A word on Exterior Wood Joinery Techniques

Charles Prowell's articles on joinery have appeared in Fine Woodworking, Fine Homebuilding, Woodwork, Old House Journal, and This Old House, among others.

There has been a fair amount of hallabaloo recently regarding the joinery best suited for not only the stile and rail garden gates developed by CPW in the early 90's, but exterior assemblies in general. Exterior joinery is all about the properties of specific woods and their corresponding dimensional stability. There are emphatic do's and don't's far beyond what the site carpenter might understand, with his stackable designs. This is all we saw, for so many centuries, and continue to see day in and day out from renderings and blueprints sent to us from architects all over the country. Levels of an assembly applied to one another in a stacking sequence. This applies most traditionally to exterior pergolas and trellises and arbors, but as well to the Wood Fence and the Wood Gate. In time, and very little time at that, the various applied levels separate from one another. They cup and bow and warp and open to unsightly joints, while offering virtually no integrity to the lateral stability of the design itself. Large cumbersome 45-degree corbels and braces are incorporated into the design to help serve this end, and yet because they are jointed by nothing more than nails or screws, their effectiveness is, well . . . limited, not to mention downright unsightly.

So who actually understands anything beyond site principals? Very few, unfortunately. The carpenter and contractor and architect have been raised under the limited scope of a status quo laid out in their respective apprenticeships and education that falls short of such complexities. They understand stress loads and span limits and appropriate species and the scope of the larger site projects, but they have no experience with the subtleties of joinery.

The cabinetmaker or traditional woodworker understands the complexities of controlled joints intended for interior usage. Their levels of tolerance regarding the tightness of their joints is we assume above reproach, and yet this tolerance has no understanding whatsoever of exterior limitations and concerns. The variables encountered when joints are exposed day in and day out to sleet, hail, rain, snow, blizzards, and the dramatic changes in humidity within a single day in areas such as Florida and Hawaii..

Charles was raised from boyhood through college building homes with his father and furniture with his grandfather. These two very different apprenticeships lasting for decades until the much anticipated break of five years in college, choosing to study design under Buckmister Fuller in Southern Illinois. Eventually, exiting into the world with such equipment, he found himself drawn to both genres of site work produced in the controlled environment of a woodworking shop located in San Francisco back when a woodworking shop could actually afford to be located in San Francisco. A mecca of woodworkers, actually, stretched along Folsum Street in what is now known as the trendy SOMA district. There was more work than anyone could possibly digest, from Victorian remodels to public art assemblies to one-off furnishings, supplied to a city residency unlike any other city in America.  A city inhabited by affluent liberals who respond to innovation in a way that conservatives never have.

Years passed, experimenting and gathering techniques and slowly learning to combine the apprenticeship that began as a boy, sharing his tutelage between the ancestry of a builder and that of a woodworker. Developments were accomplished. Innovations in design were made. Backlogs in Charles' scheduling throughout the 1970's were running almost two years out and covering a scope of genres that translated into the most interesting and challenging life imaginable. By the mid-80's, children were arriving, cute as buttons, and Charles relegated himself to the shop exclusively, closer to the 'Buttons', and concentrated on developing a line of studio furniture, along with the help of the newest Prowell apprentice, #2 son Benjamin, swaddling about the shop in his diapers.

Missing the encounters of site work and all that that entails, Charles returned to the landscape of job sites in the mid 90's with the eventual development of what began with the stile and rail Garden Gate, and expanded quickly to the accompanying Fence Panels, Driveway Gates, Arbors, and on and on. Each of them rooted in the principals of joinery and structural integrity innovated on the strength of, well . . . decades of imprisoned apprenticeship. Let's have a look at a hierarchy of joints, some of them common and recognizable, and some of them closer to performance art.

A common Gate, illustrating the Butt Joint and diagonal bracing.

Early on, while more or less still in test mode, this joint was utilized on all the CPW gates, illustrated thoroughly in an article for Fine Homebuilding magazine. But we came to realize the limitations of the half-lap.
Advantages:
1) A fairly simple joint to fabricate, providing a large mating area, or what might be called the 'glue-mass.'
Disadvantages
1) Wood breathes with the seasons. The red arrows indicates in what direction the wood will breath, expanding and contracting in wet and dry weather. This breathing occurs perpendicular to the direction of the wood grain. The stile, which is the vertical stock of the gate, breathes left and right, or horizontally. This is why there is a 3/8" clearance configured between the net width of your gate and the post or jamb. Now . . .the rails, which are the horizontal stock between the stiles, is going to breath vertically, or perpendicular to the horizontal rail. In direct contrast to the stile's movement.
------In furniture design, this is a principle that cannot be slighted, as the result can often lead to a tug of war that results in one of the two members suffering the severe damage of checks and fissures. Why? Because one of the two members is trying desperately to simply breath, which is it's organic right, and yet it is being held in check by the half-lap of the corresponding member. The result, again, is stress. Stress is bad. Stress is at the root of not only schizophrenia and paranoia and a whole bevy of nervous disorders, but it can also result in this half-lap joint opening, loosening, and the opposing member checking and cracking just as a seemingly sane soul begins to slip from sanity.
2) The second issue that arises with the half-lap joint is how the top of the joint is essentially exposed to the rainfall. This in itself is not the most favorable scenario in that a golden rule in exterior carpentry and woodwork is to avoid allowing end-grain to ever be exposed to the vertical fall of the weather, such as rain and snow and the heat of sunlight.
3) The third issue is that the top of the exposed half-lap joint has nothing to thwart it's placement beyond the strength of the adhesives. As we see in the far right illustration, once the joint opens from the exposure to direct weather, then checks and cracks from the opposing grain directions, the rail begins to crack and check under the stress and drops, or sags, with nothing to prevent the lure of a beckoning gravity.

Here we double the glue mass, or mating surface, by having each side of the tenon, or male segment, mating to the inside surfaces of the milled stile. Double glue mass is good.
Advantages:
1) Double the mating surface--an improvement over the half-lap.
2) Easy to fabricate.
Disadvantages:
1) We have already covered the issues with grain direction and seasonal breathing, and this joint is not exempt from the faults discussed in the above example. Like furniture-making, dimensional changes are not pervasive, but if you are in the business of creating high-end work, you avoid techniques that allow even the slightest margin of error. The Through Tenon joint is the sort of joint you'll see from carpenters and cabinetmaker who have not met the rigors of a proper woodworker's apprenticeship.
2) We have also covered the issue of nothing between the top of the two joint seams and the falling weather.
3) And finally, the right side illustration resulting in our gate giving way, once again, to the madness of gravity. The Through Tenon has no top shoulder, which is a major component of a joint that weighs against the omnipresence of gravity. Whereas the leg-to-apron joint of a dining or coffee table will perform admirably as a through-tenon, the table, unlike the gate, displaces its weight to the floor.

The traditional furnituremaker's joint. Here we have a male tenon milled from the same stock of the horizontal rail. The tenon consequently mates to a corresponding mortise milled into the vertical stile. Assuming a good tight fit has been fabricated, this is a reliable and long-standing favorite among woodworkers and furniture-makers, as well as traditional timber frame homes.
Advantages:
1) The top of the joint is concealed from the falling weather.
2) The top of the joint has what we call a 'shoulder' above the tenon, preventing, by a matter of simple physics, the tenon dropping to the ravages of gravity.
3) No exposed joints or wood seams. If you're going to expose a joint, be sure it's a world-class joint.
Disadvantages:
1) The issue of breathing in opposing directions is debatable. Not so dramatically as our first two examples, but nevertheless we have a tenon that is an extension of the horizontal rail and this tenon is going to expand and contract in a direction that is perpendicular to that of the stile. To many who have not lived and breathed wood principles since childhood, this is dealt with by a shrug and a raised eyebrow. It is relative, in that there are trades, and then crafts, and them artistry. The tolerances tighten with each hierarchy, and the graduations from one to another does not come overnight, nor without proper tutelage.
3) The strength of our joint between the rail and the stile is now resting solely on the properties of the tenon, which is commonly cedar. Not exactly a species exhibiting dense properties.

A natural graduation from the Blind Tenon is the Floating Tenon Joint.  Here we improve upon the more porous cedar tenon by selecting a multiple ply tenon in Russian Birch.
Advantages:
1) Eliminates the issue of the tenon breathing perpendicular to the stiles and rails.  The tenon does not breath.
2) Excellent strength and stability, with it's multiple plys.
Disadvantages:
1) Repeated slamming of an untended gate can conceivably result in a lock stile that cracks from where the Floating Tenon ends and the edge of the stile. 
2) Only a very specific species of marine ply is acceptable. 

  Carrying the above joint two steps further, we have a Full Through Tenon, secured with wedges.  Utilized by Japanese temple builders, the joint is self-sustaining in that it plays to the dimensional changes occurring from season to season.  Not only does the full tenon strengthen the gate's stile against repeated slamming when exposed to wind tunnels as well as increase the lateral integrity, but the wedges, in theory, eliminate the need for glue.
Advantages:
1) The top of the joint is concealed from the falling weather.
2) The top of the joint has what we call a 'shoulder' above the tenon, preventing, by a matter of simple physics, the tenon dropping to the ravages of gravity.
3) The joint is created with dry stock between 5-9% moisture, and at it's smallest dimension.  The wedges tighten the joint to the dry conditions of the shop and once exposed to any weather--as in rain, humidity, even fog--the tenon expands and the joint grows tighter and tighter.  In the drier weather, the joint returns to the conditions in which it was created in the shop. 
4)  A glueless joint. 
5) There are temples in Japan constructed with this joint that have been standing for a thousand years.
Disadvantages:
1)  The edge of the gate's stiles can no longer be easily planed by the on-site carpenter. Planing is often required if the dimensions provided to CPW turn out to be in error and one or both of the posts or columns are in fact out of plumb.  The site carpenter must plane an edge to create the uniform clearance, and lesser experienced carpenters are going to be tempted to reach for their plastic suitcase housing a collection of nifty battery-powered tools bought at Home Depot and if he pulls out the nifty Circular Saw, we can liken the ensuing procedure to a battlefield medic performing surgery with his bayonet.

 Single and two-sided high-strength band-aids.  Colorful commodities that are also available with little actions figures

An in-progress look at Joint #6.   The first known use and still standing in near perfect condition today are two temples in Japan, built in cypress cedar in 413 A.D and 607 A.D.  Much later, and also still standing, a church in Oslo built in the 13th century. Both utilizing joint #6. 

Our methods were learned from Grandpa, furthered by Dad, and then Charles, and now Ben, who at 22 is in his 15th year of apprenticeship and currently enrolled in North Bennett Street's veritable school of woodworking in Boston, where for 175 years or so, they have been teaching the art of hand-cut joinery in the same building situated in the old North End. 

Although no one anticipates an assembly that will stand for 1500 years, the difference, beyond the aesthetics of a trained and practiced eye for subtle design, is a product that lasts, well . . . that lasts a few decades beyond the product of a lesser trained, less patient approach.

Below left we have a structure you've all seen before, growing off the side-wall of the residential architecture like a festered boil. In short order, the cross beam, primary joists and secondary top joists will twist and cup and warp into an insightly assembly. The joinery is a combination of metal fasteners from post-to-beam, and toe-nailing the joists.

To the right we have a CPW Pergola designed and built in the early 90's illustrating the flush joinery that provides not only a departure in aesthetics, but a far stronger assembly designed to last several lifetimes. The overhead units are a series of small panels joined and set into place with a louvre affect. There are no metal fasterners, nail, or screws anywhere within the assembly.

Below left a common carpenter's fence and adjoining arbor assembly, again illustrating a stacked, applied approach to joinery with both the fence and arbor.

To the right we have a CPW Fence, gate, and complimenting Arbor as a married assembly that provides us not only a visual confluence of the three elements, but each of them once again joined without metal fasteners.

Below left: a few common examples of lattice we've all seen far too much of.

To the right, A Prowell fence-line in San Francisco, illustrating 150-running feet of flush joinery.

Because Charles has written a number of articles over the years on the art of joinery for such publications as Fine Woodworking, Fine Homebuilding, Old House Journal, This Old House, and Woodwork Magazine, among others, it seems only appropriate to carry our discussion a step further, skipping ahead to a few advanced examples in an effort to illustrate not just the sheer perplexity of this procedure, but the alluring temptation of the joint as a work of art in itself.
Below is a look at an interlocking dovetail with stepped haunches, or shoulders. A spline, essentially, that might, to the layman, more closely resemble a Rubic's schematic but to the careful and discerning woodworker, it becomes closer to performance art.

A look at the glue-less joint, where the interlocking planes and wedges are such that the joint is self sustaining. The example here is often utilized in Japanese temples for joining the rafter tails to cross-beams. Take notice of this on your next trip to a Japanese Tea Garden. Perhaps this is what CPW uses. It's an extraordinnry joint, relying soley on the execution of the Jointer's steady hand and without the aid of glues and sticky stuff.

An assortment of variations on the traditional mortise and tenon.

As a small part of an extensive CPW interior design, the use of the traditional, yet modified, Finger Joint adds an alternative flavor to the standard Butt Joint more commonly used here.

The above finger-joint wildly popularized by Greene and Green in 1917. Brothers and architects whose greatest asset was a lifelong association with a top notch team of woodworkers who realized their designed with flawless workmanship.

An early CPW design from approximately 1985 that still draws commissions. The exposed joinery of a trestle table employing both the Exposed Wedges of the Through-Mortise & Tenon Joint, but also the Primary Vertical Wedge joining the trestle board itself.

Left: Prowell oversses the early apprenticeing of Ben (on the table) and his older brother Sam. These two were sooo cute at that age. All along, actually, from newborns until this very day, as young men with the social graces of an innate courtesy toward all. That's a good thing, right?

Another look at a CPW Table using the Vertical Wedge to join the trestle board.

Joinery of this nature becomes a defining feature of the work's inherent design. This same joint went on to be designed into innumerable patio and terrace tables exposed to the elements. A joint with a great deal of forgiveness.

The hierarchy of joinery takes us to a series of intricate lamp joints, and with the Stand-Up Desk on the right, the near-obssesive use of the joint as an ebony wood hinge.

You are expecting an essay on Gate joinery, and instead have been lead on a meandering diatribe that ends with the portrait of a craftsman who often cuts the joints to your gates standing on his head, blindfolded, and with one hand tied behind his back.