Changing from asphalt shingles to metal

The winter surprise

Metal roofs are much better products compared to asphalt shingles when considering durability, performance, and resistance to the weather.  However, one aspect of metal roofs that often surprise new owners is the difference in behaviour during the winter, if the metal is a smooth metal like painted steel, painted aluminum, or native copper.

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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

Roof Leaks – causes and fixes

The most common symptom of a roof leak is bubbling paint or stains in the ceiling gyproc.  Sometimes the leak appears at the ceiling/wall joint, sometimes it is becomes visible at the seam tape between the gyproc panels.  At times, it is not subtle at all, with a large amount of water running down the walls or dripping onto your floors.  Other times, the leaks are very subtle, and don’t show up as interior damage at all, but do let water into the attic or the cold spaces.

Leakage from ice dam appears in several places.

Leakage from ice dam appears in several places.

The first thing to figure out is whether this problem is new, or whether it has happened before.  If you’ve lived in the same home for more than a few years, and you’ve never seen it before, then chances are excellent that something changed recently, and therefore contributed to the leak.  If you’ve been in the house for under two years, then it is possible that the former owner did not disclose the history of prior leaks, and therefore the leak you see for the first time, may in fact be a long-standing issue that was hidden by new gyproc and/or paint.

There are many possible causes of roof leaks.  For “new” leaks, common causes are:

  • ice dams
  • gutter/eavestrough icing
  • wind damage to roof cover
  • failure of caulking or sealant
  • physical damage to the roof surface by falling objects or ice
  • failure of a waterproofing layer
  • installation of a bathroom, kitchen or drier outlet into the attic
  • Installation of pot lights or recessed ceiling lights

Other leaks, especially if they have been present for a while, are often caused by:

  • poor installation of waterproofing layers
  • poor design and/or installation of flashings
  • use of inappropriate products to cover or waterproof the roof
  • incorrect amount of ventilation and/or insulation for the roof design
  • poor detailing of roof penetrations (air vents, plumbing vents, electrical poles, etc.)
  • poor design of roof/wall flashings
  • leaks in caulking at window and skylights
  • poor placement of vents and other roof openings relative to wind direction
  • condensation

If there were short-cuts taken during the installation of the last roof, then these usually contribute to making the original problem worse.  For example, some roofers advocate leaving the old roof on because it gives “extra protection”.  When a leak DOES happen, then the layers of old roofing provide an additional path for water to run in different directions.  Other times, they “save you the cost of new flashings” and reinstall the old flashing – this works until the water finds the holes.  Still other times, the various materials were not installed according to manufacturer directions, and therefore did not provide the appropriate waterproof seals that were expected (this is a common installation failure for waterproofing membranes).

Old shingles left on, contributed to channeling leak to different location

Old shingles left on, contributed to channeling leak to different location

It is common to find that there is not a single point of failure, but a series of weaknesses that contributed to allowing the leak.  For example:  The primary cause is an ice dam, which allowed water to find a poorly-caulked joint, when then ran onto waterproofing membrane until it found an open seam, and then into the roof.  Once it entered the roof, it ran on the underside of a beam until it came across a supporting strut, at which point the water dripped down onto the insulation, and through the insulation onto the vapour barrier.  Then it pooled on the vapour barrier until there was enough to direct it to a gap in the vapour barrier, and onto the gyproc.  It finally showed up as bubbling of paint on a gyproc seam.  Where it came in can be quite far from where the leak actually occured.  So when looking at a source of the leak, it is necessary to play detective, and follow the water trail.  Sometimes it is quite easy.  Usually, it is not.

In the above example, the solution (short-term), would be to remove the ice dam, and to dry out the accumulated water on top of the vapour barrier.  However, the long-term solution is to add enough insulation to prevent the melting of snow in the first place, then have enough ventilation to remove any residual heat leaking past the ventilation and any moisture that may be entering the attic.  When the roof gets redone, the membrane needs to be properly applied (right product, right installation method, right location), following by the appropriate flashings, and finally counterflashings and caulking.

In some cases, we’ve seen roof covering deteriorate to the point that water is freely running on the underlayment, and every gap in the underlayment allows entry of water through the nail-holes and other breaks in the underlayment surface.  If the decking is made from OSB (Oriented Strand Board) of the Exposure 1 grade, then there is a high probability that the decking will be damaged extensively as well, and will need to be replaced.  If the leakage has occured over a period of time longer than one year, then there are also mold issues to consider.

In short, a leak is telling you that there are potentially major issues to be dealt with.  Before throwing money at the problem, take the time to figure out what’s actually going on, and what the priorities should be in terms of fixing the problems in both short and medium terms.  If you contacted your last roofer and he claims that there’s no problem (that he’s responsible for fixing), maybe it’s time for a second opinion.  After this, you will have a better idea of what the problems may be, and what would be a reasonable approach to getting them resolved.

(c) 2013 Paul Grizenko