Attic/Roof Ventilation - Installing a Ridge Vent

Attic and roof ventilation is probably the least understood element of a healthy home. Proper ventilation is absolutely necessary, not only to your home's health, but to yours. This article will show you how to install ridge vents using some unique roofing and hand tools.

Although I no longer do professional contracting, I am in the process of renovating my own newly-purchased, 100-year-old building for use as both my residence and my office. As part of that renovation, I too must install proper attic ventilation. So, I thought this might be a perfect opportunity to share my ventilation knowledge and experience with you.

Types of Vents

There are two kinds of vents: inlet vents and outlet vents. Having only one or the other is the equivalent of having neither. Therefore, for proper attic and roof ventilation, both types of vents must be present, and in equal amounts of net free airflow.

How Many Vents Do You Need?

What is net free airflow? Simple; it’s the amount of air that can pass through the "gross" opening you create, minus the area of the screens, mesh, louvers, or other blockages that reduce that airflow. Each manufacturer rates their various vents by the net free airflow capacity. A basic rule of thumb is 1/300, meaning that for every 300 square feet of attic floor space being ventilated, one square foot (144 square inches) of net free airflow must be present in both inlet and outlet vent capacities. For areas insulated without a vapor barrier, that ratio doubles to 1/150.

Preparing the Roof

Before we begin, here is a helpful hint: if your roof is really hot, spray it down with water periodically. The evaporation of only one gallon of water dissipates 8,265 BTUs of energy. The sheen in various photographs is from wetting down the roof on the 90-degree day on which I was installing these vents and is the reason the roof appears to be changing colors. Use a THOR® Shingle Clip to secure your hose to the roof, when not in use.

Note: The photographs in this article are a mixture of three different sections of roof, including a 24-foot section over the intersecting gable roof, a 12-foot section over the hip roof of the garage, and a 44-foot section over the main gable roof. I chose the photograph most appropriate to depict each step in the ridge-vent-installation process, just in case you become confused about the varying backgrounds. The last three photographs are of those three completed roof lines, respectively.

Mark Vent Locations

After the existing roofing caps have been removed, snap two chalk lines marking the area on the overlapping shingles to be cut away and removed for the vent opening. This process will also mark the locations where you will cut away the roof sheathing, allowing airflow out of the attic space.

Make Vent Openings

Always try to cut the sheathing with a plumb cut, meaning that the blade should be as vertical as possible. Most manufacturer's instructions will provide the necessary dimensions of the cut size and will tell you to use a circle saw, but I prefer using a reciprocal saw with a bi-metal blade, which will cut through any nails I failed to remove in the cutting path.

Now, you have your "gross" opening cut through your roof sheathing. If you have soffit vents, hot air will immediately begin to flow out of this opening, just like heat flows out of a chimney. If you do not have soffit vents, like me, you will feel only slight flow. Remember, you need both types of vents to achieve the needed airflow in the attic, so you’ll need proper intake for proper exhaust and vice versa.

After cleaning up the roof from the sawing process, strike two more lines, marking both outside edges of the ridge vent to be installed.

Choose a Vent

I am installing "Shingle Vent II" by Air VentTM. My friend Roger B. Lyle, President of R.B.L. Builders, Inc. from Mahtomedi, Minn., said roofers in the heavy snow-belt areas have had the fewest problems with these vents. Having poor ventilation in this region can lead to damaging ice dams in the winter.

"Shingle Vent II" comes in four-foot sections, 10 sections per box. Pre-drilled holes are present to accommodate the initial aligning and nailing to the roof.

Mark the Caps

Since the caps being used are a standard five-inch reveal (12-inch high) shingle, a standard five-inch reveal THOR® Roofing Layout Tape was used. I am simply stapling the tape to the top of the vent at one end, and then unrolling it to the other end, instantly and accurately marking the location of the caps, without any manual measurement. This is done as quickly as I can unroll the tape and staple it to the center line of the vent, about 15 seconds.

Finish the Installation

With the cap tops instantly marked by the roofing layout tape, I am ready to complete this project. I simply place the tops of the caps on the center markings of the roofing layout tape, identifying the five-inch increments, and nail them in place.

Always start the roofing layout tape at the end of the vent that requires you to unroll the tape into the direction of the prevailing wind. That trick will ensure that the caps will always be installed with the best wind-resistance orientation and will therefore be less susceptible to weather infiltration.

Finally, the 44 linear feet of newly-installed ridge vent on the main roof of this 100-year-old building, constructed with individual board roof sheathing, is perfect!

Note: Ridge vent manufacturers recommend removing gable vents and other outlet roof vents, such as the nine-inch vents visible in several of my photographs. Their presence interferes with and reduces the ability of the ridge vent to remove heat and humidity from the attic or roof space. I will be removing my old nine-inch vents.

As a postscript to this article, the unique roofing and hand tools utilized in this ridge vent installation included: Magichalk®, THOR® Double Chalkers, THOR® Shingle Clips, and THOR® Roofing Layout Tapes.