Installing two roofs – why bother?

Most people assume that the visible external roof covering is the “real” roof, which functions to keep the water and weather out of the roof system.  This is true for flat or low-slope roofing materials, but not true for steep-slope materials (which is anything over a 3:12 roof slope).  Steep-slope materials are designed to shed water.  That means that water is supposed to run off the surface, and if properly layered, the water stays on the outside.  However, if wind or a physical obstruction interrupts the water flow, then it will go under the roofing material.  This is true for all steep-slope roofing material, whether it is made of asphalt shingles, or metal shingles, or metal panels, or cedar shakes, or slate.

It’s the unappreciated and unloved underlayment layer that’s below the top layer that has the job of actually keeping the water out.  That layer is the “true” waterproof roof, which is composed of layering of flashings, membrane, underlayment, and sealants.  It’s also the layer which is easiest to skimp on when the contractor is being pressed to come up with a low price.   If you can’t see it, and you don’t even know that it needs to be there, then why would you want to pay for it?

How does the saying go?  Ah yes.  Penny-wise, pound foolish.

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Unintended consequences – Renos that make ice dams.

That is a curious result that happens.  Someone finally invests some serious money in upgrading their kitchen (or bathroom), installing beautiful cabinetry, gorgeous ceiling lights, and finally get the attractive and bright kitchen (or comfortable bathroom) that they always wanted.  The following winter, an ice dam forms.  The following year, the ice dam gets worse and the ceiling starts to leak after every thaw.  What’s going on?

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Stripping is a good thing. Really.

Roof stripping, that is.  This is the process of getting rid of the old roof covering material, down to the deck.  In the process, if done correctly, you’ll learn a lot of things about what worked on your roof and what didn’t.  As all things that are good for you, there are costs and inconveniences involved, aspects which are exploited by those contractors who don’t have your best interests at heart.

What are some of the typical reasons you’re given why leaving your old roof on and going over is/are acceptable?  They include:

  • You’re saving money.
  • It’s more ecological (no landfill waste!).
  • It’s more secure.
  • It provides another layer of protection to your roof.
  • It’s recommended by the manufacturer.
  • There’s much less residual mess to clean up.

So many great reasons why you should just leave the old roof on, and go over!  And yet, pretty much all of these are false, based on the homeowner’s lack of knowledge of how the roof system is supposed to work, and compounded by the contractor’s desire to get the job done as fast as possible, get paid, and move on.  If you are presented with the suggestion that leaving your old roof on is an acceptable strategy, there are a number of things you should be aware of before you agree to this.

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Spray Foam – good or bad?

Closed cell-spray foam used as insulation, has been on the market since about 1979, and has gone through a number of evolutionary steps.  The product is a solid plastic, formed by the chemical reaction of two primary components:  an “A” side which is isocyanate, and a “B” side which is usually a mixture of oils, stabilizers, fire retardants, blowing agents, and colouring agents.  The reaction is exothermic (ie, generates a lot of heat), and the materials form a solid within seconds. The blowing agents are the compounds which produce the low-conductivity gas that forms the bubbles in the foam, and thereby form the primary insulation.

When applied to a minimum thickness of 50mm (slightly under 2 inches), the material acts as  insulation, vapour barrier, and an air barrier.  Newly-installed foam has R-values of R-6 to R-7.5 per inch, but this diminishes to about R-5 per inch over time as the insulating gas dissipates.  Compared to other insulation products like fiberglass bat or mineral wool bats, the product has more insulating value per inch, and resists the loss of heating value that sometimes occurs with loose insulation due to air convection with extreme temperature differences.

The ability to prevent air and vapor movement is generally a good thing, except in situations where moisture can enter a wood structure, and then cannot get out.  Therefore the short answer to the question in the header is “spray foam is good when PROPERLY installed, and BAD when installed in inappropriate places or in an incorrect manner.  There are also issues of the impact on the environment, and potential impact on the health of the people living in the homes where the product is used.  The rest of the post touches on some aspects that inform whether the installation is good or not.

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The Stack effect and roof leaks

During the month of January, I’ve been called to diagnose roof leaks on an almost daily basis.  These leaks have a common characteristic – they show up when it’s cold.  That usually signals that there is some form of accumulation of water on the roof, usually behind an ice dam, and that the waterproofing is insufficient to stop the water from forcing its way it.  The existence of an ice-dam almost always means that there is excessive transfer of heat to the roof which melts the snow on the upper sections, and the resulting meltwater refreezes on the lower section over the soffits.  To reduce this heat transfer, we rely on insulation (to reduce the amount of heat leaking into the attic) and ventilation (to dissipate the residual heat before it melts the snow on the roof).  However, there is another player in this game, and it can cancel out the efforts at insulating and ventilating.  That player is air leakage.

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Frost in Attic

Frost can be beautiful.  One of the benefits of poorly insulated windows, at least for small children, is the beautiful lace shapes that form – so delicate and yet so enchanting.

Despite this beauty, there are plenty of places where in a home you do NOT want to see frost forming.  Certainly, when found on windows, it is a clear sign that the window is not very good at insulating.  Another place where you don’t want to see frost, is inside your attic.

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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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Condensation plays the same role in “modern” roofing practice as “bad spirits” did in medieval times.  If something went wrong, you blamed the bad spirits.  Nowadays, if a roofing system is not working, it’s due to “condensation”.  So let’s get into it and understand what condensation is, when can it appear in a roof system, and what corrective action you can and should take if condensation is really the cause of an apparent leakage.

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Ventilation for roofs – why, what, when, where, how (instead of who)

Ventilation is like vitamins – everyone knows you need some, but often don’t know why.  This post discusses some aspects of ventilation that  homeowners should know about.


Ventilation is used to:

1)  reduce the heat build-up in the attic and

2)  reduce the moisture levels in the attic.

The first helps reduce the cooling requirement in summer, and reduces the snow melting (which can lead to ice dams) in winter.

The second helps in keeping the attic relatively dry and thus minimizes the possibility of condensation, which in turn reduces the support for rot and mold.  If the roof system suffers from persistent, if low-level leakage (as often happens with older roof coverings), this ventilation also helps dry out the leakage.


Ventilation is the movement of air through an enclosed space.  Ventilation can be passive (working on gravity and density only) or active (moved by a fan).  Active ventilation is usually NOT recommended for venting attics in cold environments due to the danger of pulling in moist air from the living space if the vapour barrier is not completely effective.

For passive ventilation to work, there needs to be an intake at a low point of the system (that’s the inlet for cold, dry air), an air channel through which the air can flow into the attic (where it picks up heat and moisture), and an outlet at the top of the structure (because warmed air is more buoyant than cold air, and rises to the top of the structure).  Since air has viscosity, the size of the openings determine the amount of resistance to the air flow.


Ventilation becomes particularly important in winter, when insufficient insulation often allows transfer of heat from the interior to the cold spaces.  If the vapour barrier is not very effective, there is also the danger of moisture from the living spaces to enter the cold areas and become condensation.  Good ventilation will help reduce both the amount of condensation, and the amount of heat transferred to the roof (where the heat can melt the snow and contribute to ice damming).


Ideally the intake ventilation is located along the lowest edges of the roof (which is where you will usually have the vented soffits), and the outlet ventilation is located at the top part of the roof, along the top ridge.  In between, you should have a minimum 2” air channel connecting the intake with the outflow.  The positioning of the intake and outlet vents is important to avoid the ventilation “short-circuit” which happens when multiple openings are present, and the air is drawn from the intake of least resistance, instead of the lowest ones.

Since many attics have limited space along the eaves for insulation, it is not uncommon to see insulation stuffed into the corners which ends up blocking the airflow from the soffits.  There are various products on the market designed to keep the airflow working.

The geometry of the roof, and the arrangement of interior space of the attic also affects the way the ventilation works.  Certain roof types and roof features are known to be problematic when it comes to ventilation, and require special measures to ensure correct function.


The ventilation product to be used depends on the roof type.  Simple pitched roofs are usually vented with soffit vents along the base of the roof.  Mansard roofs can have a more complicated requirement, and they usually use specialized vents to bring air into the upper portion of the mansard.  Outlet vents can be of various types, with gable vents, low-profile vents, turbines, ridge vents, and large vents like Maximums all having specific uses.

If venting is found to be necessary, but the existing structure doesn’t allow this, then it may be necessary to build a second deck with an air space between the old and new roofs.  Some metal roofs can be installed using the cross-bat system, which effectively accomplishes the same result.

It is very common for contractors selling “ventilation” to focus on the outflow vents and ignore the rest of the elements that make the ventilation effective.  In some circumstances, it is just not possible to arrange the correct ventilation, and other means need to be considered for achieving the two goals of ventilation.


There is a class of roof structures that do NOT need ventilation.  These are usually designed from the outset as sealed roof units where there is no need for ventilation as there is not movement of air into or out of the roof unit.  However, for these types of units, both the design and the execution of work need to be impeccable, to ensure that no moisture can enter the unit from either inside or outside.

Other stuff:

The ventilation should also be discussed together with vapour barriers, insulation, and the waterproofing on the outside of the decking.  They all interrelate in ways that need to be thought through if modifications to the system are being made.

Case Study:

This image shows mold growing on the underside of the plywood decking, despite the home having vented soffits and several large maximum outflow vents.

Example of mold buildup on decking due to leakage and poor ventilation
Example of mold buildup on decking due to leakage and poor ventilation

There were several issue with this house:

  • The insulation (fiberglass bats) were stuffed into the edges of the attic, blocking air flow from the soffits.
  • The insulation used in that area did not have enough resistance to heat to prevent loss of heat to the roof.
  • The consequent loss of heat caused large ice dams in winter.
  • The asphalt roof covering did not have enough Ice-and-water shield membrane along the eaves to prevent the water from the ice-dams to penetrate the roof covering to the wood.
  • There was not enough ventilation to dry out the moisture that was accumulating in the wood, supporting the growth of mold.
  • Due to the construction of the house and subsequent “improvements”, there was minimal space for adding either insulation or ventilation in affected areas.

There were several possible ways of ameliorating this situation.  They involved determining the areas where ventilation would just not be possible, and using certain techniques for preventing moisture buildup in those areas.  Other areas would have benefitted by using insulation with a much higher R-value per inch (given the limited space available), and by using certain products that would promote ventilation in tight areas.  A further improvement could have been made by using the appropriate water-proofing membrane in areas where the ice-damming could not be avoided.

(c) 2014 Paul Grizenko