Timber Frame 101

Why is that tree on the roof? (and other quirky timber framing traditions)

November 9, 2016

Timber Framing has been around, in some form or another, for over 2,000 years. So it stands to reason, some interesting timber framing traditions and customs have sprung up in connection with the design and construction of timber frame structures. We thought, in this season of tradition and home, we’d share some of the more inspired stories.

Common Timber Framing Traditions

One of the more well-known timber traditions involves nailing a bough or small evergreen tree to the highest point of the timber frame. According to Fine Homebuilding Magazine,

The ritual serves two purposes. One pays homage to all the trees that went into the construction of the house, and to the many hands that built it. The other symbolizes the establishment of the house’s roots, which will nourish a long and prosperous life. The young tree is called a “wetting bush”, likely derived from the German tradition of watering it as a sign of the home’s first nourishment.

Of course, this might refer to a home, an outbuilding, or even a post and beam shed, but Barn Raising evokes an image familiar to most of us–a community coming together to accomplish something big.

In the “olden days” the individual timbers for a house or barn would be cut and fitted by just a few men or a family. When all the individual walls were ready, the whole community would come together to raise the walls into position and set the rafters and ridge beam in place. This would be done using gin poles, ropes and pulleys, and of course, strong men. Although hard physical labor was required, “many hands make light work”, and the day ended with a celebration and party. The wives and children participated and a feast was held. There’s a real nostalgia in the thought of honest labor and participation in a feat that none of the individuals could have accomplished alone.

This doesn’t happen when the sheetrock goes up…but once the last timber is raised and pegged, the moment is documented with a photograph. Why? Perhaps because timber framers often feel that the timber frame will never be more beautiful than at that moment (before it is covered by walls and a roof), perhaps because all present sense that this structure will outlast every one in the picture.

Whether it’s a tradition, or simply good manners, timber framers always appreciate a hearty meal! So they perpetuate the superstition that feeding the crew will bring the building (and Owners) good luck…

A French tradition, courtesy of Charpentiers Sans Frontières (Carpenters Without Borders), is to give the lady of the house the honor of pounding in the last peg. She is expected to owe the crew one drink for every blow of the mallet required to drive the peg home.

Think of this as a very abbreviated time capsule! A coin minted the same year as the structure is built is placed under a post, where it (presumably) will not be found until the building is dismantled (and hopefully the timber frame reclaimed).

The date of completion is carved into the frame. This may be done in a visible and conspicuous place, such as over a doorway, but it might also be done in a less obvious spot. For example, some timber frame companies carve the date in the location on the frame closest to where the home shop is located. In our case, the date would be carved on the side of the building closest to Boone, North Carolina.

All the timber frame craftsmen (and craftswomen) who worked on the frame sign one of the timbers in a hidden place where it may lie undiscovered for a century or more.

Why Do Timbers Split and Crack?

June 23, 2016

Eric Morley on why timbers split and crack, how long it takes timber to dry, where to get dry timbers, and why gunshots in the night are nothing to be afraid of…

The Short Answer: Wood Checking

Splits and cracks (known as wood checks in the industry) occur when wood shrinks as it dries. Wood shrinks roughly twice as much along the growth rings (radially) as it does across the rings (tangentially)–and it is this uneven shrinkage that causes checks to develop.

Are Checks Bad?

Here at Carolina Timberworks, we think of them as wrinkles in a cotton shirt—it proves the timber is real. Checks are what make a solid timber look different from a boxed beam. Look at the reclaimed timber below. It is impossible to predict where a check will appear in timber, and it is the check that gives this reclaimed Oak timber so much of its character–and a big part of what makes it completely unique and different from any other beam in the world.

Ever Heard the Song of the Wood?

Have you ever taken a hot loaf of artisan bread out of the oven, and put your ear to it? If you do, you’ll hear the song of the bread–pops and crackles as the crust shrinks, cracks, and dries.

Walk into a timber framer’s shop one winter evening when everyone has gone home for the day. Throw another log into the woodstove, and listen carefully. If there are green (wet) timbers arrayed on sawhorses, within a few minutes, you’re likely to hear the song of the wood–a symphony of pops and cracks as new checks develop.

And if one night, asleep in your new timber frame home, you’re awoken by a loud crack a little like a gunshot–don’t worry. Roll over and go back to sleep–it’s just your timbers singing to you.

Why Does Wood Shrink?

It sometimes surprises people to learn that roughly half of a living tree’s weight is water. Let’s consider a 24’ long Douglas Fir log that measures 34” diameter at the large end and 14” at the small end as an example. This hypothetical green (wet) log would contain a little over 1 ton of water—or about five 55 gallon drums of water.

Sometimes when we drive a chisel into green (wet) timber, water spurts out of the wood. Wood is hydroscopic—meaning it’s like a sponge in that it can absorb, hold and release water. When freshly cut, approximately half of a tree’s weight is water. The photograph below shows water in a freshly cut Cypress timber–and how the timber is drying from the outside in.

How Long Does It Take a Timber to Dry?

It’s not exactly a fast process, and it depends on the humidity of the environment in which the timber is located, but one rule of thumb is that a timber air dries about 1” per year. Thus a 12” x 12” timber would take about 6 years to dry to the center.

How Dry Will the Timber Become?

A timber will eventually air dry to the Equilibrium Moisture Content (EMC) of its environment. The EMC is the point at which wood is neither losing nor absorbing water. A timber’s moisture content is determined by the atmospheric humidity of the timber’s environment.

That environment varies, of course. Is the timber located inside, or outside? What part of the country is it located in? What time of year is it? For example, in the dry mountain air of Denver, the outdoor EMC of wood exposed to the outdoor atmosphere in July is 9.4%, while New Orleans is 15.3% (Forest Products Laboratory: Equilibrium Moisture Content of Wood in Outdoor Locations in the United States and Worldwide, August 1988).

How Wet (or Dry) Are My Timbers?

Since we can’t see the water in the wood, moisture meters are used to measure moisture content. Inexpensive moisture meters measure the surface moisture content—which works ok for 3/4” boards, but not 8” thick timbers. Professional moisture meters use electromagnetic scanning to read the moisture in the wood, not on the surface of the beam.

What is the Average Moisture Content in the U.S.?

The majority of the U.S. has an 8% average moisture content, the Southeast and California coastal areas have an 11% average moisture content, and the Southwest desert areas are closer to 6%.

Most Timber Frames Are Built From Green (Wet) Wood

It is impractical (it takes years) to air dry timber, and we humans are an impatient species, so for the last two thousand years, people have been building timber frames from green (wet) timber. Yes, green timber will shrink, check, and sometimes twist as it dries, but timber framers and engineers understand and account for the movement. Checks begin on the exterior surface of the timber and almost always stop at the heart (center) of the timber, and are almost never a structural concern.

By the way, if a crack were to develop all the way through a timber (splitting it into two separate pieces), it would be called a split–and might be cause for concern.

How Much Does Green (Wet) Timber Shrink as It Dries?

Shrinkage depends on the species, but more than you may think!

Since many timber frames are built from Douglas Fir, let’s start by looking at a 12×12 Douglas Fir (Coastal) timber. This particular green (wet) Douglas Fir timber, dried to a final moisture content of 8%, would be expected to shrink a little less than 9/16” on each face–from 12” x 12” to a final size of 11-7/16” x 11-7/16”.

Western Red Cedar shrinks less: The same size timber in Western Red Cedar timber would shrink a bit less–about 5/16” to 11-11/16” x 11-11/16”. Finally, how about a species with a high shrinkage rate, like White Oak? It would be expected to shrink a bit less than 3/4” to slightly larger than 11-1/4” x 11-1/4”.

6 Ways to Minimize Problems with a Green (Wet) Wood in a Timber Frame:

Specify Free-Of-Heart-Center instead of the less expensive Boxed Heart grade.
Apply a wax-based end sealer to the end grain to slow the drying process. We use Anchorseal, available at https://uccoatings.com/products/anchorseal/.
Utilize housed joinery wherever possible.
Drawbore pegged joints.
The slower the drying, and the more gradual the process, the better—i.e., the worst thing you can do is enclose the timber frame and immediately turn on the heat or air conditioning full blast.
Do not apply a film-forming finish (i.e., polyurethane) to the greenwood.

I Don’t Want Wood Checks or Cracks in My Timbers. What Are My Options?

Don’t build with solid timber. Instead, ask us to price glulam beams for the timber frame, or to fabricate box beams from well-dried new or reclaimed lumber.

Or if you’re ok with some checks, but would prefer to avoid some of the characteristics of green lumber, consider purchasing your timber like you do your high-quality organic cotton t-shirts–pre-shrunk. There are two ways to buy dry beams: New timber that’s been dried in a kiln, or reclaimed wood that’s dried slowly and naturally for 50 or 100 years during its previous life as a barn or warehouse.

Where to Buy Kiln Dried Timbers?

We offer two types of kiln-dried timber: Conventionally kiln-dried (KD), and Radio Frequency Kiln Dried (RFKD). Conventional kiln drying dries about the outer 1” or so–leaving most of the timber wet. The other method, RFKD, is similar to a giant microwave and dries timber to the core.

What you need to know about RFKD timber is that it currently works only on Douglas Fir, it is dry to the core, and it is more expensive. It is, however, not as dry as conventionally kiln-dried hardwood flooring or lumber (6-8% moisture content). For example the driest grade of RFKD timber measures 15% or less moisture content 3” in from the surface of the timber.

What About Using Reclaimed Wood in Timber Frame Construction?

Reclaimed wood beams that have naturally air-dried for 50-100 years during their previous life as a barn, factory, or warehouse are often completely dry to the core–and the driest timbers available anywhere. It’s not easy to hand-scribe and connects irregular, twisted, and non-square reclaimed beams so the finished timber frame appears to have always been one frame.

We think a timber frame built from reclaimed wood is everything that today’s sleek, mass-produced, technology-saturated culture isn’t. It celebrates the cracks and character and dirt and all the other marks that time, weather, and use leave behind.

“When You Buy Quality, You Only Cry Once”

© 2016-2020 Carolina Timberworks

More Benefits of Timber Frame Construction

July 27, 2015

While watching Fine Homebuilding’s video of architect Caleb Johnson of Biddeford, Maine discuss the winner of the 2015 Best New Home award, we were struck by his profound insights into the benefits of timber framing and why we are drawn to, and love timber framing:

“Built poetry.”

“The materials go together in a fashion that you can tell he (architect Louis Kahn) cared deeply about the nature of those materials and the way they came together on a level that’s art–not just construction.”

“If we use natural materials, as time goes on, these natural materials take on a character and patina that enhances the building. Whereas when you use manufactured materials, those materials will look best the day you put them in, and they will deteriorate from there, and there’s really nothing you can do to bring them back.”

“I feel that the structure of a house can be the most expressive part of the architecture and I feel that when that structure is exposed, it’s most powerful when it’s authentic–like it’s actually bearing the load of gravity pulling the house down and the winds trying to push the house over.”

Special thanks to Shannon Richards of Caleb Johnson Architects for permitting us to reproduce Caleb’s words.

To look at their work, visit http://www.cjab.me

Finehomebuilding Presents Houses 2015 Awards: Best New Home from Caleb Johnson Studio on Vimeo.

What is a Timber Frame?

June 10, 2015

Traditional timber framing is the art of connecting two or more pieces of timber using wood-to-wood joinery. Prior to the discovery of metal, this is how timber posts and beams were connected to frame the homes, barns, and businesses our ancestors lived and worked in. Held together without nails, bolts, or metal connectors–the timber frames of old tended to withstand the test of time–lasting centuries instead of decades.

Eastern White Pine Timber Frame mortise and tenon joint — Pegged mortise and tenon joint

Although there are many joinery variations, the mortise & tenon joint has been used for well over a thousand years to join pieces of wood. The basic joint comprises two components: the mortise hole and the tenon tongue–which are locked together by one or more wooden pegs.

(You are already likely familiar with mortise and tenon joinery–it is how finely made furniture is built).

Timber framing began to die out in the mid 1800’s after balloon framing and mass-produced nails were invented, but was still used for barns and mills into the early 1900’s.

The renaissance of timber framing began in the 1970’s when carpenters in New England began to study the many timber frame structures that still exist on the East Coast. Timber frames, if kept dry, can and do, last for centuries (the oldest surviving timber frame in North America is the Fairbanks house in Dedman MA–built between 1637 and 1641).

Today, with the revival of the craft well under way, and with the help of modern engineering, we as an industry are creating some of the best timber frames that have been built (literally) in ages.

Differences Between Decorative and Structural Timber Framing

June 2, 2015

A homeowner building in the Outer Banks emailed a great question this week:

“I was wondering if people typically use your trusses as structural members or just as decorative members attached to the existing frame structure?” We build and install both decorative and structural timber framing.

What Is Decorative Timber Framing?

Non-structural, or decorative timber framing, is frequently timber frame trusses in a cathedral ceiling or beams on a flat ceiling. Our engineers call this timber framing non weight-bearingin other words, the timber isn’t holding up or supporting anything above it. As an example, suppose a great Room is designed with 2×10 ceiling rafters framing and holding up the roof. If you choose to install timber frame trusses under the 2×10 rafters, the timber frame trusses wouldn’t be structural because the 2×10 rafters are actually doing the work of supporting the roof.

Over the years, we’ve built a lot of non-structural timber framing. Our yardstick for this work is simple: to look right, it must look as if it is structural.

Back to our great room example. Structural trusses usually have between 6’ and 8’ spacing between trusses. So if a great room was 16’ wide and 24’ long, we’d design three trusses for the room — one at 6’, one at 12’, and one at 18’. Sometimes, we might include two more — one at each of the gable ends (at 0’ and 24’).

Often we make these end trusses half-thickness so as to appear partially embedded in the wall. In addition to truss spacing, in order to look right, the trusses must be built using appropriate timber sizes. A structural truss spanning a 16’ wide great room would probably need to be built from timber at least 6” wide – i.e. we might use 6” x 12” timber for the rafters and lower chords.

If the truss were built from smaller material, say 4” x 6”, it wouldn’t look structural, and that’s what would make it not look right. Conversely, to look structural, there shouldn’t be too many trusses, or built from timber sizes that are oversized–that doesn’t look right either.

Our yardstick for designing and installing non-structural timber framing involves a mental picture: at some point in the future, the homeowner has a guest visit the home. This guest may be an architect, an engineer, or perhaps another timber framer. We want that guest to look up, and stand there for many minutes trying to figure out if the timber is structural or non-structural, then turn to you and ask.

Pros of Decorative Timber Framing

A great place to use reclaimed beams. Reclaimed beams are normally not graded or grade-stamped, which is required by engineering for structural timber framing.
They can be (and often are) retrofit from below after the roof is in place. The benefit is that the timber is protected from the weather, and since it is installed later in the process, it is unlikely to be damaged by other trades. As you might imagine, this costs a bit more than setting heavy timber from above with a crane (picture each 400 lb. timber being unloaded and carried by hand into your great room).
Pre-finished. We frequently apply a finish to the timber because most builders agree that it’s less expensive for our crew to finish the timber framing in our shop while it’s on sawhorses than it is for a painting subcontractor to erect scaffolding and apply two coats of finish on site.
Great for remodeling. We can add or retrofit timber framing and beams to existing houses and have done so many times as a part of a remodeling project.

Cons of Decorative Timber Framing

Need to carefully consider spacing, timber sizes, and proportions so that it looks right (structural).
Added weight: timber is heavy. For example, a single 6” x 12” x 16’ Douglas fir timber weighs about 4 lbs. per board foot or 384 lbs. An engineer needs to be consulted to determine that your framing is designed to safely support the added weight of non-structural timbers.

What Is Structural Timber Framing?

Structural timber framing, as you might expect, is when the timber framing is weight-bearing, or doing the work of holding up part of the house above it. If we return to the great room example, your architect and builder might decide to support the roof using structural timber frame trusses. In that case, the timber frame trusses would be designed and engineered to support the weight of the decking (usually 2×6 tongue & groove) or sheetrock ceiling, the insulation, the roofing, and the wind and snow loads.

Pros of Structural Timber Framing

It looks right. The aesthetic concern of looking right is addressed automatically because the timber framing has been engineered to be structural.
It replaces some structural conventional 2x framing, which can help offset the cost.
Durability. If kept dry, timber framing can (and does) last for centuries.
Safety. We don’t like to think about it, but houses sometimes burn. Unlike steel or 2x4s, timber framing provides more time for the family to safely get out of a home in a fire (see the FOR ARCHITECTS section on our website for an article on the subject).

Cons of Structural Timber Framing

More expensive than conventional 2×4 framing.
Must be engineered; however, this isn’t a major cost in most projects.
Installed earlier in the building process — meaning it has to be planned in advance and protected during the balance of the construction process.