Roofs – More Than You Wanted to Know

by Steve Eno

There are many features that make up a good bluebird box — such as hole size, distance from the hole to the floor, floor size, ventilation, etc. One very important feature that is often overlooked is the ROOF.

There are several types of wood and other materials that can be used for roofs. Cedar has a long life and is the wood most commonly used for roofs on commercially sold bluebird boxes. Redwood, pine, and Douglas fir are other woods that are often used. If pine, Douglas fir or exterior plywood is used, it is recommended that two coats of paint be applied since the roof is the part of the box most exposed to the elements. In addition, using a light color paint will help reflect heat off the roof. While wood roofs are the standard, there is a new concrete-fiber siding product that has a 50-year warranty. Although it has not yet been extensively used for box roofs, it has been field tested and it looks like a promising alternative to wood.

Since the roof is usually the first part of the box to deteriorate, it is important to check the condition of your box roofs in the spring and throughout the season. Check for cracks that may allow rain to enter the box. A light-colored shingle may be placed over a cracked roof. A wood preservative applied annually will extend the life of the roof. (Of course, any preservative should be applied before or after the nesting season.)

Bluebird nestlings, with an adequate food supply, can normally survive cool or even cold weather conditions. But when nestlings get wet, the chance of them dying from hypothermia greatly increases. This fact emphasizes the importance of a solid roof with adequate overhangs. A box with large roof overhangs will not only help shade the box during hot weather but it will also keep rain from blowing into the box through the entrance hole or through the ventilation holes or slots. But just how much overhang is necessary to keep a wind-driven rain out of a box? Curiosity got the best of me and I began a personal “quest’ to find an answer to that question.

At what angle does rain fall during a wind-driven rain? I thought it sounded like a simple question that any weatherman would be able to answer. So I first posed the question to Lincoln’s “Dan, the Weatherman” (who I knew had an interest in bluebirds). His immediate response was “Tough question. I have no idea.” and he referred me to a meteorologist at the University of Nebraska whose answer was almost identical to what Dan had said. The meteorologist then referred me to Professor David Stoksbury, a regional climatologist at the University, who has a background in physics. Professor Stoksbury’s initial response was once again similar to the two previous answers I’d received. But after giving it a little more thought, he found the question interesting and made it his “challenge” to find the answer.

Professor Stoksbury told me that he first had to come up with the size of an average rain droplet during a thunderstorm. That task was complicated by the fact that the size of the droplet increases with the severity of the storm. As the size of the droplet increases, so does the speed in which it falls. David admits there was a fair amount estimation in his efforts, but he feels there is a close degree of accuracy to what he determined. The chart below takes into consideration the droplet size, the speed at which it falls, and the wind speed to determine the angle at which rain falls:

Wind Speed    Angle at which rain falls
10 mph       22 degrees
20 mph       39 degrees
30 mph       53 degrees
40 mph       61 degrees

Almost as important as the overhang in keeping driving rains out of the box, is the direction in which the box is ‘facing. David said that in the Midwest prevailing winds during a thunderstorm are most often from the west or south. So, facing your boxes toward the east or north will also help keep rain out.

After reading this, I hope you don’t feel compelled to try to determine the angle of the roof overhang on your bluebird boxes. Generally speaking, an overhang of 2 Y2inches on the front of the box and 1 inch on the sides of the box is usually adequate. Because there is such a difference in box designs, there are no set rules — but, once again generally speaking, ‘the more overhang, the better.”

Originally printed as a supplement in the Winter 1998-99 Bluebirds Across Nebraska Newsletter BANner