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A wooden garden bench with through-tenon joinery is a practical intermediate woodworking project. The joint is strong, visible from the outside, and easier to inspect than a concealed mortise and tenon. It also gives the bench a traditional look: the tenon passes completely through the leg and is secured with a wedge on the far side.
This plan is for a bench about 48 inches long, 18 inches deep, and 18 inches high at the seat. It uses common outdoor-friendly lumber, but the design can be adapted to hardwood or reclaimed stock. Work accurately, especially on the mortises. A through tenon that is slightly loose will allow the bench to rack, while one that is too tight can split the leg.
Contents
Materials and Dimensions
Use straight, dry stock with no large knots near the joints. Cedar and redwood are light and weather-resistant. White oak lasts well if maintained, but it is heavier and harder to cut. Pressure-treated pine is inexpensive and suitable for an outdoor bench, though it often moves as it dries and can be unpleasant to work until fully dry.
| Part | Quantity | Finished size | Notes |
|---|---|---|---|
| Seat slats | 3 | 48 × 4 × 1 inches | Leave a 1/4-inch gap between slats |
| Legs | 4 | 17 × 3 × 3 inches | Use matching pairs where possible |
| Side rails | 2 | 15 × 3 × 1 1/2 inches | These carry the seat between each pair of legs |
| Front and rear supports | 2 | 42 × 3 × 1 1/2 inches | Optional, but reduce seat flex |
| Wedges | 8 | About 3 × 1/2 × 1/4 inches | Cut from durable hardwood |
Buy roughly 20 percent more lumber than the finished dimensions require. Outdoor boards frequently contain splits or twist, and trimming away defects is preferable to building around them. A water-resistant exterior wood glue is useful, but the wedges and tight-fitting joinery should provide the mechanical strength.
Tools and Joint Layout
You need a saw, drill, chisels, clamps, a square, measuring tools, and a way to cut or flatten stock. A table saw makes the tenons easier to size, but a hand saw and shoulder plane also work. For chopping mortises, use sharp chisels at least 1/4 inch wide. A handheld woodworking router can remove most waste quickly, followed by chisel cleanup.
Mark every part with a face side and an outside edge. This prevents layout errors when parts are flipped. Each rail receives a tenon about 2 inches long and 1 inch thick. Center the tenon on the rail, leaving shoulders around it. The mortise in each leg should be 1/16 inch deeper than the tenon length, allowing glue and debris to escape.
Position the side-rail mortises about 5 inches above the floor. If the seat height must be exactly 18 inches, account for the 1-inch seat slats before setting this location. Keep at least 1 inch of wood above and below each mortise in the leg. Less material risks splitting when the wedge is driven.
Cutting and Preparing the Parts
Mill the legs and rails square, then cut the seat slats to length. Ease exposed edges with a 1/8-inch round-over bit or a sanding block. Do not heavily round the tenon shoulders; crisp shoulders make the frame seat firmly against the legs.
Cut the tenons slightly oversized and sneak up on the final thickness. The tenon should enter the mortise with firm hand pressure and should not fall through under its own weight. Test each tenon in its own mortise rather than assuming identical parts are interchangeable.
Cut the mortises from the outside face of each leg so the openings are clean and easy to inspect. Drill a row of overlapping holes inside the layout lines, remove the waste, and pare the walls square. Keep the mortise ends flat. A rounded hole will leave gaps around a square tenon.
Dry-Fitting and Wedge Layout
Assemble each side of the bench without glue. Check that the legs sit parallel and that the rail shoulders close evenly. Measure diagonally across the frame; matching diagonal measurements indicate a square assembly. If one corner remains open, correct the mortise or tenon before applying glue.
Once the fit works, mark the tenon for the wedge slot. The slot should run across the tenon, perpendicular to the grain, and begin about 3/8 inch in from the tenon’s end. Cut the slot only partway during the first test. The final slot should be about 1/8 inch narrower than the wedge’s thick end, creating expansion without splitting the leg.
Do not cut the slot all the way through the tenon before assembly unless your layout is very accurate. A stopped slot leaves the tenon stronger and prevents the wedge from being driven too far. Make the wedge with slightly tapered sides and round its leading corners.
Glue-Up and Assembly
Glue one bench side at a time. Brush glue on the tenon cheeks and shoulders, but avoid flooding the mortise. Excess glue trapped in a blind area can prevent the joint from closing. Press the parts together with clamps, checking the leg positions and diagonal measurements.
Drive the wedges into the slots while the glue is wet. Use a wooden mallet and stop when the wedge is snug and the shoulders are closed. If the leg begins to split, stop immediately; the wedge is too thick, the slot is too close to the edge, or the wood contains a hidden defect.
After the side assemblies cure, attach the front and rear supports with screws from the underside or with additional mortise-and-tenon joints. Predrill all screw holes, particularly in cedar and oak. Outdoor screws should be stainless steel or corrosion-resistant coated fasteners. Ordinary steel screws will rust and stain the wood.
Installing the Seat and Finishing
Set the seat slats across the supports with 1/4-inch gaps. Seasonal movement is normal, so do not glue the slats together. Fasten them from below with screws, using oversized clearance holes in the supports if the boards are wide. Leave the slats slightly proud of the frame, then ease their top edges.
Sand to about 120 or 150 grit. Finer sanding can reduce finish absorption without improving outdoor durability. Remove dust, soften sharp corners, and inspect the underside for rough edges that could catch clothing.
A penetrating exterior oil is easy to renew and does not peel, but it usually requires annual maintenance. Exterior paint offers better coverage and color control, though it can fail at end grain and joints if the wood is damp. A penetrating exterior wood finish is a sensible low-maintenance choice for cedar or redwood. Seal end grain carefully, keep the bench off wet soil, and inspect the wedges and joints each spring.
The most important inspection is simple: sit on the bench, shift your weight from side to side, and look for movement at the tenons. A small amount of seasonal looseness can be repaired, but a visibly opening shoulder means the joint needs attention before the bench is used regularly.