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Why So Much Leaking?


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For a roof that looks acceptably installed when walking on it, why would it be so severely leaking when viewed from the attic?

Not a trick question; I've come across three roofs in the last 2 weeks that certainly took me by surprise.

All three roofs had quite a bit of moss growth. Any connection?

Other ideas?

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For a roof that looks acceptably installed when walking on it, why would it be so severely leaking when viewed from the attic?

Not a trick question; I've come across three roofs in the last 2 weeks that certainly took me by surprise.

All three roofs had quite a bit of moss growth. Any connection?

Other ideas?

Hmmm, was it raining during the inspections? Perhaps those new shoes you bought 2 weeks ago, the ones with the long sharp spikes, aren't the best for walking roofs?

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Hi Randy,

If there's a lot of moss on the leaking areas the moss might be forming a dam and preventing the water from draining off the roof and some of it's backing up under the shingles. Underlayment only remains weather resistant if it's not constantly wet and can dry out between rains. If you have a situation where the moss was causing water to back up under the shingles and it has been going on long enough the underlayment would be saturated through and through and water would pass right through it. How steep was the slope on those roofs?

I'd be more inclined to think it was condensation. If there's a lot of moss on those covers the moss has to either be on the north or east sides or the houses have a lot of high trees around them that prevent sunlight from reaching the cover, killing the moss and drying out the roof. I often find roofs with a lot of moss on the north plane where the underside is dripping with condensation but the south plane will be moss free and the underside nice and dry. That's because the UV kills the spore that gives moss its start and the heat from the sun can dry out that south roof plane during the day. The north slope doesn't get any sunlight so it stays damp, the spore isn't killed on that side and the spore is fed by the dampness on the surface and eventually the moss takes off in a big way. The underside of those roofs stays cold and condensation is the norm if they don't have a really well ventilated attic.

Sometimes the contrast from one side to the other is stark. With a little investigation I typically find that insulation has been pushed in tight against the bottom of the insulation baffles and has collapsed them so that only a small fraction of the air that's supposed to come through them is actually passing. Then, if there are gable end vents installed in conjunction with the frieze vents, very little air actually moves up the underside of the roof to remove all of that dampness.

Lots of air will get into the attic around recessed light fixtures with open cans, wiring coming up through the top of wall plates, unsealed areas around chimneys and vents, disconnected ceiling fans, etc.. and lots of times I find the that when there is that much wetness the whole house ventilation system isn't even functioning - especially if it's controlled by one of those mechanical 24-hour timers - because the two little springs inside often break. People have those systems set up to vent the home but they don't realize that the timer is broken and that the home isn't being ventilated. Found two like that in the past week - on one of them the underside of the roof was, besides being wet - all covered with black leopard spots of fungi - on another the moisture was feeding what looked like giant peach fuzz on the underside of the deck. In each case there were issues with attic ventilation and with whole house ventilation systems that weren't functioning or weren't being used.

ONE TEAM - ONE FIGHT!!!

Mike

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Good thoughts.

I remember walking on certain areas and noticed--when I thought to look down at my feet while walking--it seemed like water was being squeezed out of the shingle. Sort of like stepping on a water-soaked sponge.

Maybe a dumb question, but can a shingle actually hold water rather than repel it?

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Good thoughts.

I remember walking on certain areas and noticed--when I thought to look down at my feet while walking--it seemed like water was being squeezed out of the shingle. Sort of like stepping on a water-soaked sponge.

Maybe a dumb question, but can a shingle actually hold water rather than repel it?

Not unless the shingle formulation is seriously screwed up, but if they's used the wrong kind of felt - building paper instead of roofing underlayment for instance - the paper will wrinkle up when it gets wet and that might feel like a sponge when walked on.

ONE TEAM - ONE FIGHT!!!

Mike

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Good thoughts.

I remember walking on certain areas and noticed--when I thought to look down at my feet while walking--it seemed like water was being squeezed out of the shingle. Sort of like stepping on a water-soaked sponge.

Maybe a dumb question, but can a shingle actually hold water rather than repel it?

Last week I worked on a leaking skylight for a friend. When the roof covering was replaced 10 years ago, the roofer nailed all step flashing down first then nailed down the shingles on top. To keep it from leaking, he sealed it on all four sides. When the sealant bond broke on the sides, water entered and stopped at the factory adhesive strip on the backside of the first shingle course below the skylight. When I removed this course, water dripped out and most nail heads were nothing but rust.

Like Mike O said, the shingle isn't absorbent - not that I agree with it - but water can get trapped in-between courses because of the adhesive strips. Stepping on it might force some water out.

Marc

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I rarely see any moss growing on a roof in Colorado. Does heavy moss grow root hairs into the space between the shingles? That could account for water squishing out when you walked on it.

Yep, it literally wraps itself around the crushed stone granules. If you pull a chunk off the roof you'll typically find a bunch of granules stuck to the bottom of it and the area where the moss had been, if it's been left too long, will be softer to the touch because moss secretes oxalic acid to digest whatever it is growing on.

ONE TEAM - ONE FIGHT!!!

Mike

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Next you're going to tell us that someone discovered that moss could be used to bleach wood...

Nah, moss will eat wood. It uses the oxalic acid to break down the cell structure into something it can feed off of.

Oxalic acid on the other hand is used to bleach wood without turning it white and it can also be used to age new asphalt shingles so that they'll blend in with an older roof better when you make roof repairs. It's also great for cleaning concrete - but I'm guessing you already knew all that.

ONE TEAM - ONE FIGHT!!!

Mike

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  • 2 months later...

Roof shingles are the perfect place for moss and other algae to grow as this part of your home receives the most rainfall and has higher levels of humidity, making conditions perfect for moss to thrive. If there are large trees near the home that shade parts of the roof, moss can grow in abundance as there is a lack of direct sunlight. The accumulation of leaves and other organic debris can also aid in giving moss the nutrients it needs to grow.

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