Build a Bat House
Outdoor Project: Bats are unrivaled for controlling pests like mosquitoes and for both pollinating plants and dispersing seeds. This simple project will give them a nice home.
Bat houses are wide, tall, and relatively flat. They have a 3″ to 6″ tall landing pad at the bottom, and are fully open on the underside. It’s essentially a box with internal partitions. These partitions are staggered at the lower edge to give the bats something to grasp as they enter their home.
In this house there are four baffles, spaced 3/4″ apart. When the front and back walls are added, there are five “chambers” that run the full height and width of the house. The chambers seem narrow, but our bats like small spaces.
You’ll need some kind of saw, galvanized finishing nails, glue and caulk. I used Titebond III, which is waterproof. Caulking all exterior seams and nail holes is important to preserve heat.
I used cedar boards, but exterior grade plywood would work too. It’s okay if your measurements differ somewhat, as long as you respect the 3/4″–1″ spacing between the chambers. Rough sides of boards should be on the inside to help the critters get a grip – more on this later. The roof is angled at 22.5 degrees to shed rain and snow. The sides, and all pieces that reach the roofline, are cut or beveled at the same 22.5 degrees so the roof sits tightly across all members.
Both sides of the partitions, and the insides of the front and back panel, have a series of shallow grooves on them. The grooves allow the bats to gain a secure grip when they are inside the house.
The bottom edges of the partitions are offset so the bats can enter the house easily. Once it’s attached, the back panel will drop down a few inches lower than the sides.
Into the groove
The time-consuming part of this project is cutting the shallow grooves on all the large surfaces of the partitions, as well as the interior surfaces of the front and back, before assembly. You can avoid this step if you apply 1/4″ plastic mesh to all inside walls. Grooves are about 1/16″ deep and should be approximately 1/4″ apart running horizontally from side to side.
Grooves can be cut with a variety of hand and power tools. I preferred the mitre saw. Note to the novice: don’t become complacent when you are doing repetitive cuts. Even when a table saw has teeth barely peeking out of the table, there is a possibility for serious kickback. Don’t ask me how I know this.
Machine your side panels, then cut them in two, across the grain. They will be positioned one directly above the other, but with a 1/4″ gap between them for venting. Spacing strips that run down the interior of the sides will hold the resulting two parts of side panels together. I suggest cutting all the side spacing strips slightly oversize, and with a 22.5-degree bevel on their tops, then trimming them to length once they’re in place. The front and back pieces are also cut in two to create the 1/4″ vent. They are held together by the exterior vertical trim. The outside corner trim should be ripped to width last in case your measurements differ slightly from mine. Let me warn you that once the box begins to take shape, it becomes very awkward to maneuver. As it comes together, laying it on its side is the only choice.
Add spacing strips
Laminate boards together as needed to make your walls and partitions. Don’t forget that if you’re using standard fence boards, with rounded edges, you have to trim those edges off to get a square, flat edge for gluing. Rip 3/4″ wide strips for the interior partition spacers that run down the sides and for some outside trim. Glue and nail the narrow strips along the side panels, spaced so the partitions will fit nicely between the strips. Nail from the inside out wherever possible. Glue the panels between the sides, into the spaces you have prepared. Lastly, add the vented front and back, the roof, and a landing pad. Add Trim. Caulk. You’re done.
A bat house needs to be hung 15–20′ high, facing south or southeast, and needs at least six hours of direct sunlight per day. The further north you are, the darker you should stain or paint your bat house to preserve heat. Hanging it on a tall metal pole works well. If it’s hung in a tree or on a wooden pole, then a 12″ tall sheet-metal ring around the base will discourage evil climbing predators. What’s important is plenty of open space in front of the bat house for the bats to fly in and out, and to foil predators lying in wait. Hang it in late winter and 10 degrees off of vertical to help keep babies from falling out. Fresh water should be available in lakes, ponds or rivers within a quarter of a mile. When deciding where to hang it, remember that bat poop or “guano” will be dropping from the bottom of the bat house. Guano, by the way, has been used for centuries as a rich fertilizer, and wars have been fought over it.
I asked Professor R. Mark Brigham, Head of the Department of Biology at University of Regina, and Canada’s go-to bat researcher, if he recommended people putting up bat houses in their yards. “Yes,” he said, “not all will get used but they will stimulate fun conversation.”
I am grateful to Edgar Nickel of Qualicum Bay, who kindly scrolled my little bat decoration on short notice.