*Optional: recommended for high wind areas or where required by code.
Skill Level: Beginning woodworker
Time to Complete: One weekend (allow additional time if you include concrete footings)
Estimated Cost: $1,200 (with footings)
First, select a site that is reasonably level, and drive a wood stake at each corner (each wall measures 93 inches). To ensure a square footprint, adjust the stakes until the diagonals measure the same length (131 1/2 inches). If you live in a windy area or your local code requires concrete footings for a structure this size, dig holes for four concrete piers under the side. Level the cardboard forms in the holes, fill with mixed concrete, and insert a post base bracket in each while the concrete is wet. For our project, we also trenched for the sides and back wall so the sill plates would nest flush with the ground. Use the trench digger to dig the trenches several inches deeper and wider than necessary, then add crushed rock or pea gravel to the trenches to improve drainage. Let the concrete footings cure for at least three days.
Build the Window Frames
Although the window assemblies are installed later in the process, building them first lets you space the beams and collar ties correctly. The finished window frame sizes will depend on the window sash you buy. For the frame stock, use the circular saw to cut 1X4 cedar boards to the required lengths and attach them to the sash as shown in the Window Assembly Detail illustration. Then use a table saw to rip the extra 1X4 stock into 3/4-inch molding strips and attach them as shown, using 5d finishing nails. Set aside for now.
Cut and Assemble the Wall Frames
Start by cutting the sill plates (A) to length; because they rest directly on the ground, these timbers should be pressure-treated pine rated for ground contact use. Then cut the other frame parts (B - D) to length. For the top wall plates (D), mark the half-lap notches as shown in the Half-Lap Joint Detail illustration, and use the portable circular saw to make multiple cuts in the notch portion. Break away the waste material with a hammer, then use a chisel to pare the surface flat and smooth. Note that these end notches are the same for all four top plates, but are oriented differently -- set the front and rear plates with the notch faceup and the side plates with the notch facedown. On each corner post (B), mark the locations for the sill plates (A) and crossbeams (C). The vertical distance between the two beams should equal the height of the taller window assemblies. On a flat surface, such as a patio or driveway, fit together parts A - C of one side wall. Use the 12-inch-long drill bit to drill a 1/4-inch hole through the posts and into the beam/sill ends at each joint as shown. Then switch to a 3/8 -inch bit and enlarge only the holes in the posts. Tap a 3/8X6-inch lag screw into each hole until it seats firmly, then tighten with a ratchet driver. (Use this technique on the roof frame joints, too.) Set some scrap blocks of wood across the wall trenches, then recruit a helper to lift the first wall assembly into position; use ground stakes and temporary braces to stabilize it. Repeat for the second wall section, then use more lag screws to connect the two walls with the rear sill and beams, creating a U-shape structure. With at least two helpers, remove the scrap blocks from the trenches and lower the frame assembly into place; check for level, plumb, and square, and attach the sills to the post base brackets in the concrete footings. Use 3-inch deck screws to attach the top wall plates (D). Backfill the trenches with gravel.
Cut and Assemble the Roof Frame
Rather than precut all the parts for the roof frame, work in stages so you can test-fit the pieces as you go. Start by attaching the center beam (E) to the front and rear top plates (D) with lag screws, just as you connected the wall crossbeams to the posts. Next, use the power miter saw to cut four rafters (F) with 45-degree miters at the base ends; leave the opposite ends a few inches longer than listed. With a helper, set two rafters up at the front gable end, with their angled ends placed at the corners and the upper ends slightly offset so they bypass each other. Mark each rafter end along the upper edge of the opposing rafter, then square-cut those ends and cut half-lap notches as shown, similar to the top wall plate ends. Refit and attach with 3-inch deck screws to the top wall plate corners as shown. Repeat for the rear rafters. With all the rafters in place, fit and attach the ridge beam (G) and the purlins (H) with lag screws as shown. Using the smaller window assemblies for spacing, cut and fit the collar ties (J) into the end gables as shown; attach with 3-inch deck screws. Then cut and attach the two ridge blocks (K) the same way. Finally, cut a 45-degree miter on one end of each roof brace (L) and do a test fit to mark the lower end; cut to fit, and attach with lag screws as shown. With the frame complete, apply a coat of exterior-grade clear sealer or semitransparent stain if desired.
Install Roofing and Windows
Use the table saw to rip two cedar 2X4s with a 45-degree bevel cut to make the roof plates (M); use 3-inch deck screws to attach them to the top wall plates, with the bevel facing out and aligned with the rafters as shown. Starting at one end, place a corrugated metal roofing panel along the rafter edge, with a slight overhang and with its top edge slightly below the peak of the ridge beam. Attach with washer-head screws. Overlap the next panel at least one full ridge, and fasten. Repeat to install the remaining roof panels, then attach the cap flashing along the ridge. To install the window assemblies, simply fit them into place and drive 1 5/8 - inch deck screws through the 1X4 stock into the crossbeams, top wall plates, and/or collar ties.