The Anatomy of a True Mortise and Tenon Joint in a Custom Gate

A mortise is a precisely cut rectangular cavity, usually in a vertical stile. A tenon is a matching tongue machined onto the end of a horizontal rail. The tongue slides into the cavity. When fit correctly, the joint is so tight that the rail will hang from the stile without any glue at all — a test we run on every gate frame before final assembly.

The strength of the joint is mechanical, not chemical. Glue helps, but it's not what holds the gate together over decades. What holds it together is the long-grain contact surfaces locked inside the mortise, transferring load along the wood fibers rather than across them. A mortise and tenon joint moves with the wood as humidity changes; a steel bracket fights the wood and eventually loses.

On a typical 6-foot pedestrian gate, each corner joint has about 9 to 12 square inches of glue surface — more than enough to resist the cyclic loading the gate sees every day for the next 40 years.

Outdoor gates live through the worst conditions wood can experience. In Lafayette, the same gate that sits in 105°F valley sun on a July afternoon will see 38°F overnight in January, with humidity swinging from 15% to 95%. Every cycle, the wood shrinks and swells. Every cycle, the screws holding metal brackets to that wood loosen a little more.

Steel and wood expand at different rates. Steel barely moves; hardwood can grow and shrink several millimeters across a stile. Over time, the screw threads chew through the surrounding wood fibers, the brackets get loose, the corners begin to rack, and the gate starts to sag at the latch. By the time you notice it, the damage is done.

Mortise and tenon joints don't have this problem because they don't rely on fasteners holding two materials together. The wood is the fastener.

Modern joinery often relies on long clamp time and modern glue to make a mortise and tenon work. Traditional joinery solved the problem differently: the drawbore pin. After cutting the joint, the joiner drills a small hole through the mortise cheek, then drills a matching hole through the tenon — but offset slightly toward the shoulder.

When a tapered hardwood pin is hammered through, it pulls the shoulder of the tenon tight against the stile and locks it there mechanically. The joint cannot loosen unless the pin shears, and even modest oak pins shear at loads far beyond anything a residential gate will ever experience.

Every Heartwood mortise and tenon joint on a gate over 4 feet wide gets at least one drawbore pin per corner. On large estate gates in Alamo and Danville we use two. It adds time in the shop — but adds decades to the life of the gate.

Not every mortise and tenon joint is identical. A through-tenon passes completely through the stile and is visible from the back, often wedged for additional mechanical lock. We use through-tenons on craftsman-style and Greene-and-Greene-inspired gates where the joinery is meant to be expressed as a design feature.

A blind tenon stops inside the stile and is invisible from the outside — the right choice when the gate is meant to read as a single continuous surface or when both sides will be planed flush. This is our default for contemporary gates in Walnut Creek and Berkeley.

Twin tenons split the joint into two parallel tongues with a bridge of wood between them. We use twin tenons on wide horizontal rails where a single tenon would be too tall to be reliable — common on the bottom rails of automated driveway gates that need to span 7 to 8 feet without flexing.

Tenons are cut on a dedicated tenoning machine that produces a four-shouldered, precisely dimensioned tongue in a single pass. Mortises are cut on a hollow-chisel mortiser that drills a square hole — a process that takes about 90 seconds per joint on a stile in 8/4 Sapele.

Every joint is then hand-fit. We pare the cheeks of the tenon with a chisel until it slides into the mortise with firm hand pressure — tight enough to support its own weight, loose enough to seat fully without splitting the stile. A joint that's too tight is more dangerous than a joint that's too loose; it can split the stile when temperature and humidity change.

Before final assembly, every joint gets a dry-fit and a load test. We hang the frame from a hoist and check for any movement. Only then does the gate go into final glue-up.

A gate built with real joinery is not a cosmetic upgrade. It's the difference between a piece of millwork that will outlast your front door and a fence panel that will need replacing in eight years. In a city like Lafayette, where most properties are intended to be held for decades and resale matters, the construction matters.

We've serviced gates we built fifteen years ago for clients who have since moved twice, and the joints are still tight. We've also serviced gates we did not build, where the bracket-and-screw construction had failed inside of five seasons.

When you're commissioning a custom gate, the single most important question to ask is: how are the corners joined? If the answer involves screws, brackets, or steel plates, you're buying a different product. If you're not sure how to evaluate the answer, see our companion piece on how Heartwood builds steel sub-frames for hybrid gates: the steel sub-frame + Sapele cladding method.

Real joinery has limits. On extremely wide automated driveway gates — anything past about 14 feet in a single leaf — the cyclic motion of the operator generates forces that can outpace what an all-wood mortise and tenon frame can handle. For those gates, we use a hybrid: a fully welded steel sub-frame that carries all the operator load, faced with a Sapele or white oak cladding that gives the gate its appearance. The cladding is itself joined with floating tenons.

Pure mortise and tenon construction also requires hardwood thick enough to host the joint — typically 8/4 stock or thicker. Light cedar or pine fence-grade lumber can't reliably hold a mortise. This is why we don't build in those materials.

The 90% of gates where mortise and tenon is the correct answer are exactly the gates most clients are commissioning: pedestrian gates, courtyard gates, garden gates, and driveway gates up to roughly 14 feet per leaf.

Every gate we build for a client in Lafayette, Walnut Creek, Orinda, or anywhere across the East Bay starts with the same conversation about joinery. We'll walk you through the joint type we plan to use, why it's the right one for your site and your gate's geometry, and how we'll cut and fit it. If you'd like to see the workshop, we welcome shop visits.

To start your project, request a free design consultation through our contact page or read our custom gates service overview. If you want to dig deeper into materials before booking, our guides on Sapele vs. redwood and Sapele vs. teak in Northern California are the next two we'd recommend.

A gate is one of the longest-lived purchases you'll make for your home. The joinery is what makes that true.