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Timber-Frame Roofs for Great Rooms - Continued

Drywall ceilings. Some clients prefer gypsum-board ceilings. In those cases, we use moisture-resistant GWB, laid out so the seams occur over the purlins and rafters. Next comes the tricky part: laying out and fastening 3/4-inch T&G plywood on top of the GWB, and screwing it to the purlins and top chords of the trusses. Once this is accomplished, we fasten the gypsum board to the plywood with drywall screws from the underside. The two layers of foam and the rest of the installation are the same as if T&G boards had been used.

What if there are no purlins? In a truss roof without purlins, we provide nailing for the built-up roof in one of two ways: We either add an extra layer of 2x3 strapping across the rafters, then a crisscross layer of strapping on top of that, or we use a stout nominal 3-inch-thick T&G product called Lock-Deck (Disdero Lumber Co., 800/547-4209, www.lockdeck.com) for the ceiling.

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Here, with the assistance of the crane, the gable truss is held in position while the tenoned purlins are dropped into place, stabilizing the roof structure.

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Lock-Deck is a great product because it comes with engineered design values that allow your designer to take into account wind-loading on the roof. It's also available in thicker sizes if you're in a heavy snow or high-wind area and need the extra strength or greater nail penetration. And, because this material is tongue-and-groove on all four edges, there's minimal waste.

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    Ganged 2x6 posts carry the point loads to the foundation at each truss location. It's important for the framers to observe a "no nail zone" in the post so that a pocket can be chiseled for the knee brace tenon (see upper detail). A 2x6 "mini wall" makes up the difference between the double top plate and the top plane of the truss (lower detail).

Cost

How much is this going to cost? The short answer is that it will be noticeably more costly than a simple drywall cathedral ceiling. The upside is that you'll have a dramatic exposed timber ceiling that is also well insulated. Exact cost depends on several factors, including timber species and grade, how big and complex the job is, and the level of craftsmanship the customer wants. Any timber-frame company, once you've provided your floor plans and elevations, can walk you through the variables and present a number of options.

As an example, take a great room 16 feet wide and 24 feet long, with a 10/12 pitch. Let's say we go with two strutted king-post trusses and three bays of purlins and ridges, two of which die into stick-framed gable ends. The species is Douglas fir and the grade select structural. Let's further assume that the joinery is fully housed, the edges stop-chamfered, the timber dead smooth and finished, and the frame itself sealed by an engineer.

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Working on the "no nail zone" a carpenter prepares the knee-brace mortise in a 2x6 support post.

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The knee brace is then persuaded into place.

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On this job, the top-sloped "mini wall" was added after the trusses.

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The cost for all that, FOB, might range from $9,000 to $13,000. Crane time on a job that size would run about four to six hours. As for the built-up roof, we figure about $1.20 per square foot for the 3-inch-thick extruded polystyrene, and around $1.10 to $1.70 per square foot for the 1-by T&G, depending on species and grade. Lock-Deck runs between $3.50 and $5 per square foot.

In Answer to the Purists

So, is the hybrid approach true timber framing? Of course it is. The same joinery is involved, members are larger than 4x4, pegs are used, and the timbers provide structural support. The only difference is that by selectively limiting the areas where you want the timber framing, you maximize impact and minimize costs.

A good number of years back, I encountered the first of my clients to request that only a few areas of her house be timber framed — and not because of budget limitations, but because that was what she wanted. At the time, being somewhat of a purist myself, I reacted badly. I felt that she was misguided, and pushed for a whole-house frame. She stood fast. Later into the job, she insisted on whitewashing the Douglas fir timbers, rather than going with my preference, tung oil. Again she stood fast.

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The author's built-up roof method provides both ventilation and insulation. First, T&G ceiling boards are installed face-down over the trusses and purlins.

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Next come two staggered layers of 3-inch-thick polystyrene insulation.

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Followed by flatwise 2x4s, which get nailed to the timbers with ring-shank pole-barn nails.

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A layer of 5/8-inch sheathing completes the buildup.

As time passed and the project came to a close, I came to realize that her taste and perspective were, in many ways, superior to mine. We've since married. Now, when we find ourselves at an impasse, she takes charge. And I've stopped reacting badly.

Anthony Zayais the president of Lancaster County Timber Frames, Inc., in Lititz, Pa.