The standard pergola -- literally, an arbour -- is a four-sided decorative garden structure often used as a host for climbing-vine plants such as grapes and roses. But a three-sided, or triangular, pergola also is possible, and can be a good project for the corner of a house or an odd-shaped garden.
Decide in advance how large you would like your pergola to be. This will help establish the spacing of each post for your pergola. Hammer a garden stake in the ground with a mallet at the location of your first post.
Using a measuring tape, measure the distance to the second post. Place a second stake in the ground at this location.
Place a third stake in the ground at the remaining edge of the triangle.
Measure the total length of your 10 cm by 10 cm (4 by 4 inch) posts with a measuring tape. Using post-hole diggers, dig a hole at the location of your first post that is one-quarter the total length of the post.
Pour gravel into the bottom of the post hole to a depth of 5 cm (2 inches). Then place one end of a post into the post hole.
Brace the post with 5 by 10 cm (2 by 4 inch) boards so that it is perpendicular to the ground. Check that the 10 by 10 cm (4 by 4 inch) post is level with a carpenter's level.
Mix concrete with water until it is the consistency of a mud pie. Pour concrete into the post hole surrounding the 10 by 10 cm (4 by 4 cm) post until the hole is filled. Allow the concrete to cure up to 72 hours.
Repeat each step two more times to set the three posts of the pergola in the ground.
Measure locations where the posts will attach to your deck, and mark them with a piece of chalk.
Attach deck-post brace brackets to the deck in the locations where your posts will attach with wood screws.
Place 10 by 10 cm (4 by 4 inch) posts into the brackets and secure them with wood screws.
Measure the length of space between two posts.
Cut a 5 by 10 cm (2 by 4 inch) board to the same size as the length of space between the two posts. This board will be one of the beams of your pergola.
Lift the beam to the top of the posts, and attach using wood screws and a wood connector bracket at each end. Repeat this step two more times to attach the remaining two beams to the top of the pergola.
Decide which direction the rafters will run along the triangular decking surface of your beams.
Lay out the 5 by 5 cm (2 by 2 inch) rafter boards in the correct configuration on the ground. Measure the distance between beams on the decking of the pergola. Add 10 cm (4 inches) for overlap to the edges of each beam distance.
Cut the rafter boards into the correct length to fit their location on the beam. (For example, the beam on the corner where two beams come to a point will be much shorter than the beam on the side of the triangle that is parallel to the rafters.)
Lift the rafters and place them in their correct spot on the top of the beams. Fasten the rafters to the beams by hammering a nail through the beam and into the rafter where the beam and rafter touch.
To avoid cutting your 5 by 10 cm (2 by 4 inch) beams, set your 10 by 10 cm (4 by 4 inch) inch posts 23 feet and 8 inches apart. The length of a standard 5 by 10 cm (2 by 4 inch) board is 7.30 metres (24 feet). Setting your posts at this distance will allow you to install the beams without first having to trim them to the correct size.
Always wear a safety helmet when lifting heavy items such as wooden boards over your head. Wear safety goggles and gloves when operating machinery such as a saw or drill. Always wear protective eye-wear, breathing protection, gloves and protective clothing when mixing concrete.