Roofing a Slanted Round Roof

What To Know Before Starting

• Roofers should know three facts about the roof before beginning work: the diameter of the cone's base, the length of the rafters and the widest piece of shingling that will be used. The diameter multiplied by pi -- approximately 3.14 -- will provide the circumference of the cone's base, while the rafter length provides the cone's height along any one side. Taken together with the preferred shingle size will give the roofer a fairly accurate measure of how many shingles and other materials will be required to complete the job. The smaller the shingle, the rounder the roof will appear. Also, because the pitch is often too steep to work from, roofers should have a scaffolding around the base of the roof, which will create a safe area to work from and a convenient place to store materials.

Underlayment

• While underlayment on traditional roofs is typically applied in a horizontal manner, a round roof requires it to be applied vertically so that it is perpendicular to the eave and parallel to the flow of water. Asphalt plastic cement should be used to adhere the underlayment to the roof, with no overlap greater than six inches. The excessive overlap at the roof's peak should be trimmed prior to placing any shingles down. This protection is particularly crucial in areas with heavy rain, sleet and snow.

Ventilation

• Proper ventilation is crucial in cases where the roof opens into an attic area. A common rule of thumb says there should be one square foot of net free ventilation for every 150 square feet of attic space when only exhaust vents are used, or every 300 square feet with both intake and exhaust vents. For completely circular roofs, a roll-type ridge vent can be installed at the peak. Adding dehumidifers and ceiling fans can also assist in reducing mold and moisture-related issues under the roof.

Flat-Sided vs. Round-Sided Cones

• Flat-sided cones may be pyramidal, hexagonal or octagonal, while round cones have no flat surfaces at all. Roofing a flat-sided cone is much like roofing a standard hip roof, although a chalk line should be snapped around the cone to ensure that each course, or row, of shingles is level with those on the adjacent sides. However, a completely round cone is much more challenging due to the lack of a horizontal eave line with which to use as a guide. Also, less and less material is required for each course as the circumference of the cone gets smaller toward the peak, requiring a variety of shingle sizes that can give a sloppy, random appearance if there is too much horizontal offset. Standard three-tab shingles must be trimmed to get proper vertical alignment and then be applied with precision.

Vertical vs. Horizontal Alignment

• For proper vertical alignment of the shingles, snap two chalk lines from the base of the cone to the peak, with each line about 36 inches apart at the base -- or whatever the length of one shingle is. Use these lines as a vertical guide when laying the shingles. Full shingles will fit between the two lines in the first couple courses, but must be trimmed as the roofer makes his way up the cone. Also, snap several horizontal chalk lines from the base to the apex, each three courses deep. These lines not only assist with horizontal alignment, but also help the roofer gauge the size a shingle should be at any given section of the roof by allowing them to compare it with other shingles in that section.