Post-and-Beam: An Old Method Resurrected; Installing Sloping Roofs

As homeowners spend more time in their backyards—and desire structures that make gardens more enjoyable—they are discovering the advantages of post-and- beam construction. For the supporting framework, it uses a few big pieces of lumber, widely spaced, instead of the many closely spaced 2 studs, stiffened with sheathing, normally employed today for homebuilding. The standard construction method until the early 19th Century, when machine-made nails and mill-sawed lumber made the stud wall more economical, post-and-beam is particularly adapted for outdoor uses.

To construct a post-and-beam frame work, you anchor two parallel rows of wooden posts, connect the posts in each row with crossbeams to form a wall and then tie the walls together with rafters. Left as is, the post-and-beam framework can be used as an arbor or a trellis; roofed and sheathed with any of a variety of openwork materials, it be comes a breezy garden shelter. With weatherproof siding and roofing, the structure can be a shop, shed or studio.

The intended purpose determines the foundation of a post-and-beam building. For an open-roofed structure, a simple concrete slab or set of precast concrete piers is adequate. A closed- roofed structure, particularly one that must bear the weight of winter snows, should have a reinforced turned-down slab or concrete footings.

The size of the posts for an unroofed structure is determined by its width—the distance between the two walls. Long, narrow structures are easier to build, for if the width is less than 8 ft., 4-by-4 redwood or fir posts suffice. If the structure is wider than 8 ft. but less than 12 ft., use 4-by-6 posts.

Determining the size of posts for a roofed post-and-beam structure requires more precise calculations. As a general rule, a roofed structure should be able to support a load of 50 pounds per square foot. To determine the size of post you will need, multiply the area of the structure by 50, then divide the answer by the number of posts you plan to put up. If the result is less than 8,000, use 4-by-4 posts, which can each support as much as 8,000 pounds; if the figure is greater, use 4-by-6s, which can hold as much as 14,000 pounds each.

The size of the beams is determined by the span between posts. The width in inches of a 4” beam (sometimes referred to as a “4-by” ) should equal its span in ft.. Thus, a 4-by-6 beam can span distances up to 6 ft., a 4-by-8 up to 8 ft., and so on.

Rafter Sizes for an Open Roof

74 Anatomy of post-and-beam. Metal anchors hold together the basic post-and-beam frame work. The posts are attached to post anchors fastened in a concrete slab, as above, or to pre cast concrete piers. The post anchors are attached with lag bolts and lead shields.

Rafters to bridge the beams can be spaced as far apart as 48” in an unroofed structure; use the table below to determine the spacing and lengths of rafters for an open roof. If you plan to roof the structure, the rafters should be set no more than 16” apart. Use 2-by-4s for a structure up to 5 ft. wide, 2-by-6s for up to 9 ft., 2-by-8s for up to 11 ft. and 2-by-10s for up to 14 ft..

At the tops of the posts, metal post caps secure the beams. Rafters are attached to beams with metal rafter anchors. The beam ends overhang the posts below them, and the rafters overhang the beams. Diagonal 2-by-4 cross bracing is lag-bolted to posts and beams.

Spacing | Maximum rafter length 8ft.

Assembling the Structure

75 1. Setting the post anchors. The U-shaped anchors are bolted to the concrete slab or pier with an offset washer that permits post positions to be shifted for alignment. Place anchors and washers at each post position so that you can out line the washer holes, then drill a ¾” hole 4” deep at each mark and drop a 3 lead shield into the hole. Replace the post anchor and washer over the hole and , with your fingers, screw a ½” lag bolt into the shield. Set a post support inside each anchor.

2. Raising the posts. Nail a post cap to the top of each post and , while a helper holds the post upright on its post support, nail the flanges of the post anchor to the bottom of the post. Use eight-penny galvanized nails.

76 3. Plumbing and bracing the corners. Attach 2-by-4 bracing to hold the corner posts plumb while a helper checks with a level. The top of the bracing must be at least 20” below the post tops. When corner posts are plumb, tight en the lag bolts under them .

Mark the tops of the beams for rafters, spacing them as desired but making the first mark so as to position the outside edge of an end rafter flush with the outside edge of a corner post.

4 Attaching the beams. Set each beam atop a row of posts, marked side up, aligning the outer most marks with the outside edges of the end posts, and have a helper hold the beam steady while you nail the corner post-cap flanges to the beam. Plumb the inner posts with a level, nail • them to the post-cap flanges, then tighten the lag bolts under the posts.

77 5. Putting up rafters for an open roof. Nail a rafter anchor on the top of one beam, to the right of an end-rafter mark. Nail another anchor on the top of the other beam, to the left of the mark. Mark the side of a rafter 12” in from each end, set it against the anchors and nail the rafter to the anchors with eight-penny nails. Similarly attach the remaining rafters.

Mark positions for the cross braces on the posts and beams. On the outside of each post except the end ones, set the mark at least 12½” from the top of the post. Mark the top of the beam at least 18” to the left and right of the center line of the post. On each end post, set the mark at least 18” down from the top of the beam, and mark the top of the beam at least 18” from the outside edge of the post. Cut 2-by-4 braces to fit the marks, angling the ends at a 45° angle.

6. Mounting the braces. Tack the braces in position, then secure them with 3/8” lag bolts 3” long, fitted with washers.

To make post-to-rafter braces at the ends of the structure, mark the end posts at least 12½” down from their tops and mark the tops of the end rafters at least 21½” in from the outer edge of each beam. Cut braces to fit, tack them in place and attach them to the posts with 3 lag bolts 3” long, and to the rafters with 3/8” lag bolts 2½” long. Re move the tacking and temporary bracing.

Installing Sloping Roofs

To provide a weathertight roof for an open beam structure, install sloping rather than horizontal rafters and cover them with sheathing and roofing material. No special calculations are needed; simply estimate the length of the rafters after determining the desired pitch of the roof. This informal kind of carpentry is less precise than the techniques that building a dwelling demand but will serve admirably for a simple out door structure.

For a shed roof, build a post-and-beam structure with the posts higher on one side than on the other by an amount determined by the desired roof pitch; in the example at right, the rise is 1 inch for each ft. of an 8-ft. run—a difference in height of 8”.

A gable roof is a simple alternative and an appropriate covering for a post-and- beam structure, which has sides of even height.

Marking rafters for a shed roof. Have a helper align a rafter board, on which a chalk line has been snapped down the middle, so that the top of the board touches the top of the higher crossbeam and the chalk line touches the top outer edge of the lower crossbeam. Tack the rafter to the upper crossbeam, and mark along the outside and top edges of the lower crossbeam for a bird’s-mouth cut, a notch that fits the rafter snugly to the crossbeam. Then mark the rafter along the inner face of the upper cross beam for the ridge cut, the cut that fits the rafter to the upper crossbeam. Using a level, mark an overhang cut on the outward end of the rafter.

78a Using the marked rafter as a template, cut the remaining rafters. Toenail them on 16” centers to pre-marked spots on the upper crossbeam, and secure the bird’s-mouth cut to the lower beam with metal rafter anchors. To brace the structure, lag-bolt 2-by-6 collar beams to each pair of end posts; toenail another collar beam between the center posts and reinforce it with 3” metal straps.

Framing a Peaked Roof

78b 1. Marking rafters for a gable roof. Have a helper align the rafter board to a crossbeam and a marking guide and , after tacking it to the marking guide, mark the bird’s-mouth cut. To construct and place the marking guide, attach a scrap piece of wood the same thickness as the ridge beam and long enough to extend beyond the apex of the roof you envision, to a plank long enough to span the structure. Attach a diagonal brace to both the plank and the marking guide and position the plank on the cross- beams so that the marking guide is equidistant from them. Finally, tack the plank to both crossbeams. Mark the ridge cut and , using a carpenter’s level, the overhang cut, and use this board as a template to cut the other rafters. Re move the plank and temporary marking guide.

79 2. Assembling the frame. On the ground, cut a 1-by-B ridge beam to the same length as the crossbeams, mark it for rafters at 16” intervals and spike precut end rafters to one side of it with three 16-penny galvanized nails. Toe nail the other end rafters to the opposite side of the beam. Brace the structure temporarily with 2-by-4s spiked across the opposite rafters.

3. Setting the frame in place. With three helpers—two at the far end of the roof, one at your end—lift the frame into place, setting the bird’s-mouth cuts of the rafters onto the cross beams. If necessary, remove the temporary bracing so you can adjust the fit of the rafters, and replace it when they are correctly positioned. Spike the rafters to one side of the ridge beam and toenail them to the other side, using three 16-penny galvanized nails, then nail them to the rafter anchors ().

4. Fitting the collar beams. Set a 2-by-6 cut to the width of the structure atop the crossbeams and against a pair of end rafters, and mark it along the top of the rafters. Cut the board at the marks and use it as a template for the other collar beams. Nail collar beams to the end rafters with six eight-penny galvanized nails.

Mount the rest of the rafters, nailing a pre-cut collar beam to each pair of rafters as you go; then remove the temporary bracing.