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.

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Some myths about roofing

Most people think they have a good grasp of how roofs work – the shingles (usually asphalt) keep the water out, and that they do that by being the waterproof layer that stops water from getting in.  That’s more myth than fact.

Another myth, is that caulking is an acceptable way to achieve waterproofing.

Another myth, is that if you have ice-and-water shield on your roof, you’re good.

What all these myths have in common is that they misunderstand how a sloped roof system works in reality.

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Skylight leaks – no, they don’t have to.

The skylight is a wonderful enhancement for homes, bringing in natural light to dark areas, and increasing the feeling of “openness” in a house.  However, skylights also have a bad reputation due to problems of leaks and condensation.  Fortunately, if the skylight is of good quality and is properly installed, then you should have all the benefits without suffering the usual problems.  This post discusses some of the issues in preventing problems with skylights on steep roofs.

Why do skylights have a bad reputation for leakage?  It’s probably due to the fact that the roof and the skylight were installed by different people, and the demarcation line between the two is rarely clearly indicated.  That means that one or the other probably didn’t take all the possibilities into account when doing the installation and waterproofing.

How a skylight fits in a roof.

How a skylight fits in a roof.

For a skylight to work effectively, without causing condensation or leakage, the light well must be sealed with a vapour barrier, and insulated at least to the same level as the ceiling.  Then, the dew point will be located in the insulation, and will not contribute to condensation moisture.

The pink area shows the presence of the interior air which is warm and humid.  The cyan area represents the cold attic (and wall cavity) air.

The key elements in keeping the skylight (#3) from causing condensation problems are shown below:

1. Vapour barrier which prevents moisture from internal air from leaking into the cold  spaces.

2. Insulation appropriate to the climate zone, which is of sufficient thickness and effectiveness to keep the dew point inside the insulation.

3. The skylight itself should be at least double-paned, with a sealed interior space to prevent condensation within the skylight.

4. Outside curb wood

5. Insulation between inside and outside curb structure.

6. Inside curb wood.

If your skylight well is properly insulated and has a good vapour barrier, then condensation should not form inside the well itself.

Once the skylight is mounted on the roof, it has to be well sealed.  An example of appropriate installation methods is shown at the Velux web-site (here:  The waterproofing consists of a good application of membrane (Grace I&W Shield or equivalent) around the entire curb box of the skylight, with field-formed flashings sealing and protecting the membrane.  Then the skylight flashing kit is usually used on the outside to complete the installation.

If a skylight is installed after the roof is already in place, then ensuring that enough waterproofing is installed becomes a key concern.  As mentioned in other articles and blog posts, membrane has to be installed in a correct manner for it to work.  That includes priming non-wood places and/or where the membrane may not bond directly to wood.  There must be proper overlap between membrane layers, and the membrane needs to be fully adhered to ensure water-tightness.  Once the membrane is in place, custom flashings are usually field-formed to envelop the skylight curb and to protect the membrane. Then the skylight trim flashings get installed over this assembly.

Therefore, one common cause of skylight leakage is the failure of the flashings and/or waterproofing around the skylight curb.   Another is the use of the incorrect flashing for the roof type you may have.

Skylights sometimes also fail because the seals that are installed at the factory, become dislodged, dry out, or shrink, causing a place where water can enter.  If this is the case, it is usually most prudent to replace the skylight unit.

Since skylights form a rather large obstruction to water/snow/ice flow on a roof, it also happens that a local ice dam forms at the top of the skylight, and if the waterproofing was not run far enough both up and laterally, you can get leaks.

If you have a “leaking” skylight, the first thing to investigate is whether the leakage is condensation.  Once that has been ruled out, then the seals around the skylight glass need to be checked.  After that, it is the exterior waterproofing that is looked at.  However, the latter is possible only when the roof is dry and not covered by snow or ice, as the flashing covers of the skylight have to be removed to see the exact condition of the waterproofing and the flashings.  If you’re not sure what’s going on, give us a call.

(c) 2013 Paul Grizenko

Waterproofing membranes for roofs – Miracle product or junk?

The last time you had your roof done, you wanted to be sure that the leaks you had before would never happen again, and you made sure that your roofer installed waterproofing membrane to protect your roof.  Now it’s winter, and you’re having some leakage issues.  WHY?

Let’s start by examining what are waterproofing membranes, and how are they should be used.

There are a number of products that are marketed as waterproofing membranes for sloped roofs.  There are different grades, performances and price-points.  For the membranes to work properly, they need to be properly installed, under the right conditions, at the right place(s) and for the right purpose.  As is true for all building materials, they have both strengths and weaknesses, which must be taken into account by the installers.  From their method of application, they are also known as “peel-and-stick” membranes.

In general the membranes work by bonding (melting into) the wood decking, and thereby providing a waterproofing bitumen layer which keeps the water out.  The membranes bond well to each other, and to wood (provided it is dry and warm enough).  When pierced by a fastener such as a nail, the material acts as a gasket around the nail and resists water penetration.  Therefore, if the membrane is applied to the right place, in the correct way, at the right temperature, and to the right materials, it works.

Membranes, however, sometimes fail.  It is useful to know the different ways failure can happen.

  • The membrane was not installed over dry, solid wood.  This meant that the membrane could not bond to the wood, and thereby establish a waterproofing layer.
  • The membrane was installed over non-wood materials.  This is a common failure, found when the installer did not properly prime the non-wood surface to ensure both adhesion of the membrane and the creation of a waterproof seal.
  • The membrane was not overlapped sufficiently.  Each membrane has a minimum overlap requirement, and the installers MUST ensure that the bond between successive layers is active.  This kind of failure also happens if the membrane was laid vertically when it should have been laid horizontally.  The vertical joints are more susceptible to leakage.
  • The membrane was cut flush with the edge of the roof.  This is a common installation failure, which allows the water that may be on the surface of the membrane to enter the decking at the edge of the roof.  Proper installation practice is to run the membrane past the roof edges by at least 1-2 inches to ensure the water stays outside the roof system.
  • The membrane was installed over damaged wood or on a joint.  Fasteners piercing the membrane at those points will not have the solid wood support that ensures the dimensional stability of the puncture point, and therefore, water will enter the roof system through the nail holes in those locations.
  • The membrane was installed over a non-ventilated space.  This may become a very serious issue if the space is NOT sealed, and may contain moisture or water vapour.  This is known as “trapping” the moisture, and will lead to both wood rot and mold (potentially toxic) in the spaces.
  • The membrane cracked or torn, letting in water.  This can happen if the membrane was left exposed to the sun for longer than the manufacturer recommended, or if there was movement in the structure that put tension on the membrane.  This can also happen more easily with cheaper membranes.
  • The wrong kind of primer was used, leading the materials to “melt” the membrane, instead of providing a suitable attachment point.

There are, of course, many more ways the membranes can fail to work, but as can be seen above, if a membrane doesn’t work, it’s almost always because the installer failed to do the installation properly.  Proper application of the membrane takes time, and ensuring that the overlaps are all properly sealed, and that the junctions to various roof penetrations are properly primed and joined, is detail work that cannot be rushed.  As well, as in painting, if the proper preparation is not done, then the adhesion of the waterproofing layer is going to be compromised.

In our experience, the failure of the waterproofing layers is a very common reason for replacing a roof prematurely.  Unfortunately, once the roof covering is on, it’s impossible to verify whether this critical element was properly installed.  In practice, if the cause of a roofing failure is suspected to be linked to the membrane, it is often necessary to disassemble the roof system and to do an “autopsy” to determine the cause of the failure.

So, how do you ensure that the waterproofing step is properly done?  There are a number of steps that help you get to the desired result.

  • Check the products that will be used.  Get the brand names, the product names, and the installation specifications.  Read carefully the parts about what NOT to do.  Make sure that the manufacturer’s suggested use covers what you want to use it for.
  • If you’re having someone else install the products (your roofer, for instance), ask them to explain where they would use the products, what kind of preparation they will do, what kind of verification or quality control they will do to ensure the materials will work as intended.  Ideally, they should be able to show photographs of prior installations where they did the membrane application.  Probably more important, is asking them under which circumstances the products did NOT work, and what they would do if those circumstances were found on your roof.
  • Once the installation starts, you need to check how the material is being installed.  How well was the roof preparation carried out?  Are the non-wood surfaces being primed with the appropriate primer?  How are the overlaps sealed?  Is the edge of the roof being overlapped?  Imagine water running on the surface.  Where will it go?  Is there a chance for obstruction?  Is there any apparent damage to the material during the course of installation?
  • Once the installation is complete, but before the covering or flashing is put over the membrane, the surface needs to be verified as to its adhesion, overlap, and coverage.  It is a good idea to request (in the contract) that photographs be taken of all roof penetrations, roof joins (such as valleys, endwalls, sidewalls), and roof terminations (rake or gable end, eave, hip, ridge) to show how those details were executed.  Alternatively, hiring a third party to conduct an inspection at this stage is another way to ensure that this critical step in the process is properly done.
  • Occasionally, especially with complicated roof lines and assemblies, it is appropriate to conduct a water test to ensure that there are no weak spots in the water-proofing coverage.
  • If membrane will be installed over the entire roof, it is really, really important to ensure that EITHER the roof system below the membrane is sealed and impermeable to water vapour, OR that it is well vented, so that any moisture trapped under the membrane can escape.  If a roofer or contractor agrees to install the membrane over the entire roof without make sure that either condition exists to a satisfactory level, they are being at best ignorant, and at worst, willfully negligent.

Of course, the membrane protection is only part of a well-constructed roof system.  Good installers will build in several layers of over-lapping protection to ensure that there is no single point of weakness that can undermine the efforts put into building the roof system.  If you’re not sure what needs to be done with your roof, give us a call.  We can give you an intelligent analysis of what issues are important in your situation, and what you need to do to make sure your system works.

(c) 2013 Paul Grizenko


Below are two membranes that we use routinely, and that have been, in our experience, proven to work well, provided they are used correctly.

Grace Ice and Water Shield Membrane:

Interwrap – Titanium PSU membrane:

Further information:

If you are interested in knowing more, use the contact form (under the Contact Us menu selection), or give us a call at 514-636-2300.