Roof Leaks, the “invisible”kind

What’s a “roof leak”?

For most people, a roof is leaking when the ceiling gyproc starts to discolour, or bulge, or when the paint starts to bubble.


This usually happens after a heavy rain, or at the end of winter when the snow is melting off the roof.  Yes, those are usually leaks.  They are easy to notice and impossible to ignore.  However, the sad thing is that the cause of the leak is usually several years in the making, and by the time the leak is noticed, there is lots more damage that has occured.  In other words, the apparent and visible leak is preceded by “invisible” leaks that start (often) many years before the leak is apparent.  This post is about these invisible leaks.

Let’s define our terms.  A roof leak is the entry of water past the roof covering, waterproofing, and underlayments into the roof decking, and into the attic interior.  It is NOT condensation, and it is NOT related to gutter overflow.  It CAN be a wall or siding leak that becomes a roof leak if the siding or wall ends on a roof line.  The key point is that the roof waterproofing is no longer working at keeping the water away from the roof structure and the interior.

The usual suspects

Just as in the movie Casablanca, where the police chief instructed his assistant to “round up the usual suspects”, we can first focus our attention on the usual places where roof leaks often start.  These are:

  • Caulking around roof penentrations (air vents, plumbing vents, skylights, chimneys, etc.)
  • Flashings around skylights, chimneys, roof joints.

Deteriorated caulking around plumbing vent


Sidewall flashing detached from siding

These can be checked by external inspection, and by conducting a water test on the suspected point of water entry.  The latter can be done by two people, one in the attic at the suspected entry point, and another person on the outside, watering the suspected area with water from a garden hose.  NOTE:  any work on roofs can be dangerous, so anyone doing this should use fall protection, secure footing for ladders, and avoid contact with electrical wires.  For those inside attics, a respirator mask is a good idea to protect against the dust, and insulation fibers that are present.  If there is mold, then respirator masks with filters that can screen out mold spores would be a very good idea.

The hidden failure points

Once we eliminate the usual suspects, we can focus on the hidden points of failure – those connected to the waterproofing layer underneath the roof covering.  What’s this, you ask?  If you’ve read my post on roofing myths, you’ll already know that the roof covering is NOT waterproof, and that true protection against water infiltration rests with the waterproofing BELOW the roof covering.  That is to say, the membranes, underlayments, flashings, and sealants that form the true protection.

It is unfortunately not uncommon to find that under the roof covering there is NO underlayment of any kind.  Once water blows past the shingle joints, it’s on the roof decking.  Next, is the “reuse” of old underlayment which is full of holes from the previous roof.  Yup.  The new nails holding the new roof will magically find the exact same nail holes used by the previous roof.  Then, there is underlayment which is torn, or cut, or laid down with gaps which means it cannot do its job of keeping the water away from the decking if there is water on the underlayment surface.


Leakage caused by ice dam and inappropriate use of underlayment (#15 felt instead of at least one width (3′) of peel and stick ice-and-water shield membrane.

Another common failure, is not using the waterproofing membranes at the right places, ie, in places where water can be expected to form under pressure, such as along the eaves (due to ice dams – see image above), or along valleys, or along endwalls and sidewalls.

When the waterproofing layer of the roof is not well installed, or not present, any water that gets past the shingles is now on the roof surface.  It is quite common for us to find a roof that looks like new on the outside, and leaking on the inside.  How do we know?  By examining the decking from inside the attic.  There are signs that tell us that the water has been entering the roof system for many years, signs such as rust stains around the nails that hold the shingles onto the roof.  The rustier the nails and the larger the rust stain, the longer the leak has been happening.

Leakage into decking_4558

Underside of plywood decking, showing rust stains from water penetration through nail holes holding shingles to roof.  Note rusted H-clip towards lower center.  Persistent moisture is allowing mold to grow on the wood.

There are other signs as well.  It is not uncommon to find that the leaks have allowed the water to run down the roof sheathing to the joints, and the connecting H-clips are often quite rusty or corroded.

So if there’s a leak, why can’t I see it inside?

For that, we can usually thank the vapour barrier plastic that’s above the gyproc.  Water dripping down from the roof will enter the insulation and work its way down to the vapour barrier, where it pools.  If the vapour barrier is well-installed and properly sealed, then the water stays up there, contributing to a moist environment.  Over time the ventilation in the attic will gradually dry it out, until the next episode of heavy rain or ice damming.  However, during that time it contributes to wood rot and mold formation.  The wood rot often attracts carpenter ants as rotting wood is their food source.

Water on vapour barrier_0916

For those homeowners who have OSB (oriented strand board – a form of presswood) with glue rated to Exposure Grade 1 as their roof decking, this persistent moisture also presents the problem of damage to the glue holding the wood chips together.  This is often revealed by the wood chips flaking off the interior of the decking.  If the leaking is localized, then the board in the location of the leak is usually compromised as the glue is dissolved away leaving a rather weak collection of wood chips.

In short, it is not enough to look at the roof from the outside to determine if it has leakage – that has to be determined by examining the decking from inside the attic.  This inspection should reveal if the waterproofing layer is still working, and if it is not, the extent of damage that has been caused.

Consequences of invisible roof leaks

These are some of the consequences of invisible roof leaks that we have encountered and had to repair:

  • Mold and mildew
  • decking damage
  • rafter and joist damage
  • wall damage
  • compromised insulation
  • compromised and weakened decking (leading to breakage with snow loading)
  • carpenter ants
  • various entry points for insects
  • easier entry points for animals like squirrels and raccoons

A roof system that is compromised becomes a serious liability to the health of the inhabitants of the home, and to the decrease in resale value when the property is put up for sale.  It’s clearly a case of doing it right up front, or paying for the consequences down the road.

Next time you have someone give you a quote for a roof, ask yourself whether they took the time to find out the true state of your roof system, and if they put into their quote the appropriate level of waterproofing, insulation and ventilation for your roof system to work as it should.

(c) 2016 Paul Grizenko


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