Pinwheel Tree Bench

A cedar surround with a different spin

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Until recently, the idea of building a tree surround left me a little baffled. Most of the ones I’ve seen are the functional equivalent of eight or 10 chairs placed in a circle with their backs facing one another — a painfully antisocial arrangement. In addition, they tend to be complicated to build, with a lot of compound miter angles and some very tricky joinery. And what happens to the thing when the tree grows?

My wife and I have a lovely elm tree in the backyard, and she brought up the idea of building a tree bench around it. I scoffed at first, pointing out my three reasons why tree benches don’t make sense. She thought about each reason carefully and offered solutions, and by the end of the conversation  the tree grows. And individually adjustable legs let you level the benches on almost any terrain.

The bench we built will wrap around a 2-ft.-dia. tree and still leave enough overhang at the end of each bench for a comfortable seat. If your tree is larger, adjust the plans by lengthening the benches. As a rule of thumb, multiply the diameter of the tree by 2.5 to find the minimum bench length, and add a bit to allow for growth. If possible, check standard deck board lengths with your lumber supplier before finalizing your plan so you can choose a bench length that minimizes waste.

Support frames
To begin, cut the seat supports (8 in., 48 in. and 55 in.) to length from 2x4 cedar. You’ll need one seat support of each length per bench, along with an end support, a short cross support and a pair of longer cross supports. Everything fits together with butt joints except for the half-lap joints between the middle seat support and the two longer cross supports. Use a sliding compound miter saw set to cut 1-3/4 in. deep to make the half-lap notches (photo 1). It’s easiest  if you gang all of the middle seat supports together and then cut the notches in multiple passes. You could use a circular saw and straightedge, a table saw or a router if you don’t have a sliding compound miter saw with adjustable cutting depth.

Cut the notches in the longer cross supports the same way; then make the half-lap joints. Reinforce each joint with polyurethane glue and a couple of 3-in. screws. (We used stainless steel deck screws that won’t discolor the wood; photo 2.)

Next, attach the end supports and the front cross supports to the frame assemblies, squaring the frames as you work. Then attach the outer seat supports to the frames and add a 2x4 cleat at the square end of each frame, flush with the top edges (photo 3).

Make the legs
The bench legs consist of two sections of 2x4 stock sandwiched between 14-in.-long 1x4s. The upper 2x4 is fixed between the 1x4 outer legs, flush with the tops. The lower 2x4 fits loosely into the gap between the two outer legs, and a 3-in.-long slot routed through it allows it to be adjusted up or down on a carriage bolt.

To make the leg units, first cut the outer leg sections to length from 1x4 cedar, and bevel one end of each outer leg at 45 degrees (photo 4). Cut the inner legs to length. Then join two outer legs and one inner fixed leg to make 16 leg assemblies. The tops and edges of the leg assemblies should all be flush, with the bevel placed at the bottom of each outer leg. Insert an adjustable lower leg into each leg assembly so that it butts against
the bottom of the inner fixed leg. Then drill a 3/8-in. guide hole centered side-to-side through all three leg parts, 1-3/4 in. up from the bottom of the outer leg bevel (photo 5). The hole in the adjustable lower leg will be used as a starter hole for cutting the leveling slot.

Next, cut the slots in the lower legs on a router table with a 3/8-in. straight bit. The slots should start at the guide hole and stop 1 in. from the top edge of the leg. Because the leg is 1-1/2 in. thick, we had to rout the slot into one face and then flip the board and finish the cut from the other side (photo 6).

Attach the upper leg assemblies to the frame using polyurethane glue and 3-in. screws (photo 7). Then slide the lower legs into place and hand-tighten the nuts onto the carriage bolts to hold them there — use a washer on the nut end of the bolt.

Make the seats
Once the legs are in place, cut the seat boards to the full 60-in. length from cedar decking. (Cut a 10-ft. board in half to make two boards.) Although they’re billed as 5/4 x 6-in. boards, the stock we found had actual dimensions of 1 x 5-3/8 in. Arrange three seat boards spaced 1/2 in. apart on each frame assembly, and allow for a 1-in. overhang at the front and back edges. (Getting the overhang correct is more important than consistent gaps between boards, so make any compensations to the gaps, not the overhangs.) The seat boards should also overhang the end supports by 2-1/2 in., revealing 1 in. of the cleat (photo 8). Attach the seat boards to the supports using 1-5/8-in. deck screws.

When all the seat boards are attached, use the grid drawing shown on p. 12 as a guide to make a template for the profile. Trace the shape onto the seat boards and cut along the lines with a jigsaw (photos 9 and 10). Ease all the edges, and make sure all the screws are countersunk. If you want the benches to retain their cedar color, apply a coat or two of UV-resistant wood protectant.

Set up the benches
Once you’ve completed the benches, simply arrange and level them around your tree. Start with all the adjustable inner legs fully raised into the gaps between the outer legs. If the benches are not level, find the high point of the assembly, and lower the other inner legs as needed to bring the seats above them up to level. You’ll have about 3 in. to play with, which should be enough for most dips. If necessary, slip a couple of flat rocks into deeper dips.

When the seats are roughly level, clamp the seat boards to the cleat on the adjoining benches, making sure the benches fit together flush. Make final leveling adjustments; then tighten the nuts on the carriage bolts (photo 11). For added strength, run a couple of deck screws through the outer legs and into each adjustable leg after the benches have had a chance to settle.

Drive a few screws through the seat boards at the square end of each bench and into the cleats to pin the assembly together. As the tree grows, you can loosen these screws and shift the benches farther apart. And if the adjustable lower legs ever start to rot from ground contact, you can replace them. Just measure the distance from the bottom end of the damaged adjustable leg to the bottom of the inner fixed leg, cut a new 2x4 to that length, and glue and screw it in place — no need to bother cutting a slot.

 
STEP-BY-STEP INSTRUCTIONS

Step 1: Cut notches for the half-lap joints with a sliding compound miter saw or a
circular saw and straightedge. Gang similar parts together first. Clean out waste wood with a chisel.

Step 2: Reinforce the half-lap joints with 3-in. screws (exterior-rated) and waterproof glue, such as polyurethane glue. Drill pilot holes when driving screws into edge grain.

Step 3: Attach a 2x4 cleat to the square end of each completed frame, flush with the top edges. The reveal of the cleat will fit under the overhanging seat boards of the adjoining bench.
 
Step 4: Bevel the bottom edges of the 1x4 outer leg components at 45 degrees to soften the appearance.
 
Step 5: Drill a 3/8-in. hole for the carriage bolts through all three layers of each leg. The hole in the adjustable inner leg is used as a starter for routing the leveling slot.

Step 6: Cut a 3/8-in.-wide x 3-in.-long slot in each adjustable leg. A router table simplifies the cutting job, but you’ll still need to make two cuts, one from each face, to
get through the 2x4 stock.

Step 7: Install the inner adjustable legs with carriage bolts, nuts and washers. Then attach the completed leg assemblies to the bench frames using polyurethane glue and 3-in. screws.

Step 8: Attach seat boards to the supports. Leave a 1-in. overhang on the front and back, and leave 1 in. of the end cleat exposed.
 
Step 9: Make a template using the grid drawing, and trace the curved profile onto the ends of the seat boards.

Step 10: With a jigsaw, trim off the ends of the seat boards along the profile outline. For best results, cut just a hair outside the lines and then sand or file up to the line.

Step 11: Adjust the benches until they are roughly level, then clamp them together. Make final adjustments, and then tighten the carriage bolts to secure the legs at the correct height. Finally, screw the benches together at the cleats.