Dome Magazine: Summer, 1993 Vol. 5 No. 4
Although the dome appears to be radically different from the conventional roof, it is shingled using the same basic techniques as conventional construction. The many faces of the dome may be considered as a number of small, triangular roof planes and treated as such.
One of the most often heard objections to domes is their propensity to leak. This is no idle lament, as the problem of keeping the water out of the dome is very real, unless the dome is properly sealed.
Our first experience was in 1975, when the company for which we were working as salesmen was recommending and selling a plastic roofing that was supposed to be the end-all of roofing materials. It was not. The plastic lasted about two years, and water started to enter the structures. As sales agents, we looked pretty bad. Since that time, we have been approached by roofing material companies, pushing their roofing products. When asked about their warranties, their answers have been far from satisfactory, at least when measured against conventional residential roofing.
This experience prompted us to revert, if you will, to the old standard of roofing materials: shingles. Though many roofing materials have been promoted in the past in conventional housing, the use of asphalt or fiberglass based shingles has proved the most economical and the most satisfactory. Some homes have been shingled with cedar shingles, but these are difficult to install and exceptionally expensive.
Standard Building Materials
The use of standard building materials is generally recommended because the material costs are generally lower due to the economics of scale. The more generally used, the lower the cost.
To make certain that the dome is sealed properly, we recommend that all of the intersections be caulked prior to shingling. This is done for two reasons. First, the plywood on the dome exterior will shed water because of the roof angle, and can be exposed for a period of time without any serious damage. However, if the dome floor does get wet, being flat, and usually plywood, there is always a possibility of damage. Sealing the intersections of the primary dome surface assures that the floors will remain dry. There is a natural time delay between installing the details of extensions, doors and windows and the final application of the roofing material. The sealing of the intersections insures that the floor will remain dry.
The second reason for sealing the intersections is that it provides a third barrier against water damage in the event of the failure of both the roofing felt and the shingles. This is a good time to have all of the above details flashed, and the eave edge installed.
Accessibility
One of the problems of shingling the dome is its accessibility. Because of the dome’s spherical shape, it is necessary to use a little different approach than with the square building. The least expensive and the easiest to handle seems to be the use of three ladders with ladder jacks and connecting planks.
Basic Steps for Shingling
Beginning with the base course of the point-up triangles, apply a layer of fifteen-pound roofing felt, lapping the upper layer over the bottom sheets, a minimum of two feet, and lapping the end joints at a six-inch minimum. Once the point-up triangles are finished, the next step is to apply fifteen-pound roofing felt to the point-down triangles. The edges should over lap the point-up triangles edges about six inches.
The first step is to apply a starting strip of shingles, as recommended by the manufacturer. The start is made at the eave edge detail of the point-up triangles.
The point-up triangles are shingled first, using the fastener type and spacing recommended by the manufacturer. The overlap over the intersection should be four to six inches These shingles may be cut, but it is not necessary.
Beginning with the eave edge on the ventilation detail, single all point up triangles in the row. Maintaining level and lapping adjacent triangles by at least 4″. Do not cut any shingles along these lines.
Next, do the point-down triangles. A line is drawn parallel to the intersecting roof plane and offset about four feet. Another horizontal lie is drawn, using a level, about halfway up on point-down triangle. The shingles are then cut to the line drawn parallel to the intersection. This gives a double lap at the intersection, and the horizontal line helps keep the shingles level.
Shingle all point down triangles by first snapping chalk lines 4” over the seams (where the panels join) of the shingled point up triangles. Then using a level, draw a line with a pencil about a third of the way up on the point down triangle
Hold the shingle level with this line and cut either to the left or right edge where the shingle intersects with the chalk line.
Slide the shingle down to the point where it intersects with the opposite chalk line. Check to make sure that the shingles are level.
In this photo, you will note that the shingle line over the entry door is definitely not level.
This error is almost certain to cause trouble later. Keeping the shingle line level is a basic requirement of proper installation; it keeps the water running downhill.
Drawing 3 represents the shingling of the side pentagons.
Drawing 4 shows the necessary procedure to shingle the top pentagon. Following these instructions will assure that the dome will be waterproofed, and the story of leaking domes can be put to rest, once and for all.
Beginning at the pentagons you will encounter horizontal diamonds. These will be shingled as one unit.
Snap two lines 4” over the point down triangles below and using your square, off the vertical strut. Apply shingles cutting where they intersect the lower chalk lines and overlapping where they meet the adjacent point down triangles.
Build up the top course with rolled roofing. Fit the edges flush and tar them down. Then shingle each top panel, butting the panels at the seams and tarring them down. Then apply a Boston ridge over each seam.