Build a Bat Box

Do you have bats in your vicinity? Perhaps bats in your home that you would prefer to move elsewhere? Build a bat box for those little flying insect eaters. (One mosquito-eating bat can consume 2,000 insects per night!)

There are many designs for bat houses available on the Internet. Review several, then build one that fits the materials that you have. This article focuses more on principles than exact dimensions.

Steps

Preparation

  1. Consider the requirements of a bat box:

    • Bats need enough space for a small colony.
    • They need a place to land to enter the box.
    • They need rough surfaces to cling to.
    • They need an airtight enclosure as they will regulate the interior heat by moving up or down in the chamber.
    • The box should be installed in the right location (somewhere where they will get 4-5 hours of morning sun, 15-20 feet above the ground).
  2. Review several plans. Some are better than others. Think about the concepts which are common to all.
  3. Gather materials and tools and clear a suitable workspace.
  4. Think about the work plan; you will measure, cut and fit materials, and then assemble.

Example

Note: The images are for a slightly different box than the measurements given it the article.

  1. Cut a {{safesubst:#invoke:convert|convert}} section of 1x8 rough sawn board into the following:

    • Three pieces {{safesubst:#invoke:convert|convert}} long (creating the sides and back of your "house")
    • One {{safesubst:#invoke:convert|convert}} long (front of the house)
    • Two {{safesubst:#invoke:convert|convert}} pieces (two partitions)
    • One {{safesubst:#invoke:convert|convert}} piece (the third partition)
    NOTE: Set the saw blade to be very shallow and cut grooves (kerf cuts) in both sides of the partitions, the inside of the front board, and the inside of the back board. The pictured design includes a "landing area"; cut grooves in that also. For an airtight seal, don't cut all the way to the sides of the boards. The purpose of these cuts is to provide the bats something to cling to. Some designs suggest using a screen, but "roughing up the wood" is cheaper and more durable.
  2. Measure {{safesubst:#invoke:convert|convert}} on one side of two of the {{safesubst:#invoke:convert|convert}} pieces.
  3. Cut diagonally from the 17.25 mark to the closest end corner on the other side of the board. Repeat for the second marked {{safesubst:#invoke:convert|convert}} board. This makes the sides of your bat house.

  4. Set your miter saw for a 33 degree cut.
  5. Angle cut the "top" ends of the following boards:

    • Remaining {{safesubst:#invoke:convert|convert}} board
    • 17.25 piece
  6. Measure and mark the two side pieces parallel to their long sides as follows:

    • {{safesubst:#invoke:convert|convert}}
    • {{safesubst:#invoke:convert|convert}}
    • {{safesubst:#invoke:convert|convert}}
    • The last line "should" end up {{safesubst:#invoke:convert|convert}} from the opposite edge.
  7. You may wish to cut and test fit all the parts before you start caulking and drilling and screwing.

  8. Assemble the sides and front and back pieces with their angled ends up.

    View image at full scale to appreciate the kerf cuts in the back board.

  9. Nail or screw the front, back and two sides together. (Side pieces overlap the front and back pieces at their edges.)
  10. Insert the partitions by laying the house on its side and sliding them into the box.

    It may be easier to fit the partitions a set distance apart by using a small "spacer stick" as depicted.

    Since the stick is 3/4" thick, this gives the proper thickness to each chamber.

  11. Center each partition on a line and nail or screw into place.

    • Position each partition so that its angled edge will lay flush with the angled roof.

      Two partitions are 3/4" apart, the larger back chamber is 1 1/2" wide (two sticks).
    • The shortest partitions should be at the front of the box, while the {{safesubst:#invoke:convert|convert}} should be positioned at the back. Cut a passageway through the partitions to allow movement from one section to the other.

  12. Place the 10 inch 1x10 on top of the angled section so that it lies flush with the "back" of the bat house and overhangs the "front" of the bat house.

  13. Nail or screw the roof in place.

  14. Caulk all exterior seams. It is good to add a little bead of caulk along the seams as you are assembling the parts. The box needs to be air-tight in the top part.

    • Side view of bat box interior arrangement.

  15. To hold in additional heat (and make the home last longer), staple a black roofing material to the top.

  16. Drill two ventilation holes about 4" from the bottom of the box. The holes should be drilled in the front of the box, upward at a 40 degree angle. This angle is used to prevent rain from running in.

    A chain can be added by attaching to two screw in the top of the back part.

  17. Mount the bat box {{safesubst:#invoke:convert|convert}} off the ground. Bats prefer a temperature range of {{safesubst:#invoke:convert|convert}}. Locate your bat house accordingly. (orientation to sun, prevailing winds, etc). They like morning sun on the box. This box was hung up in a tree which catches the morning sun.

Tips

  • Do not paint the interior of the house. It may repel bats and it will fill in the nooks and crannies the bats need to hold on to.
  • Nails are cheap, but they are harder to install precisely, tend to split the wood, and do not withstand the rigors of the elements. Use screws if you want your box to last for more than one season.
  • You could paint the bat house exterior light or dark to either cool down or warm up the interior. If you use a black water-based stain on the exterior, it will help hold heat and will help hold bats.
  • Use rough sawn wood, preferably cedar. Rough sawn wood gives the bats something to hang from and crawl on. Cedar will not deteriorate as quickly as some woods. Additional roughing up will be appreciated by the residents.
  • Bats enter the house from the bottom. Leave it open. Cut a longer board for the back and use the lower part of it for a "landing pad".

Warnings

  • "Dry fit" all parts together after cutting. This will ensure that everything fits right, and may give you ideas on where screws should be placed to hold it all together. It is harder to make changes after it has all been drilled, caulked, and screwed together.
  • Measure twice, cut once. Save materials by double checking everything before cutting.
  • Take sensible precautions when using tools.
  • If you find a bat in your home, speak with your doctor about your risk for rabies, since even an unnoticeable scratch may transmit the disease.
  • If you live in the Ohio River Valley, avoid bat droppings, since they often carry Histoplasma capsulatum, a fungus that causes lung disease.

Things You'll Need

  • Power saw with miter adjustment
  • Hammer (alternatively, screws and screwdriver)
  • Ruler / tape measure
  • Pencil
  • {{safesubst:#invoke:convert|convert}} section of 1x8 untreated, rough sawn cedar
  • {{safesubst:#invoke:convert|convert}} section of 1x10 untreated, rough sawn cedar
  • 20 or so nails or {{safesubst:#invoke:convert|convert}} exterior wood screws
  • Two drills are handy; one to drill holes, the other to drive screws.
  • Caulking material to seal the joints.
  • Black roofing material.
  • An adjustable work table is useful for holding boards for cuts and when assembling the parts.

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