Everything you need to know to design and build a rooftop ventilator.

Figure 1
A gravity-type ventilator, shown in Figure 1, is a roof opening protected against rain and snow.

The protection begins with a curb that is placed around the roof's opening and properly flashed. The purpose of the curb is to keep any rain, snow and ice that accumulates on the roof from entering the dwelling.

On the top, the curb is a series of flashings that allow you to attach the hood or cowl of the ventilator without allowing any water to penetrate the curb. The gravity ventilator is designed to either allow fresh air to enter or to exhaust air from a dwelling.

The hood, otherwise known as the cowl of the ventilator, can be made in several different ways. The most popular, illustrated in this article, is a 135-degree gooseneck, similar to those shown in architectural manuals published by the Sheet Metal and Air Conditioning Contractors' National Association.

There are three separate sections to this ventilator hood that need to be fabricated and assembled, as shown in Figure 2.

c The base of the hood that attaches to the curb.

c The elbow that extends from the base, designed to keep out the rain and snow.

c The screen to keep out birds and other debris such as leaves.

The base of the hood includes the first straight section and the first 45-degree section of the bend, which is shown in Figure 2. Begin laying out the cheek by drawing two parallel lines the width (Point A to Point B) of the fitting and extend a minimum height of 12 inches.

From the top of the straight section shown as Line AB, extend the line to C; the distance from Point B to Point C is one-half the width of Line AB. From Point C, make a line at 45 degrees to extend near Point E, as shown. Set your trammel points to the distance of Line CB and using Point C as center, draw an arc from Point B to Point D.

Figure 2

Trammel points

Reset the trammel points to the distance from Point C to Point A, and again using Point C as center, strike an arc from points A to E. Add 1 inch parallel to Line DE for the 90-degree bend as shown in the detail. There is no change in width from A to B and D to E; however, the width of the discharge opening (F to G) should be increased by 2 inches to make up for the free area lost using a quarter-inch mesh screen. This second elbow is no different then your basic change elbow, shown in Figure 3.

The radius from Point C to Point D is the same radius as previously used from points C to B in Figure 2. However, as in all change elbows, shown in Figure 3, to make an arc from points G to E, a new point (H) must be established. To establish Point H, set the trammel points the distance of G to C and using G as center, make an arc at Point C, as shown.

Then, using the same distance on the trammels and Point E as center, make an arc that intersects the previous arc. This intersection becomes Point H.

Again, leaving the trammels to the same distance and using Point H as center, strike an arc from G to E. Using the same center for the radius as in H and C, add or subtract the necessary allowance for the flange. This is typically a quarter-inch. Depending on the type of flange necessary, this measurement can change.

Figure 3

Laying out the heel

Once the two cheeks are completed, begin the layout for the fittings' heel and throat by obtaining the measurement of lines EG and DF from the top piece, and BD and AE from the bottom piece. You would add 12 inches to the length, as shown in Figure 2.

The last step is to install some sort of screening. Typically, this material would be of quarter-inch mesh. Figure 2 shows the additional 2 inches of material added to the end of the cheek.

When installing the screen in this manner (known as being "clinched"), it becomes more permanent and harder to service. A better method is shown in Figure 4. It allows the screen to be removed and serviced much easier. The end view illustrates the use of angle iron attached to the hood outlet. The edge of the screen is clinched between a metal striping and fastened to the hood with screws.

Figure 4

Weathertight installation

Figure 5 shows how the hood should be mounted to the curb to maximize a weathertight installation. The curb flashing in Figure 5 is not directly fastened to the curb, but instead is held on the curb by a second flashing that actually hooks to the main flashing. This second flashing, shown as Note 1 in Figure 5, is attached to the inside of the curb. The hood is then attached to the curb flashing from the side and sealed.

Figure 5

Flashing and the curb

Also take note on the clearance between the flashing and the curb itself. This clearance is necessary for the roofing material to be attached to the curb. There are as many variables as there are curbs to be flashed. So there are no particular allowances on the flashing itself. SMANCA has much material on these standards and for those who are not familiar with them, it would be wise to invest in some of their materials.

(For more information on manual sheet metal layout and exercises, visit www.thesheetmetalshop.com).