The underlayment should be applied vertically, perpendicular to the eave, as shown in this figure from the ARMA Technical Bulletin titled “How to Shingle a Cone Roof.” (Copyright Asphalt Roofing Manufacturers Association, reprinted with permission.)
Cone-shaped roofs are one of those projects that contractors either love to do or avoid like the plague.
A prominent architectural feature on Queen Anne- and Norman-style homes, cone-shaped roofs are also found on Armenian and Georgian churches and medieval towers and castles. Their sloping and curved geometric surfaces can be difficult and labor intensive to shingle, especially for roofers who are accustomed to working only with straight lines.
Whereas a simple pitched roof typically has two or more sides and a hip roof has at least four sides, a conical or turret-style roof can appear to have an infinite number of sides. Some cone-shaped roofs have three to eight flat sides that create more of a geometric shape, such as a pyramid.
So, the challenge is: How do you install flat shingles on this intricate, rounded surface?
Thanks to their flexibility, modern asphalt shingles can be installed on roofs of any shape.
To begin shingling a cone roof, you need to know three measurements: the length of the rafter, the diameter of the cone and the widest piece of shingle you’ll be using.
To determine the distance around the base of the cone, multiply its diameter by 3.14. For example, if the diameter is 20 feet, the perimeter would equal 62.8 feet. With a 12-inch-wide shingle, you would need 63 shingles in each row around the cone.
Precise calculations are necessary because shingle pieces will need to change shape and become narrower as you move from the base of the cone up to its peak.
Cutting the shingles is a task you can do ahead of time, by creating a template, or when you get to a particular part of the installation.
On a flat-sided cone roof, use the standard hip and ridge installation method. (Copyright Asphalt Roofing Manufacturers Association, reprinted with permission.)
Because cone-shaped roofs are usually steep and high off the ground, consider hammering footholds into the roof for stable support while you work. Better yet, use scaffolding, which not only provides a platform for leaning a ladder onto the roof, it also serves as an easily accessible shelf for your roofing materials and tools.
Underlayment and Ventilation
With preparations complete and safety equipment in place, you’re ready for the fun part: installation.
First, start by applying a good quality underlayment to the deck per the manufacturer’s instructions.
The underlayment should be applied vertically, perpendicular to the eave, following the flow pattern from the cone’s peak to its base. This process will help to prevent the material from wrinkling or buckling. You should end up with an overlap near the peak, which can be trimmed during underlayment application and before installing shingles.
Continue to overlap the underlayment vertically as you progress up the cone and use asphalt plastic cement to cement the lap edge. Alternatively, you can use a peel-and-stick underlayment. A self-adhering underlayment protects the eaves and flashing from wind-driven rain and covers any possible gaps between abutting shingles.
Next, check the ventilation. If the cone is open to the attic area, it should be part of the ventilation system. To accommodate static ventilation in the main portion of the attic, increase the requirement for the net-free area by the same square footage as the cone-shaped room. If the area is open to the living space, a ceiling fan can help force moisture and heat from the cone-shaped room to the main living area for dispersal. Using a room dehumidifier may also be helpful.
When shingling a rounded cone roof, divide the roof into three distinct zones. (Copyright Asphalt Roofing Manufacturers Association, reprinted with permission.)
When working with a completely circular cone, use an off-peak, roll-type ridge vent at the peak for positive ventilation. The formula for cone-shaped rooms is consistent with any other residential area:
Equal intake and exhaust vents: 300 square feet of attic area = 1 square foot of net-free vent area
Exhaust vents only: 150 square feet of attic area = 1 square foot of net-free vent area
In cases with no ventilation, make the homeowner aware of potential issues with accelerated wear and how it can affect the product’s warranty. For more specific requirements, contact the shingle manufacturer.
Shingling Flat-Sided vs. Rounded Cones
After installing underlayment and addressing ventilation, you can start applying shingles.
If you’re working with a flat-sided cone roof, you can use the standard hip and ridge installation method. Snap vertical chalk lines from the cone tip to the eave center on each of the flat sides. Then apply shingles to the flat areas, cutting at the hips or joints. Use a standard hip and ridge shingle to complete the hip joints.
To ensure a continuous roofing line, snap horizontal chalk lines around the cone so that shingles will line up on adjacent sides.
Shingles on steep-sided cone roofs — those greater than 21/12 slope — may need to be hand sealed with asphalt plastic cement. Check the manufacturer’s instructions for steep-slope application.
When shingling a rounded cone roof, you won’t have a horizontal line to follow because of the curvature. If you try to create a line, butting the sides of the shingles together, the shingles will gradually curve downward and won’t correctly align when you encircle the cone.
Side overlap of shingles is more noticeable in the upper portions of each cone. Trim shingles at an angle to make the joint parallel to water flow. (Copyright Asphalt Roofing Manufacturers Association, reprinted with permission.)
To make installation easier, divide the roof into three distinct zones. Start applying shingles to zone one, at the bottom of the cone, and then work your way up to zones two and three.
While you are nailing, have another crew member help hold the shingles down around the curve so they are flush against the surface.
Because the cone shape tapers from the base to the peak, succeeding courses require less material.
The degree of horizontal offset and varied shingle cutouts will create a random appearance. When using standard three-tab shingles, trim each shingle for proper vertical alignment. A simpler alternative would be to use a randomly applied shingle that doesn’t need to be vertically aligned.
Shingling a cone-shaped roof may be challenging, but with the proper knowledge and execution, you can restore this architectural focal point to its full glory.
For more information from Atlas Roofing, including technical bulletins, installation instructions and product data sheets, visit atlasroofing.com.
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