Whether for a tiny house or a green house, a wooden arched roof is a great DIY way to give extra head clearance while providing a strong overhead structure as well as being economical with materials. Arches have been such a classical and commonly used component of architecture because of their ability to distribute weight from the apex of their curve down into the vertical structure of the walls....or to clarify, any force put on the top of the arch is transferred directly into the walls and then the ground. Since arches are so awesome, let's make some!

Step 1: Tools/Materials

For this project you will need the following components:

1cm or about 1/2" plywood sheets.
- I used sheets that had been previously used for concrete forms. How many you need depends on how many arches you can get out of a sheet and how many you want/need. My arches were 10cm wide and three sheets of plywood thick and arched a span of 2m.
5-6 beams that will end up being the trusses for your roof. One of these should be at least as long as your structure and the others can 4 or 5 beams can be made up of multiple pieces that join on each arch.
Wood glue
A jigsaw (you will definitely need this).
A drill and some 25 or 30mm screws
Measuring tape.
Some string (should be as long as the distance you want the arches to span)
A square or other tool to measure right angles.
Hand saw (optional)
A bit of sand paper.
Pencil(s)

Step 2: Drawing an Arch (Archery?)

Don't worry, drawing the arch will not require you to do any math. You will need a string, a pencil and a couple of nails or screws or tacks...possibly tape.

The images for this step should clearly illustrate the process but I will also describe them:

Measure out the span of your arch on a sheet of your plywood and cut a piece of string to the same length (represented in the images as the line which connects A to B).
Find the center-point of your first measurement on the plywood and draw a line at a 90-degree angle from that center point until you reach the desired height of your arch (represented by point C in the images).
Now find the center of your string and while holding its center-point to point C, stretch each end of the string so that it touches the line connecting A and B while being as taught as possible. Mark the two points that the ends of your string touches. We will call these new points, 1 and 2.
Use a tack, screw, nail or tape to secure the string to points one and two.
Now with the string secured to points 1 and 2, you should be able to slide your pencil along the inside of the rope between points A, B and C in order to draw your desired arch. By keeping the string taught, you can draw a line on the board to mark your arch and the string will self adjust to keep the appropriate distance from the center point.
Repeat steps 1-5 with your new A, B and C points being 10cm closer to the center than the previous ones.

Step 3: Cut Out Your Arch

Ok, this step is pretty straightforward.

Use a jigsaw to cut the arch that you drew out of your plywood.

*You can also draw your first arch on paper, cut it out with scissors, and then use that to trace multiple sections of arch out of smaller pieces of plywood which you will later piece together into a full arch. My full arches were made up of two or three segments in a length and then glued and screwed on top of each other to create a full piece.

Step 4: Sanding

Sand the edges of your plywood arches or arch segments.

Step 5: Production

Trace out your arch or arch segments onto the rest of your plywood. Cut them all out and sand their edges, just like you did with the first.

Step 6: Segments

Regardless of whether you cut out full arches or segments, you will need to glue at least three layers of arch, or arch segments, together in order to make your final arch piece.

Apply glue to one side of a layer of arch (segments) and place the next player on top. Screw them together while the glue dries. Add glue the top of these two laminated arch layers and then add one more layer, screwing it afterwards.

Repeat this to make as many arches as you need (or have wood for).

Step 7: Trimming

Depending on how you drew and pieced together your arches, their ends maybe a bit strangely shaped. You should flatten them out so that they will be easy to attach later on.

Do this by leaning them up against a wall so that they stand up at nearly a right angle. Place a spare piece of wood on the floor right in front of one end of the arch. Draw a pencil line along the top of that piece of wood and repeat this at the opposite end of the arch. Providing that the sides of your wood are parallel, you should have a straight line marked on both ends of your arch. Trim off the two ends where you just marked them.

Repeat this with all of your arches.

Step 8: Arches

Now you should have several full arch pieces to work with like those in these images.

Step 9: Notches for Trusses

To make a space for your lateral trusses, you will need to cut notches in your arches. Trace out a space for your trusses at each end of your arch and also at the center. Probably you will just need one truss between the apex and and end trusses but if you have a really big arch you will need more than one between. In the case of a really large arch, measure out your trusses so that each one is no more than 50cm from its two neighbors.

Use your first arch (with all of the notches cut out of it) to trace where notches will go on the rest of your arches. Cut out all of your notches.

Step 10: Installing Arches

So now you have a pile of wooden arches. Well done! Since you have them, you should install them somewhere. How you choose to install them is ultimately up to you but I will suggest the following way...

Start by clamping the two end arches to your structure. Each end of each arch should sit on your walls at a right angle to them. You can see how the end arch is clamped in the picture and the others are upside down, waiting to be put into place.

With your two end arches clamped, use screws to toenail* them to your wall frames.

Now you can place your center truss into the slots at the apex of either arch and screw it into place. The ends of the truss can go beyond the end arches or they can stop at each arch, that depends on your preferences.

*Toenailing or skew-nailing is a popular technique that carpenters use regularly to fix two timbers together by slanted application. The fasteners (nails or screws), used in pairs, are driven in on opposing angles. This locks the timbers together, to create a stable framework, e.g. in stud walls (partitions) or roof framing. The angled nailing makes later dismantling difficult or destructive. One of the most common places to toenail a framing member is where a rafter meets the top plate of a wall at the birdsmouth. Alternatives to toenailing include the use of hurricane ties, joist hangers, and other engineered steel connectors that permit nails to be attached perpendicular to a member's surface.

When toenailing, nails can be driven from the inside or outside of the joint, depending on access available to use the hammer. Skew nailing is also a technique used by other woodworkers, for example a drawer or box can be glued and skew-nailed with finer nails or panel pins. Skew nailing will fasten the joint, while the glue sets, avoiding the use of clamps. A variation of toenailing is to use screws for the same purpose, which might be called toenailing or toe-screwing. - Wikipedia

Step 11: More Arches

Install the rest of the arches, making sure that each one stands at a right angle to the wall frames. you can clamp them into place on the center truss while you toe-nail them into place on the wall frame.

Step 12: Angles

To add extra support, I used some extra pieces of plywood that I cut into right triangles. I toe-nailed each of those plywood right triangles into the wall frames and also into the arches so that they prevent the arches from pivoting at their base.

Step 13: Finishing Trusses

You should still have a few trusses to screw into your arches at this point.

All you have to do is pop your trusses into each notch that you've cut for them and screw them into place.

You will need these additional trusses to create an even curve in your roof cover.

You can also go ahead and cover your roof. This can be done with whatever material you like...

You could use some kind of large fabric or plastic that you just have to staple into place.
You could also create a hard top by bending thin plywood over your arches and screwing it into place. Repeating this process with multiple layers that have glue between them will create a super strong roof that you can coat with something weather-proof such as tar and asphalt tiles.
Another option is that you could get a thin sheet of steel/tin/aluminium and screw it into your arches to create a curved metal roof but it could get very hot in the summer months.

Step 14: Roof.

Now you should have a fully functioning, light, and sturdy wooden arched roof. Congratulations!
I hope that it serves you well and you had fun in the process.

If you have any questions, statements or concerns about this Instructable, please leave a comment and I will get back to you as soon as possible.