5 Most Common Causes of Roof Leaks On Sloped and Flat Roofs

January 9, 2023 kellymacdonald

Most Common Leak Causes


Roof leaks can occur for a variety of reasons. In some cases, there may even be multiple sources
contributing to an interior leak. Over the years and through thousands of thorough investigations these
have been the top culprits for leak causes on both sloped and flat roof surfaces.


Sloped Shingle Roofs

Missing Shingles1) Missing or Damaged Shingles

One of the most common leak issues we encounter with sloped roof surfaces is missing or damaged
shingles, regardless of the exact type of shingle. Even relatively new roofs can be impacted when severe
weather events occur or if the shingles were not installed as per manufacturer’s guidelines.


It is incredibly rare that we encounter homes that have a waterproofing underlayment over the plywood
roof deck and beneath the shingles throughout the entire roof surface, meaning that when the shingles
come off or become damaged the plywood is exposed to the elements. Ontario building code only
requires underlayment to be installed in minimal areas, such as three feet from the eaves. If rain or
snowfall occurs while the area is left exposed, the home is left highly susceptible to potential interior
water infiltration.


Monitoring roof surfaces for missing or damaged shingles and having localized repairs completed as
soon as defects are observed will present the best chance to avoid major interior damages from leaks.


Attic Condensation2) Attic Condensation

Oftentimes, when investigating an interior leak, the root cause isn’t from defects on the roof surface but
rather from excessive condensation in the attic. Lack of adequate airflow in the attic can lead to
condensation so severe that active water ingress occurs in the interior of the home.


Not only can attic condensation lead to costly interior damages but also presents the risk of dangerous
mold growth in the attic space.


The issue we see most often that leads to the heaviest condensation is when interior ducting has
become detached or wasn’t installed correctly originally. In these cases, hot air from interior spaces like
kitchen or bathroom fans is venting directly into the attic, rather than out of a roof vent.


Having a professional attic inspection completed if there are suspicions of condensation or if ice
damming is occurring can help to identify any ventilation issues and determine required action for


Exposed Nail Heads3) Exposed Roofing Nails

At quick glance, a roof surface may appear to be in great condition, with beautiful, new shingles.
However, when taking a closer look, if there is a presence of exposed roofing nails throughout the roof’s
surface, leaks may be imminent.


When installing shingles, the tabs are meant to overlap, covering the nails used to secure the shingles to
the roof deck. When these nails are left visible, an exposure is present where water can eventually
enter. This issue can be made even worse if the nail is installed on an angle or if it begins to rust and
corrode overtime.


Accessing the roof surface to inspect work completed won’t always be feasible. When having
replacements or localized repairs completed, ensure you are always receiving a photo report outlining
all completed work and allowing you to keep an eye out for potential leak issues.


Missing Sealant4) Missing or Deteriorated Sealant

Sealants like caulking act as a first line of defense against the elements, sealing transitions and
exposures that may lead to water infiltration. If that caulking has become deteriorated to the point of
splitting and cracking, or if caulking was never applied in the first place, leaks may occur.


On sloped roofs, the areas we regularly see missing or deteriorated caulking that leads to water
infiltration is along flashing seams or plumbing gaskets.


Completing regular inspections and maintenance of the roof surfaces can help to identify areas where
caulking application or renewal may be required to mitigate the presence of interior leaks.


5) Plumbing and Vent Flashing

Plumbing and vent pipes going through the roof’s surface have protective flashing installed at the roof
opening to prevent water infiltration. If the flashing is not adequately roofed-in with shingles, if the
flashing begins to lift or if damages occur to the flashing, leaks may occur.


During regular inspections and maintenance, vent flashing should be checked to help identify if any
issues are present that may lead to water ingress if left unattended.


Flat Roofs

Membrane Deterioration1) Membrane Defects

Regardless of the type of roof system installed, membrane defects are the number one cause of leaks
we encounter on flat roofs. The types of membrane defects can depend on many factors.


With tar and gravel roof surfaces, the membrane defects are most commonly attributed to age. When a
roof system is reaching the end of its serviceable life, damages like splits and cracks in the membrane
can occur more regularly. Rapid and severe weather fluctuations, which Ontario often faces for six
months of the year, can exacerbate these types of defects.


On single ply membrane systems, such as TPO, EPDM or PVC, small punctures in the membrane are the
most common type of defect encountered, with a variety of causes. Lifting membrane seams at corners
or transitions is also a frequent sighting, which may be the result of an aging membrane or improper


Membrane Damage


Membrane defects are also commonly discovered on liquid rubber member systems, commonly applied
beneath an inverted roof system which features insulation, filter cloth and gravel ballast or paving
stones over the membrane. This type of system can make tracing and identifying particular leak issues
difficult at times, since the various layers of the roof system need to be removed to expose and inspect
the membrane. Defects encountered can range from large areas of deteriorated to small pinholes, both
of which can lead to major interior leaks.


Improper Installation - Hardware too Big2) Improper Installation

Regularly, we conduct investigations on roof systems or localized repairs that were very recently
completed. If the work is brand new how could a leak be occurring? Unfortunately, there are too many
contractors out there not following precise steps or using the right materials for the specific job.


Following installation guidelines outlined by the particular manufacturer, using materials that are
compatible with each other and are the correct type and size, and ensuring installation surfaces are
sufficiently prepared are just some of the steps required to ensure a repair is completed properly.


When steps are missed and corners are cut, the integrity of the repair is compromised and can lead to
defects that result in leaks immediately after the work is completed.


The improper installation we see most often is with 2-ply SBS membrane systems. Whether the
membrane is torch or cold applied, if the proper steps are not followed the membrane may not adhere
properly, leading to open and lifting seams. The stripping or flashing membrane installed along building
upturns is vitally important, however, we have encountered many situations where this membrane is
not installed at all or is not to manufacturer guidelines, leaving exposures at the upturns.


Selecting the right size of flashing for areas like vents, gas lines and plumbing pipes it crucial, as flashing
which is oversized will leave gaps that can lead to leaks.


Again, ensuring detailed reports are received when repairs or new installations are being conducted are
extremely helpful in reviewing the work completed and ensuring work was done properly.


Vent and Pipe Flashing3) Vent and Pipe Flashing

As with sloped roofs, vents and pipe protruding through flat roof surfaces have flashing installed at the
base to protect the opening in the roof. If the vent flashing becomes damaged or deteriorated, the area
may be susceptible to interior leaks.


Commonly, we encounter instances where the vent flashing corrodes, leading to holes where water can


4) Drainage

Both flat drains and overflow scupper drains are the cause of many leaks we encounter. Issues can arise
when the drain inserts become corroded or if the seal of the membrane begins to deteriorate. In these
instances, exposures become present where water is able to infiltrate.


With any type of drain, it is critically important that they are checked regularly to ensure they are clear
of blockages that may lead to roof flooding. Over the years we have seen hundreds of cases, in both
Ontario and British Columbia, where blocked drainage leads to disastrous floods and subsequent leaks.
During the winter months when heavy snow and ice accumulation followed by subsequent melt occur
has often led to extremely high volumes of leaks.


Having the drains checked at roof level can also help to identify the first signs of deteriorating
membrane which can assist with leak prevention.


5) Sealant

Just like with sloped roofs, areas where sealants like caulking are missing or have become severely
deteriorated can be impacted by devastating leaks. Generally, flat roofs have many more areas where
caulking may be required, compared to a sloped shingle roof.


Metal cap and wall flashing, vent and unit seams, wall penetrations, and other areas can all be
monitored during routine maintenance and inspections to determine if caulking renewal is required.



Regardless of why a leak is occurring, having a qualified roofer investigate and address the issue as soon
as the first signs are observed is always recommended. Prolonging the investigation can only increase
the severity of interior damages and risks potential soaring costs for remediation. Detail Roofing is on-
call 24/7/365 to assist with any leak issues you are experiencing. Contact us to inquire or schedule a
service crew.

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