin­gles lose their gran­ules and begin to de­te­ri­o­rate, some changes in ap­pear­ance may oc­cur. Dark­coloured shin­gles may fade, and light-coloured ones may darken with dirt or de­bris, with more of the dark­coloured bi­tu­men ex­posed. Nei­ther of th­ese is­sues are par­tic­u­larly prob­lem­atic, and if that is the cause of the dark ar­eas on your roof, there is noth­ing to be con­cerned about. Also, de­bris from nearby veg­e­ta­tion can cause var­i­ous ar­eas to be­come dis­coloured. If you have lots of trees near the roof or over­hang­ing, sap drip­ping from the trees will of­ten cause stains on the roof­ing. This can be min­i­mized by wash­ing the roof with a gar­den hose or with a pres­sure washer on a low set­ting, but care must be taken not to dam­age the shin­gles when clean­ing. The roof should al­ways be washed from the top down to pre­vent driv­ing mois­ture un­der the shin­gles, which may cause dam­age and leak­age. You are cor­rect that a lack of proper at­tic ven­ti­la­tion can cause pre­ma­ture de­te­ri­o­ra­tion to shin­gles, but this is nor­mally seen by curl­ing, lift­ing or shrink­age. Faint changes in colour or vis­i­ble cos­metic de­fects are not re­ally a symp­tom of that de­fect. While you should look in your at­tic and on your roof to en­sure the vents are ad­e­quate and not blocked by de­bris or ex­ces­sive in­su­la­tion, I doubt this is your is­sue. Pe­ri­odic in­spec­tion of th­ese ar­eas is al­ways a good idea, but will likely not pro­vide much in­sight into your prob­lem. The next thing to con­sider is en­vi­ron­men­tal con­di­tions in the area where your home is lo­cated. Are there any un­usual industrial sources, such as power plants, fac­to­ries or re­finer­ies nearby? Pol­lu­tants emit­ted from th­ese sources could cause dis­coloura­tion of your roof from chem­i­cals in the air. Var­i­ous forms of air­borne pol­lu­tants can cause roof­ing or other house com­po­nents to dis­colour of de­te­ri­o­rate. Do you have a wood-burning ap­pli­ance in your home or nearby? Soot par­ti­cles from fire­places and wood stoves will of­ten cause dark stain­ing on roof­ing, par­tic­u­larly light­coloured shin­gles. Again, this may be mostly cos­metic in na­ture and not some­thing that needs any ma­jor re­me­di­a­tion. Fi­nally, a de­fect in the man­u­fac­ture of the roof­ing is the other pos­si­bil­ity. Var­i­ous man­u­fac­tur­ers over the years have pro­duced shin­gles with var­i­ous de­fi­cien­cies. Th­ese may range from de-lam­i­na­tion of lam­i­nated shin­gles, ex­ces­sive gran­u­lar loss, curl­ing, crack­ing and other forms of pre­ma­ture wear. While it is dif­fi­cult to ac­cu­rately as­sess whether this is oc­cur­ring, a slight dis­coloura­tion on the shin­gles would only be a mi­nor con­cern. If you are see­ing ex­ces­sive curl­ing or shrink­ing along with the dark spots, more se­ri­ous eval­u­a­tion may be needed. While se­ri­ous pre­ma­ture wear and de­te­ri­o­ra­tion to bi­tu­men-based shin­gles is some­thing to be in­ves­ti­gated and fixed, slight dis­coloura­tion may be noth­ing more than a cos­metic is­sue. Dif­fer­en­ti­at­ing be­tween the two will be im­por­tant to de­ter­mine whether any re­me­di­a­tion is needed, and I doubt your is­sue will be lit­tle more than a vis­ual one. Ari Marantz is the owner of Trained Eye Home In­spec­tion Ltd. and the past pres­i­dent of the Canadian As­so­ci­a­tion of Home and Prop­erty In­spec­tors — Man­i­toba ( He can be reached at trained­eye@in­ or 204-291-5358. Check out his web­site at


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