Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
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The following drawings:
The plans have a List of Materials, showing a breakdown of materials for the floor and roof, including the skylight and its flashing.
The Instructions include the following topics:
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Orient the floor plan and elevations so you choose which side of the gazebo will be the front side. The front and back will look a bit better than the sides, since the overlap of materials doesn't show on the front and the beam is shown along its face rather than its ends, as on the side. I went with footings and 8x8 square concrete posts. Also acceptable for the gazebo is a footing with 12" round concrete piers formed with sonotubes. The beam needs support in the middle of it so go with a 12 x 12 concrete pad or footing with a double 2x4 post supporting it. Nail the beams together with 3" common galvanized nails 16" apart, top and bottom. The concrete piers or posts should extend at least 6" above any grade and the bottom of the footing needs to extend below the frost line. The 6x6 posts should be attached to the top of the piers with the use of a post saddle rather than embedding the post itself into concrete. Allow for the concrete posts correct width so that the wooden 6x6 post will be 12' exactly to the outside of the gazebo, that is 12' - 2".
The gazebo floor is supported off concrete pads and 2x4 wooden posts under double 2x8 beams. The gazebo roof is supported independently off triple 2x10 beams on 6x6 posts on a post saddle embedded in 8x8 concrete columns on 16x16x6" deep pads. Notice on the Front and Side Elevations that the 2x4 posts under the floor beams share the 16x16 footings in the corners with the concrete posts.
The four concrete posts should be poured with their footings in place. Their elevation is not critical at this point, you can level the top beam by cutting the posts off level with the shortest one. Check each corner of the gazebo both ways before cutting.
Once the posts are installed and to the right height, the triple 2x10 beam can be installed. With any beam or joist make sure that the crown of the board is assembled on top, so when a load comes on top of it, it will settle down straight. Make sure the bearing of the beam has 3" on the post. Assemble the beam perimeter first on the outside of the post, then double up the beam on the inside, then install the third beam to each side. This will stagger the joints on the post to give a full bearing on all joints. Make sure the outside of the posts and beams are 12' on the gazebo.
Depending where you live and the amount of wind or earthquake threat in your area, we should tie the beam to the posts of the gazebo more securely than just with toenails.
Referring to the drawing, the scab should be the same size as the post, a 2x6 and nailed or screwed similar to the pattern shown. A 45 degree cut on the end dresses it up a bit. Leave a slight reveal of the square edge, as shown.
Now that the posts and beams are up and securely nailed, brace up the corners of the gazebo in both directions with temporary braces to hold everything plumb. The braces can be tied into 2x4 stakes driven into the ground. These braces will remain in place until the gazebo roof is on, sheeted and nailed.
You can build the gazebo floor now or after the roof. It may be easier to work on the gazebo roof if there is a nice level floor to work off. Nail up the beams on both sides of the gazebo to support the floor joists. Level them as you install the posts. If the beams for the roof are level, you could measure down the post from on top of the beam to about 9'-3 1/8", if you want an 8' ceiling.
Cut 2 - 2x8s 12-0" and lay them both out together for the box joists. Start at the left end, measure 15 1/4" and place a mark with an X on the right of the mark. Now move the tape to this mark and continue to mark and place an X in the same manner at every 16" mark on your tape, until you run off the 2x8. When you nail the joists onto this box joist the left edge of the joist will be on the mark covering up the X.
After nailing on all the joists on the gazebo you should be able to place your tape on the edge of any joist, except the first one, and read a multiple of 16". This will enable the plywood to end on the center of the joist.
When the joists are all nailed on, measure their diagonals to be sure the gazebo floor is square. Bump one box joist to the left or right until the diagonals are the same and the floor is square. Go ahead and nail the plywood on. When using 5/8" tongue and groove (T & G), we usually start with the tongue flush with the outside of the floor and the groove towards the inside of the floor. Since the floor will not have a wall over the edge, I would suggest you cut the groove off and add a filler to the other side. Unfortunately, tongue and groove plywood is 48" wide including the tongue, so on a 12' wide floor the sheets are going to come about 2" short of the width. I would suggest ripping the first 4' from the spare piece you have left and then ripping two other pieces with a groove in one edge. You can eliminate the grooving of a thin piece by buying another 2'x8' sheet from your local building supply. Usually a couple of passes through the table saw and a groove is easy to cut out.
Notice the stamp on the plywood, it usually says place this side down (the tongue is thicker on the bottom). Nail on your first sheet, full length and a 4' sheet end to end, snug to each other. Just use a few nails to keep the plywood in place and mark where the joists are. Keep them back from the groove side by at least 6" to allow the next sheet to slip into the groove easier. Install the third sheet, the other half of the 4'x4' and start on the left, the way you laid out the joists. Place the tongue up to the existing groove get your helper to stand on the tongue and groove together while you hammer lightly with a sledge hammer against a 2x4 against the groove side of the third sheet. The sheet should slip into the groove of the one nailed on. Now install the fourth sheet (full size) next to the half sheet. This is called staggering your sheets so the joints are not in the same line. Continue the next row with a full sheet, staggering your sheets again. Nail or screw on the plywood securely: nails spaced 6" apart on all joints and 12" apart in the middle.
Make a box for the skylight curb out of 2x12 and 49 1/2" x 49 1/2" square, outside measurements. The skylight is a 4'x4' standard unit, which means you need a 46 1/2" square curb on the inside. Scribe a line or snap one at 4" down from the top. At this line your hips and common rafters will be attached. The rest of the curb should stick up for the skylight to be fastened on. Layout the common rafters on this line as shown on the plan: the rafter in the center and the ones on each side 1 1/2" in from the corner of the curb. This places the gazebo's ceiling joist directly below the curb. Put 2x4 posts from the top of the ceiling joist up to the bottom of the curb. This will give some added support to the gazebo under a heavy snow load. If you are finishing in the ceiling, studs can be added between the posts.
The 2x8 hips will be cut with a plumb cut of 5/17 using a framing square at a compound angle of 45 degrees, which is 16 1/2 degrees. This is a bit of a trick since we want the hips to have a notch at the upper end as shown on the plan. Use a circular saw for this and set your depth at 7/8". Maybe try some practice shots first.
These measurements are dependent on the outside of the beams being exactly 12' on the gazebo and square.
Notice on the Roof Plan I put the ceiling joists in red. The purpose of the ceiling joists is to prevent the posts and beams of the gazebo from deflecting out with a load on them. On the Ceiling Joist Plan, notice the 4 circles. These denote that joist hangers are required on these intersections to provide the continuous aspect of ceiling joists being tied to opposite walls. This helps keep the walls of the gazebo from bowing out under a heavy load. Make sure these are installed. Whether you want a ceiling on your gazebo doesn't really matter, you can leave it open or install a ceiling of cedar boards or whatever.
Cut 12 common rafters with the 5/12 slope on each end. If you have a cut off saw the 5/12 angle is 22 1/2 degrees. The length is 51 3/16" to the outside of the beam and another 15 3/4" to the end of the overhang or 14 1/2" on the level. Total length of each common rafter is 66 15/16" as shown:
Cut the bird's mouth as shown by sliding the square along the plumb cut line until 3 1/2" is reached. Cut one rafter as shown and mark the others from this template. This should give you a heel height of 2 1/4".
It is good to have some help when installing these rafters on the gazebo. One person is at the curb and the other at the beam line. The curb will be heavy and awkward to keep in position until all 4 hips are in place. The bottom of the curb will be about 25" above the wall line. It may be a good idea to double up on two ceiling joists. Lay them on the flat and block the curb up on them, wedging them up a bit at a time to find the correct height while fitting the hips into position. Remember to have the curb raised about 4" above the rafter lines. Use the line on the curb for nailing the hips and commons in position. When the four hips of the gazebo are in place and the curb is level, start assembling the common rafters as shown on the plan. Keep the outside commons in from each side of the curb by 1 1/2" and the center one dead center. The rafters should be tight against the curb and tight against the seat cut on the beam.
Now we can fill in the gazebo roof with the hip jacks.
Measure the length of the space where the jacks will go. There should be a common difference of 26" for the jacks if fastened in on 24" centers. Try the first one in place, then use it as a template to mark the other ones. Subtract 26" from this first jack to arrive at the length for the second. Try it and use it as a template for marking the others.
The best way to cut the gazebo's jacks is to cut the side cut in the center of the 2x6. This way you can use the same cut on the other end of the board, turn it over and use it for the opposite jack. To cut the side cut angle that goes against the hip, if you have a compound bevel and miter saw, cut the miter 47 degrees and the bevel at 22 1/2 degrees. With a circular saw use a rafter square and mark the plumb cut - 5/12 and mark the top edge of the board with 12/11 1/16 marking the 12 side of the square. Failing this, estimate two degrees above 45 if you can squeeze a bit more than 45, otherwise, just cut the plumb cut at a 45. Notice I gave you the length of the long angle. To cut the tail of the jack, use your common rafter template and scribe the bird's mouth and tail cut exactly as for the common rafter. There are only two sets of opposite jacks since all four corners of the gazebo are the same. When nailing in the jacks install them in pairs making sure that the hip remains straight. Just sight along the hip and hammer the jack tighter to move it over a bit if necessary.
Now that all the rafters are in place on the gazebo, install the 2x6 ceiling joists to tie them all together. Watch the Roof Plan for the direction they go. Make sure you nail the joists to the side of the rafters and into the beam, as well. Try to stay on the side of the rafter that is shown, this way the joists will be uniform if a ceiling is later desired for the gazebo. Backing is nailed on the top of the beams to provide nailing for the ceiling where it parallels the beam. This backing is usually a 2x6 nailed on the flat with half of it overhanging the beam and the other half nailed onto the beam. Notice you need to cut off the top of the ceiling joist where they project above the slope of the roof. A reciprocating saw is good for this.
Install a 1x8 fascia board around the perimeter of the gazebo. The top of the fascia board is flush with the top of the sheathing. This way it hides the end grain of the plywood or OSB. Finish the eaves as desired, either leave them open or close them off with cedar boards.
Apply your choice of shingles as suggested by the manufacturer on the bundles. No need to add a heavy eave protection if the gazebo is not heated. After the shingles are applied, install the skylight flashing. This will be four pieces of front apron going up the curb and extending down over the shingles, overlapping about 4" at the corners. The skylight will overlap the flashing on the curb. Put a bead of caulking on top of the curb before mounting the skylight. Just to be safe make sure you build the curb and try the skylight on it before assembling the rafters. All my measurements are taken from the curb size I gave you. Fasten the inside of the skylight to the inside of the curb.
Remove the temporary braces and enjoy your new gazebo.
Dave(Ask Dave) (About Dave)