Building the Traditional Hewn-Log Home

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In single-story log structures without lofts, the ceiling joists were sandwiched between the top wall logs and the plates.

PURLINS AND RIDGEPOLE . . . OR RAFTERS?

A very old roof-raising technique common to round-log construction is to continue up the gable ends of the structure with a stairstep pyramid of progressively shorter logs. Purlins—heavy poles that run parallel to the ridge from gable to gable—are notched and spiked to the ends of every gable log as it goes into place (sometimes to every second or third one). A heavy ridgepole caps things off, with decking or large shakes nailed directly to the purlins. The disadvantages of this method include a heavy roof that tends to sag under its own weight as it ages, and the hassle and danger of wrestling a massive ridgepole up to the highest part of the structure.

A rafter-supported roof, on the other hand, employs several pairs of rafter poles spliced together to form inverted Vs that bear and distribute the weight of the roof. To support the roof decking, lath is nailed across the hewn tops of the rafter poles parallel to the ridge. (If the builder prefers, milled lumber can be used in place of poles.) There is no ridgepole, and since the gable ends bear none of the roof's weight, they can be framed in with light material such as board-and-batten or shakes over latticework.

With a rafter-supported roof, the plate logs must be heavy enough to bear the outward thrust of the rafters, or tied together by joists or tielogs fastened with either dovetail notches or angle irons and bolts.

GETTING AN ANGLE ON ROOF PITCH

No matter what type of roof-support structure you decide to use, the first step is to determine pitch. Probably the most practical roof pitch for owner-built log homes is 45°. The resulting 1:1 pitch is steep enough to shed snow, offers a pleasing, traditional appearance at the gables, supplies ample headroom for loft or attic, and-best of all for the builder—makes the required mathematical calculations as easy as pi.

Easier than pi, in fact. That's because when the cabin is viewed from the gable ends, each side of a 45°-pitch roof forms the hypotenuse of a right triangle with legs of equal length. With a 1:1 pitch, ridge height always equals half the exterior length of the gable-end wall logs. (For example, if your cabin is 20' from the outside of the front wall to the outside of the back wall, the ridge will be 10' above the plates.) Knowing that, we can employ the good old Pythagorean theorem to determine rafter length.

Where X stands for ridge height above the plates, Y stands for half the end-log length, and Z is rafter length (which we're looking for),

In English (or Swahili, for that matter), that means rafter length (Z) equals the square root of the sum of the ridge height squared (X2) plus half the outside length of the end wall squared (Y2).

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