Tips for Improving Ventilation on Residential Re-Roofing Projects

During residential re-roofing applications, it is important to ensure the roof system is properly ventilated. Photos: John R. Crookston

If the question is “Should I provide ventilation on this steep-slope roof?” there is a simple, one-word answer: Yes. The problem with this answer is that it would make a very short article, and I am sure that is not what was expected. Let me explain!

Ventilation is required if you have unconditioned space, and it is that space that needs to be ventilated. In most commercial applications, you are dealing with a flat roof membrane over insulation installed directly over a metal deck. With no “attic” involved, there is no unconditioned space and, therefore, there is no space to ventilate. You still have to find a way to control the moisture, but this is accomplished through the use of mechanical air conditioning and heating units, and also through the introduction of outside air and air exchanges. For this article, I want to concentrate on a typical residential steep-slope application, and the basis for most normal houses all goes back to 1 John 1:1, which goes something like this: “Thou Shalt Ventilate.”

To help all of this make sense, it is important to define some terms I use. “Conditioned space” is anywhere in the house that we are attempting to control the temperature or humidity — the living space of the house. “Unconditioned space” refers to areas of the structure where we are not attempting to control the temperature or humidity — typically attics (although some attics are treated as conditioned spaces). Unconditioned spaces should be as close to the outside temperature and humidity as possible. To accomplish this, we would use vapor barriers, insulation, and ventilation. The insulation would be anything that would restrict the transfer of either heat or cold in either direction; the vapor barrier would be anything that stops the transfer of moisture between the hot and the cold areas; and ventilation would be the method by which we allow the hot or cold air to move between the inside of the unconditioned space and the outside atmosphere.

Erecting scaffolding at the roof’s edge ensures safety and gives technicians a comfortable spot to examine and repair intake ventilation at the soffits.

Current building codes and building technologies have improved the performance of homes greatly by making them able to “breathe” and at the same time resist that transfer of energy. Examples would include the newer thermo-pane windows with better weather stripping, and house wrap to stop the wind pressure from penetrating the house. There are also truss roof systems that incorporate high “energy heels” at the plate to allow insulation all the way out to the edge of the plate and still allow a 4 inch air space at the plate to allow the air to flow freely.

To make this all work, it is important to fully ventilate the soffit area, and to combine this with a system to get the air out of the attic space. This could include a ridge vent system, regular roof louvers, turbine vents or gable end vents. It is important to remember, however, that you cannot mix these vents. We need to understand that air is lazy and will always follow the path of least resistance. If we mix the different types of vents on the roof, the air will move from one to the other and short-circuit the airflow. For example, air might flow from a roof vent near the peak to the ridge vent just a couple of feet away, leaving the rest of the attic with no airflow. In this case, more is not better.

Wide Range of Energy Efficiencies

You will find that most of the houses built after the late ’70s used truss systems that incorporated the energy heel. This also corresponded with the “energy crisis,” which saw a massive increase in the amount of insulation blown into the attics. Four to 6 inches became 12 to 20 inches, and it is important to know that the more you tighten up a house, the more important it is to increase the ventilation. A fully insulated house demands a fully ventilated house to perform effectively. About 10 years ago, we built a house and a cheese-making facility, using R-panels, which are made from EPS foam sandwiched between layers of OSB panels. They are incredibly strong and energy efficient. They can get so tight it is difficult to open or shut a door because of the air pressure. That can potentially be dangerous in the event of poor indoor air quality and pollution. In this instance, we engineered a mechanical system to completely change the air twice an hour — and at the same time, saving the energy of the heated or cooled air with an air-to-air heat exchanger. The system supplied combustion air for the furnace and the stove, and also recycled the heated air from the bathroom fans and the oven exhaust, saving the heat, but exchanging the air itself.

The soffit should be removed as part of the tear-off process.

Our projects have ranged from this extreme of efficiency to some of the older homes — some more than 150 years old — that had no insulation. They were wonders of efficiency for their time, and the builders understood all of these principles. It was common to see some of these elaborate homes with what looked like a “widow’s watch” observation tower with windows all around at the very peak of a low-sloped hip roof. Combined with a large central staircase, the owners could open the windows and inside doors in the summer, and the central hallway and the “widow’s watch” acted as a large chimney, moving the hot air out and pulling cool air in without any fans or electricity.

Many churches and other large buildings used the same principle to control the air inside, using either the steeples or large towers to act as chimneys as well as architectural and design focal points on these buildings. Problems often begin when we try and upgrade buildings to modern standards without taking into consideration how the changes will affect the design and operation of the building. Addressing these large, complicated buildings will be the subject of another article, but right now I want to specifically address the needs of residential houses built from the ’20s up through the late ’70s.

Homes Built From 1920-1979

There are millions of them. Before World War II, most were built with perhaps some minimal insulation or some aluminum foil to act as a radiant barrier, but energy was cheap and to do more would have been a waste of money. Without much insulation, there was little need for ventilation, as the house was drafty and, by definition, a drafty house is ventilating itself. After the war, the ranch-style house was the rage and I worked on thousands of them growing up. I have home movies of myself on the roof with my father when I was only four years old. By the time I was 10, I could lay out a roof and knew exactly what I was doing up there. Today, they would call that child abuse, but back then it was life. The point is that I lived and worked through this transition. In the ’50s, 2 inches of insulation in the walls was common and 3-1/2 inches in the ceilings. We would install some roof louvers in the attics and they would install some 3 inch vent strips in the wooden soffits for an intake. It was not much, but it was enough. Then the oil embargo occurred in the early ’70s and energy prices jumped.

It may be necessary to remove the bottom sheet of plywood to access the area from above. Often insulation will be found blocking the soffit.

Demand for insulation to save energy skyrocketed, and suddenly there were six pages of ads in the Yellow Pages for insulators. If some insulation was good, then more was better, and they blew insulation everywhere. Some was installed in the walls, but the biggest bang for the buck was in the attics, and it seems that all of the soffits were filled and the opening at the plate was blocked. Without this intake, the only thing that the roof louvers could do was let out some heat; the air movement stopped. The water vapor still got into the unconditioned attic space through whatever insulation was installed, and since it could not get out, it would condense in the insulation and on the wood surfaces and cause mold, rot and mildew.

Since there was no air movement at the plate, and the insulation was packed tightly against the bottom of the roof decking, in northern climates the heat would transfer to the roof surface during the winter, and ice buildup became a huge problem. In the southern climates things were reversed, and problems cropped up during the summer months when the air conditioning was running. Simply put, “You cannot fool Mother Nature!” Shingles that used to last for 25 to 30 years were now “cooked” in place in 10 to 15 years. Mold and algae became a problem on roof surfaces to a much greater extent than in years past, and most of this is and was caused by a lack of ventilation.

Tips for Avoiding Mistakes

Roofing is so much more than just installing shingles, but we have to be able to see the bigger picture to understand why. As a third-generation union carpenter, who is still working on roofs at 67 years of age, I love what I do and I am very good at it. Since I see the same mistakes being repeated again and again, I feel obligated to pass on the experience that I have accumulated over the years. I have learned some hard lessons making all of the mistakes I am talking about here, and hopefully all of us can learn from them.

Installing proper vent baffles at the soffit creates an air channel into the attic from the soffit and prevents the insulation from touching the underside of the roof deck.

Here are some tips for avoiding common ventilation mistakes at each stage of the re-roofing process:

  • Check the attic space when you figure a new roof.
  • If you can’t see light coming from the soffit into the attic, then there is no air getting in either. · At this point, you either have to become a carpenter or soffit man — or hire one.
  • Take apart the soffit as you do the roofing tear-off.
  • You may need to remove the bottom sheet of plywood to see what you are doing from above.
  • If the soffit is aluminum or vinyl, chances are that there is an original wooden soffit beneath it.
  • Tear it all out. Take out the insulation that has been blown into the soffit at the same time.
  • Replace the old soffit with a fully vented aluminum or vinyl soffit system. Vented aluminum has twice the Net Free Area as vinyl for the same square footage, but they both work.
  • Install proper vent baffles at the soffit to create an air channel into the attic from the soffit. You cannot let the insulation touch the bottom side of the roof deck.
  • Check the bathroom vents and make sure that they are vented to the outside with a flapper vent through the roof and not vented into the attic space.
  • Do the same for any kitchen vents.
  • Install new sheeting along the bottom after you have fixed all of these problems.
  • Determine how much ventilation you need to vent the attic space (square inches). Normally, this is 1/150 of the attic floor space. (For example, if the attic floor is 30 feet by 50 feet, the attic floor area is 1,500 square feet. 1,500 divided by 150 = 10 square feet of ventilation.)
  • You need this much ventilation opening at the high point of the roof, ideally at the ridge.
  • If you install a ridge vent, take out and cover over the holes of the old roof louvers, turbine vents and gable end vents. You can just install some felt or plastic sheeting over the gable vents from the inside.
  • This will give you one intake at the soffit area, and one exhaust at the ridge. Don’t worry about having too much intake at the soffit, as it will only allow as much air in as it exhausted at the peak.
  • Install the new roof as per code.

Ensuring Safety and Efficiency

Making sure the new roof system is properly ventilated will maximize the service life of the shingles installed.

We will normally erect a scaffold around the perimeter of every job we do to give us access to this important area of the roof. This may sound like overkill, but that is the area where you want to spend the most time, as that is where the problems normally occur. It can be more expensive, but what we are talking about is value as opposed to price. Quite simply, I am not interested in talking to someone whose only concern is the cheapest price. On a steeper, higher roof, you will find that this is actually a faster and cheaper way to work, too. With a catch platform around the building, you have a place to work and store materials, you can see exactly what you are doing, and you also don’t need to have harnesses and ropes to obstruct you and still meet OSHA standards. Since I sell the jobs but also work on them, this is what I prefer. It is also impressive for the homeowner and it sets us apart from most of the competition. We have been doing it like this since 1986, so I know that it is a viable option. Scaffold is expensive initially, but when you have used it once, you will wonder how you did the work without it. It is also a line item on all of my bid forms, and after it has been paid for it is a profit center, too. That is the best of all worlds.

I have read that experience is what you get when you are looking for something else. I have many years invested in looking for “something else,” so I hope that this article helps you avoid just some of the mistakes that I have made in my lifetime.

About the Author: John R. Crookston is a roofing contractor and consultant located in Kalamazoo, Michigan. He has more than 60 years of experience in the roofing industry and has written technical articles for a variety of publications under the pseudonym “Old School.”

Understanding Is the Key to Preventing Trouble Spots With Masonry Chimneys

Restoration work in progress on a historic building shows copper chimney flashing being installed on a recently repaired chimney. Photos: John Crookston

There are almost as many types of chimneys as there are cars, but for this article, I am talking about masonry chimneys, and specifically masonry chimneys protruding through a steep-slope roof. These can take the shape of a simple chimney block unit with a clay flue liner running from the foundation thorough the roof, all the way to massive stone or brick structures with two and three fireplaces built in at different levels of the house. Chimneys can serve as load bearing structures and heating systems for the house. I have worked in homes where the chimneys are constructed so that the hot gas passes up and down through the structure of the chimney to heat the masonry mass before they escape out the chimney top, allowing this heated mass to radiate the heat for hours inside the house to supplement the regular heating system.

Eventually however, the chimney has to pass through the roof and into Mother Nature’s realm, and we as roofers have to deal with keeping rain and snow from working back into the building. Most of this task is accomplished with the flashing system at the roof level, but we need to remember that water and ice can also work through the mass of the chimney itself and cause leakage inside the structure. That is the reason for specifically mentioning masonry chimneys in the title above; when one is dealing with chimneys and water, moisture can be coming from all different angles, and all of these areas need to be addressed.

As part of the repair process, a drip line was cut into the underside of the chimney cap to prevent water from migrating across the surface.

I distinctly remember being at a local supply house years ago when a roofer ordered “a five-gallon bucket of flashing.” At best, plastic roofing cement is a temporary fix or patch, and eventually it has to come off. In my long career working on roofs, I have worked on thousands of shingle roofs and many hundreds of metal, tile and slate ones, too. I didn’t know whether I laughed or cried when the guy said that, but I did want to scream, “Flashing doesn’t come in a bucket!”

Masonry walls and chimneys bear on their own foundations and they move at different rates than the rest of the building. You have to allow for that movement, and you need to channel the water in the direction you want so that it is easier for it to flow off the roof rather than into it. Permanent flashing allows for the movement with hundreds of places where the metal can move while always at the same time directing the water lower on the plane of the roof and toward the bottom edge. Whatever the type of roofing material used (shingles, tile, slate, metal, thatch, etc.) and whatever type of flashing metal you use, the flashing always has to be lapped so that the water flows in the direction of the overlap. Depending on the slope, the lap has to be sufficient to allow for the worst possible amount of water flow you will encounter. The flatter the pitch, the more overlap you need. At less than 2/12 pitch, we start to deal with “flat roofing,” and that has its own challenges, though the principles are the same.

New copper chimney flashings were installed as part of this tile roof replacement project.

I could write about a detailed method to flash a chimney, but on every bundle of shingles sold, there are very good details printed and I would dare to say that not one in a million are ever read or even looked at. In this article, I want to explain the reasons why the details specify what they do. In the preceding paragraph, I mentioned that you have to make it easier for the water to flow off the roof rather than into it. That, in essence, is the principle of all flashing — and roofing, for that matter. If you fight the water flow, you will lose 100 percent of the time. Examples of “fighting the water flow” would be for instance lapping the flashings in the wrong direction, or perhaps building a saddle on the backside of a chimney and not extending it far enough sideways so that the water was trapped in the bottom corners. It could also be something as simple as cutting the shingles or slates too tightly against the flashings. As a general rule, always leave about a 3/8-inch gap between the vertical bend of the flashing and the cut of the shingle. This will allow the water to clean out the debris while keeping the joint water tight. Also, don’t jam the counterflashing tight against the horizontal surface of the flashings. This will also restrict the water flow and could cause leakage.

As an interesting aside, one very common mistake I see with cutting valleys is for roofers to not trim back the top corner of a shingle in the valley as shown on all of the packages. The water will catch on that top corner if it is not cut back and track along the top of the shingle until it finds a way into the envelope of the roof. This applies to all valleys, but in this instance, I am specifically speaking of the valleys created when a saddle or cricket is installed behind a chimney. It is also important to not cut the valley shingles in the center of the valley or the low point. Keep the cut line about an inch out of the center of the valley so that the water can again do your work for you by cleaning out the debris. This will also keep the water away from the top corner of the valley shingle.

Problems With Chimneys

Inspecting and repairing any damage to an existing chimney is an essential part of steep-slope re-roofing projects. Loose mortar, cracks in the bricks themselves and spalled surfaces are obvious signs of water damage.

Proper flashing application is crucial, but many of the problems associated with chimney leakage have to do with the chimney itself. Until the advent of the high-efficiency furnaces, most exhaust gasses from the heating of the building went up the flue of the chimney. When more efficient furnaces were introduced, they reduced this gas and excess wasted heat, yet many were vented into the same flue. By definition, a 50 percent efficient furnace puts half of the energy and heat up the flue, while an 80 percent unit would only vent 20 percent of the heat into the same volume, heating the house with the other 80 percent. It takes heat to create the draft necessary to carry moisture out of the chimney. Any gas will cool when it expands, and we are drastically cutting the amount of heat when installing a more efficient furnace. If we don’t reduce the size of the flue, the water vapor can condense back into water before it escapes from the top of the chimney. Mostly, this occurs in the section of chimney directly exposed to the weather, which would be the part sticking out above the roof line. It was common years ago to see the face of a lot of the bricks spalled or breaking off from the rest of the brick. This was caused by the water vapor condensing and then saturating the brick, freezing, expanding and breaking off the surface. If you still have one of the 80 percent units, it is important that you have a smaller flexible metal flue liner installed to reduce the volume and increase the speed with which the gasses escape. This in normally not a problem anymore, as most units now are 95 percent efficient and are vented out the side of the building using a plastic pipe.

This granite chimney shows signs of damage caused by water migrating underneath the chimney cap.

Very few of the homes built today even have a masonry fireplace or chimney, mostly because of the type of furnace used and modern codes. Most fireplaces installed today are zero-clearance units and are basically a gas appliance similar to a gas stove. Many of the older homes that still have wood burning fireplaces have switched them to a gas burning unit, and this will cause the same problem as switching the older furnaces if a smaller sleeve is not installed to reduce the volume of the flue. The biggest problem with this switch is that the effects are not immediately apparent, but delayed, often by several years. We worked on a large condominium project in the early 70s that had dozens of large chimneys, most with several fireplaces on different levels. At some time in the 90s, they all had gas inserts installed, without changing the size of the flue liners. Not all of the chimneys had problems; only the ones that used the gas logs. No matter how often they redid the bricks on the tops of the chimneys, they kept on breaking and spalling. The flaws in the brick and cracks in the caps caused by the water vapor freezing and expanding also caused regular leakage in the chimneys by loosening the counter flashings and letting water past the step flashings and head wall flashings. The caveat to be learned here is that there is a cause and effect that occurs for every action taken, and before making a change it is important to do some research and determine what the effects will be and what has to be done to make sure that it doesn’t do more harm than good. When roofing an existing structure, it’s also important to determine what other changes have been made to the structure in the past.

Erecting proper scaffolding is often the essential first step in the chimney repair process.

Current building codes and modern engineering make the new homes built more efficient and less prone to these types of problems; however, there are millions of older homes out there that need to be retrofitted or in some case “re-fixed” or “unfixed” to make them work.

The one big advantage we are working with today is that there are very few “roof overs” done on steep-slope roofs, as most districts require that the old roof be removed before a new one is installed. This will allow for all of the flashings to be replaced. If chimneys still exist but are no longer used, the possibility might exist for them to be taken down, having the framing replaced and the opening covered and roofed. If this is done, make sure that you take the chimney down to the height of the ceiling joists, cap it at that point and insulate above it.

When re-roofing an existing structure, it’s important to inspect the roof system for damage and determine if any changes have been made to the structure in the past.

If the chimney is left in place, it is important to have the masonry mass inspected and fixed before the roof is done to avoid damaging the new roof. Install a new flashing/counter-flashing system, and make sure to follow the directions printed on the shingle wrappers. My objective here is not to reinvent the wheel, but to make sense of what they are telling us to do. Many years ago, there was a commercial with a tag line that went “It’s not nice to fool Mother Nature.” The truth is that you can’t. If you work with gravity and nature, on the other hand, you can eliminate a lot of the problems we are fighting with on the roofs and in this business. The choice is yours.

This architectural detail from The NRCA Roofing Manual: Steep-slope Roof Systems—2017 shows the proper method for flashing a masonry chimney. Detail courtesy of the National Roofing Contractors Association (NRCA).
The packaging for shingles often contains product-specific details for flashing chimneys and valleys, such as this diagram for CertainTeed’s Carriage House shingles from the CertainTeed Shingle Applicators Manual, 14th edition. Detail courtesy of CertainTeed.

About the Author: John R. Crookston is a roofing contractor and consultant located in Kalamazoo, Michigan. He has more than 60 years of experience in the roofing industry and has written technical articles for a variety of publications under the pseudonym “Old School.”