A wooden arch is simple to construct, and when built from treated lumber it makes a long-lasting support for plants. Whether the roof is pitched, as below, flat and pergola-like, or rounded, the procedure remains the same; mark out the area, assemble and erect posts and beams, then place the rafters on the top. These instructions are for a kit with 8 feet (2.4 m) posts set 4 feet (1.2 m) apart left to right, and 1 foot (30 cm) deep.
When working with lumber, make sure it is treated — including at the sawn ends — with a preservative to prolong the life of the wood. Always attach joists with galvanized nails or rustproof screws, and use metal "shoes" to prevent the bottom of wooden posts from rotting in damp ground. Ensure your safety by never sawing wooden joists in the air on a stepladder; instead move to ground level and saw on a firm bench.
hammer or mallet
4 spiked metal post supports or "shoes"
4 lengths of treated, sawn lumber for beams, as supplied
4 lengths of treated, sawn lumber for posts, as supplied
6 lengths of treated, sawn lumber for rafters, as supplied
rustproof screws or galvanized nails Wood preservative
Use a large set square or framing square to measure out the exact position of the archway: this ensures that the 2 sides of the arch are parallel to each other and that the front and back are square to the sides. Using a mallet, drive in a marking peg at 2 corners, winding the string around the outside of the peg.
Draw the string line taut along the sides of the framing square. Check that you have measured accurately (always measure from the same points), adjusting the pegs if necessary.
Using a mallet and driving tool, hammer a spiked metal support for the arch posts into each of the four corners exactly where the measuring peg was. Check each one with a level to ensure it stands upright.
First, join the pitched beams. These lengths of lumber for the back and front arch top should be cut in a half-lap joint at the correct angle for the arch: how steep this angle is determines the pitch of the roof.
If the kit is not pre-drilled, drill two holes in the join. Attach with rustproof screws. Similarly, join the beams for the other side of the roof.
Butt them up firmly to the metal supports to hold them steady and to confirm they will be exactly parallel once erected. This also allows you to fit the top of the arch to the posts while they are still on firm ground, rather than balancing high up on a stepladder. This is possible only where the posts are not too heavy to lift once assembled.
With a helper, lift up the front of the arch and lower the posts into their supports. Repeat for the back. Check that the posts are straight, then screw each one into its support.