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.

Myth 1 – shingles ARE the waterproof layer

Shedding the water…

The part of the roof you see, the asphalt shingles (the most common form of roof covering in the Montreal area), or metal shingles for that matter, are only the most visible components of a roofing system that has many elements working together.  For an overview of what makes up a roof system, you can go to the following link Roof System Article, found in the Articles portion of this web site.

Wind-blown water - normal sheddingALL steep-slope roofing materials are designed to SHED water (ie, allow the water to run off), and the installation practice is designed to allow this water to stay to the outside of the roof system.  Such materials generally shed water well as long as the slope of the roof is 4:12 (1:3) or steeper.



slope 4_12 example

How slopes are expressed in roofing

(Aside:  in roofing slopes are expressed as ratios of the vertical rise to the horizontal run.  4:12 is the ratio of 4 units vertical to 12 units horizontal).


When the roof slope falls below 4:12, the slope is now transitioning to a low-slope roof system.  In general, steep-slope materials CAN be installed on shallower slopes (down to 3:12), as long as additional installation steps are taken.  Generally, below 3:12, down to 1:12, the steep slope materials are no longer appropriate because they no longer “shed” water, but are more likely to be penetrated by water along the joints and edges.  This is one reason why you should not have conventional asphalt shingles on a low-slope portion of your roof.

Below are some links to web sites of roofing material suppliers discussing “steep slope roofing” (Note, this is not an exhaustive list – you can find more links if you enter the phrase “steep slope residential roofing” into your favourite search engine):

Certainteed roofing products

GAF Application Instructions for Asphalt shingles (Note:  slope discussions on pages 4 and 5 of the application manual)

BP Technical bulletin regarding roof slopes

“Sealing the shingles”

Some people think that when asphalt shingles are “sealed” they are waterproof.  They are not.  The “sealing” refers to the underside tar strip which is supposed to melt and “seal” the top shingle to the lower shingle, primarily to prevent wind uplift.  If you examine the underside of most asphalt shingles with a tar strip, you’ll see quite quickly that this will not work as waterproofing.  In my opinion, “Sealing” is a misuse of the word as many associate that with phrases such as “waterproof seal”, whereas a better word would be “bonded”.  However, the game in marketing is to use a suggestive word that implies a function, but does not explicitly claim it.

So if shingles aren’t waterproof, what’s keeping the water out?

The unsung heros of roof waterproofing are the components underneath the shingles:  the membranes, underlayments, base flashings, and sealers.  Together these form the true waterproof layer on the roof.  In effect, your roof has two layer:  the visible exterior layer (which is usually made up of shingles and certain flashings), and the working interior layer which provides the actual waterproofing performance.

Furthermore, the waterproofing layer has to integrate with, and tie into, all the other roof elements such as roof penetrations, protrusions, junctions, and other roofing features.

The waterproofing challenge can be quite difficult if one takes into account the pressures that can result from wind-driven rain, or water under pressure in ice dams, or concentrated water flows from heavy rain.

The insight I am offering you here, is that when you have a steep-slope roof installed, you are in fact installing two separate layers – the pretty, exterior layer (the asphalt shingles and external flashings), and the vital but invisible underlayer which provides the actual waterproofing performance.

Ok, so there’s a hidden interior layer.  Why should I care about something I can’t see?

Because, it’s the layer that provides the actual waterproofing performance and protection to your roof.  It’s the muscle and sinew of your roof system.  If you have a leak, from whatever cause, that’s the layer that has failed.  In fact, if you have this layer properly installed and you don’t bother with installing the shingles, the “roof” will work and will keep your house dry.  It may not be pretty, but it will work.

The materials needed to effectively ensure waterproofing are usually more expensive than the shingles themselves.  When you add up the costs of the waterproofing membrane, underlayment, sealants, base flashings and other components, the material cost of the waterproofing layer can be more than the material cost of the covering.

On the labour side, the proper installation of the various waterproofing materials and tieing them into the various junctions, protrusions and penetrations almost always takes more time than the installation of the covering shingles.  And this brings us to what you’re paying for when you buy a roof (or re-roof).  How much time are you spending in discussing with prospective contractors how they plan to identify the waterproofing challenges on YOUR roof, and how they plan to counter these weaknesses?  Chances are that this is a conversation you’ve never had, on the assumption that this is something the roofer should take care of.

Interestingly, if you watch the some renovations shows, such as those of Mike Holmes and Jon Eakes, and undercover consumer investigations such as done by the CBC Marketplace, you often see that the “professional estimate” can be up to twice (or even more) the cost that many roofers are offering.  The discrepancy is almost always in the roof preparation and the waterproofing layer installation.  The range of skill and ability in the roofing trade ranges from the “shingle-bangers” to the professional waterproofing experts.  The shingle-bangers will do an average residential roof in a day.   The conscientious professionals will take up to five days to do the same roof.  From external appearances the two will look about the same.  But one will work under extreme weather conditions, and the other will fail.

Myth 2 – caulking is acceptable waterproofing.

If that’s the main way someone is waterproofing, then sorry, it’s not.  Even if it is applied correctly and works effectively, it has a life of about 5-10 years, being affected by the sun’s UV rays, heat, and mechanical effects of expansion and contraction.  Once it hardens, or cracks, or pulls away from one of the surfaces it is supposed to seal, it allows water to penetrate the system.  Now, if there is a good waterproofing layer below, backstopping the caulking above, then you’re good.  But if the roofer was relying on mainly the caulking to make the assembly waterproof, then the roof will be compromised in as little as five years.  By coincidence (*cough, cough*) the labour warranties usually don’t last past five years.

Caulking is ONE of the techniques a good roofer will use, but it will be tied into other methods of waterproofing, AND it will not be relied on to keep the water out.

Myth 3 – If you have ice-and-water shield membrane installed, you are protected against leaks.

Nope.  Not even close.  Ice-and-water shield peel-and-stick membranes are truly wonderful tools available to the roofer, but they are not, in themselves, a magic solution.  To get the desired performance, one has to place the RIGHT product (membranes are not all equal), in the RIGHT place, at the RIGHT time, following the RIGHT installation techniques, and interfaced to other components in the RIGHT way.  That’s a lot of “rights” to get right to get the final benefit.  So let’s examine some of these “right” conditions.

Right product.

Membranes are not created equal.  There are some “membranes” that cost about $30 per roll (2016 prices), and there are some that cost over $200 per roll.  Higher price doesn’t always mean higher quality, especially if the membrane properties are not appropriate for its chosen use.  There are membranes that maintain structural integrity even under very high temperatures (as may be found under some metal roofs).  There are membranes that provide built-in mechanisms for ensuring waterproofing seals between layers.  There are membranes that give better nail-sealing performance over a wider range of conditions.  There are membranes that can only be installed if the ambient temperature is above 10C or less than 35C.

There are situations where membranes are not the appropriate product, and some form of underlayment (ranging from 15# felt to engineered synthetic underlayments) is a better fit for the desired use.  Some underlayments are extremely strong and temperature resistant.  Some underlayments are water-resistant but highly breathable.  Some underlayments are rather fragile (to either foot traffic, or exposure to the sun, wind and other elements), and others are the opposite.  Again, there is a very large price difference between the cheapest underlayments, and the ones with the best performance.

Each product has an array of both strengths AND weaknesses, and these have to be properly matched to their intended use.  In many cases choosing the right membrane (or underlayment) is more important than choosing the right shingle, but I can assure you that this conversation virtually never happens (except when people do their homework and understand the issues).

Right place.

It is common for me to hear that membrane was installed along the eave (3 or 6 feet) and along the valleys.  However, if it’s installed over an eave that has never had any history of ice damming or other form of possible water entry, then it’s a waste of money.  On the other hand, if it’s NOT installed below a valley runout, then that’s a common location for leaks that’s not protected.

To ensure that you put the membrane in the right places, you do need to analyse the way the roof performs, see where the roof system may contribute to ice-damming, examine the water run-out paths, and once the roof is stripped, verify that all weak points have been fully identified.  If the roofer or contractor does this type of analysis, they will know how to adapt their normal installation practice to YOUR SPECIFIC situation.

When planning the installation, it is also important to NOT put it where it can cause problems.  For example, if a roof deck is not well vented, and has multiple locations where condensation can build up, then putting a waterproofing membrane over those locations may actually create worse conditions, by trapping moisture.  So while some roofers will suggest covering the entire roof with membrane, this will be a dangerous step IF the roof venting has not been verified to be enough to prevent condensation accumulation.

When a contractor offers to install membrane in certain parts of the roof system, ask for the rationale behind why there, why the quantity, and why that specific product.  They should be able to explain the specific conditions that they are protecting against, and why they made the product choices they made.

Right time.

Believe it or not, there are times when certain membranes should NOT be installed, such as when it is too cold, or too wet.  Membranes rely on bonding with, or melting into the wood substrate to be able to perform properly, and if the temperatures are too low, this bonding will not happen.  Under these circumstances, special low-temperature membranes should be used, but then again, it will also depend on the temperature range they will experience under live conditions.  In short, proper installation requires installing the membranes under conditions that the manufacturer specifies as being acceptable.

Right installation technique.

Each membrane comes with instructions on how to install it correctly.   Here are two membranes that we use often.  Note the very detailed instructions about substrate preparation, etc.

Grace Ice and Water Shield Application Procedure

Titanium PSU30 Application Instructions

There are rules about priming, acceptable substrate, appropriate fastening methods, etc.  Failure to follow ALL the rules can mean that the protection that should have been there was not.  I’ve seen fully membraned roofs that leaked because the substrate was not continuous.  I’ve seen membraned roof valleys leak because the proper priming was not carried out.  I’ve seen membraned roofs fail because the edge preparation was not properly done or the layer overlaps were not sufficient.  What is important during the installation process is to take the time and ensure that all the conditions required for successful installation are followed.  It is common to see this step being hurried, with the result that the membrane is installed, but it won’t work as it was intended.

Interface to other components in the right way

The interface to various other components (underlayment, flashings, penetrations, protrusions) of the roof system has to be done in a way that ensures long-term waterproofing.  Since the membranes don’t automatically stick (and stay stuck) on various materials, it is important to have the appropriate primers AND amount of overlap.   This is probably the most common reason we find for the failure of membraned roofs – failure to adequately manage the interface to other components.


Your beautiful roof covering protects against the wind and the sun.  The hidden water-proofing layer protects against the water.

Since the time and effort to ensure a good waterproofing layer installation is expensive, this is usually the place where “competitive” roofers cut most corners.

Caulking is not waterproofing, certainly not by itself.

Membranes and underlayments ARE good elements of the waterproofing layer IF properly installed (which takes both time, and cost).

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