Our barns.... Purlins and Rafters

Photo: Don Perkins

A common rafter roof.

Photo: Don Perkins

A purlin roof system.

Photo:

Modern gambrel barns use sawn common rafter roof construction.

Photo: Don Perkins

Vertical wall and roof boards shed water in this purlin roof barn.

LISA

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Next time you’re in a barn, take a good look skyward. Roofs are often some of the most intricate elements of the building’s construction. There are many questions surrounding roof assembly and distribution patterns. Who said barns were boring? There’s plenty to ponder in our barns of old. Chasing down the history of their construction will no doubt keep “barnologists” like myself and others pondering and comparing historic carpentry for years to come.

There are two basic types of roof framing in an old timber framed barn, which can be broken down into general terms: purlins and rafters.

Purlin systems dictate the roof boards be laid vertically, from eaves to peak, while rafter systems require the boarding be affixed horizontally, parallel with the roof ridge. The distribution of these two types is somewhat of a mystery. But in this part of Maine, the determining factor is often a barn’s age. Older barns, especially those prior to the Civil War are usually framed with purlin roof systems. But what’s interesting is that’s just this part of New England.

One scholar, Thomas Durant Visser, addresses this in his book Field Guide to New England Barns and Farm Buildings. The book is an excellent read and has a map of the distribution of New England roof systems around 1840. Interestingly, purlin roof frames are typically not found north of Bangor, nor are they seen west of Central Vermont. This line extends southward splitting Massachusetts roughly in half. Purlin frames are generally not found in Connecticut or Rhode Island either.

Visser explains the answer lies in the settlement patterns of the early colonists, with two primary population centers landing at the shores of Massachusetts Bay and Connecticut. As these populations moved north and inland, they brought their distinct traditions with them. This is all well and good and perfectly plausible. But I like to think there were other variables. As a carpenter, I propose that the ease of construction, or a builder’s proficiency, also entered in, as did what the finished roofing material would be.

Barns are not houses but outbuildings, often left crude and unpainted. Most early barns did not even have siding, which is why vertical wall boarding was the preferred method of exterior covering regardless of geography. Vertical boards naturally shed water. If left un-sided and exposed to the weather, horizontal boarding will hold water at the top of each board. The wind will force this water to the inside of the building, thus rotting things prematurely.

Some propose that vertical roof boarding was simply a carryover of this strategy to the rooftop. Not all early barn roofs were shingled, but may have been simply covered with a couple of layers of overlapping vertical boards. A barn that was simply to have a “board roof” would naturally be built with a purlin system. Conversely, roofs that were covered with wood shingles needed a base of horizontal lath-work applied over closely-spaced vertical common rafters. With me so far?

Builder proficiency must have been a factor as well. A roof system of closely spaced common rafters, such as can be found in more modern house construction, takes less skill to construct and is easier to erect than a purlin system which rests on massive timbered rafters spaced widely with the bents and posts of the building. These “major rafters” must be tenoned and specifically fitted into other specifically mortised timbers. The raising of these massive (heavy!) timbers requires considerably more labor.

Because of economics today, closely spaced “common rafters” are the way to go. A purlin roof system is usually only found in high-end timber-framed housing these days. Large timbered trusses (King post, Queen post, and scissors, to name but a few) connected by horizontal purlins provide pleasing architectural elements with cathedral ceilings. And speaking of cathedrals, one can simply recall the old European-styled cathedrals with their timbered roof structures of elegant hammer beam trusses (Google this term) and realize the amount of proficiency needed to construct such artistry. This was not Jack-of-all-trades carpentry but apprenticed joinery.

Yet another element of whether to build rafter or purlin could have been the proximity of the nearest sawmill. Early barns were built of large timbers simply because a sawmill often didn’t exist, or the farmer/builder couldn’t easily get his logs there for milling. Thus a builder may not have even considered the option of employing many uniformly sawn rafters. We take it for granted today that we can easily purchase standardized lumber. Heck, we can even have it conveniently dropped right at our door! In the early days standardized lumber didn’t exist. It would take the railroads to bring that commodity within reach. This is why many purlins were left round with the bark still on, with perhaps a single side flattened to meet the roof boards.

Our barns in the Oxford Hills display both rafter and purlin roof styles, with older barns typically displaying purlin arrangements (but not always!).

It’s remarkable how barn history can lead to so many different segments of society (cathedrals, colonial settlement patterns) and the history of technological innovation (joiner proficiency, railroads and sawmills).

I’ll be speaking about our barns, giving a PowerPoint presentation, at the New Gloucester Historical Society on April 21 at 7 p.m. at the New Gloucester Meetinghouse, next to the Town Hall on Rt. 231.

For more on Maine’s barns visit www.ourbarns.com.