How to Build a Flat Roof Pergola


A pergola is an outdoor structure providing a large shaded area where backyard activities can be conducted in an architecturally attractive setting. It gives people the feeling of being indoors while enjoying the outdoor beauty of the lawn and garden. A pergola can also be small enough to sit in and enjoy a hot cup of coffee alone in the early morning. A flat roof pergola can offer shelter from the rain or the harsh sun of summer. While a pergola is not generally roofed over, it is not that difficult a project to undertake.

Things You'll Need

  • Measuring tape
  • Post-hole digger
  • Shovel
  • Quick setting cement
  • Mixing tub
  • Cedar posts, four 4-by-4 inch by 10 feet
  • Carpenter's level
  • Electric drill
  • Drill bit, 3/4-inch
  • Phillips-head bit, 3/4-inch
  • Bolts, stainless steel, 3/4-inch by 10 inch
  • Washers and nuts, stainless steel, 3/4-inch
  • Cross beams, four 2-by-8 inch by 10 feet
  • T-square
  • Pencil
  • Hand-saw
  • Stretcher beams, 2-by-8 inch by 10 feet
  • Wood screws, stainless steel 5-inch
  • Nails, galvanized, 4-inch
  • Plywood sheets, 1/2-inch-by-4 feet by 8 feet
  • Tar-paper
  • Cedar planks, 1/2-by-8 inch by 8 feet
  • Staple gun
  • 1/4-inch staples
  • Determine where to build the pergola. Choose a flat level spot in an attractive area of the lawn or garden that won’t interfere with the line of sight from the house. Mark four spots to form a perfect square where each post is exactly eight feet apart.

  • Dig holes 3 feet deep and 8 inches in diameter. Set the support posts one at a time in the holes and fill the holes with quick setting cement, mixed according to package directions, to within 6 inches of the top of the hole. This 6-inch space will be filled with dirt and planted with grass after the pergola is built.

  • Set the posts so that the outside of each post is flush with the other posts so that when cross beams are added they will set flush against the sides of all posts. Make sure each post is in plumb by placing a carpenter’s level to the side of it and adjusting the post accordingly. Brace the posts as necessary until the cement is set, according to package directions.

  • Measure three bolt-holes, in a straight line, from the top of the post down 3 inches, 6 inches and 9 inches, and drill three 3/4-inch holes through the posts. Use 10-inch long, 3/4-inch stainless steel bolts, washers and nuts to attach the 2-by 8 inch crossbeams to the outside of each post.

  • Measure half the width, or 3 inches up, at the end of each stretcher beam. Use a T-square to measure up 3 inches and measure in 6 inches, mark with a pencil and cut out the piece of wood with a handsaw. The notch on each end of the stretcher beams will rest on the crossbeams.

  • Drill two 3/4-inch holes through the outside of the cross beam at 4 inches and 5 inches down from the top of the stretcher. Use 5-inch stainless steel wood screws to attach each stretcher to the cross beams.

  • Nail two 4-by-8-feet sheets of 1/2-inch plywood with galvanized 4-inch nails to the stretcher beams so they completely cover the roof. Cover the plywood with tar-paper and secure with 1/4-inch staples.

  • Cover the roof with 8 foot-by-1/2 inch cedar planks, nailed closely side by side, into the stretcher beams at 1 foot intervals beginning and ending at the crossbeams. Use three nails side by side on each crossbeam.

  • Apply a waterproofing sealant or allow the red cedar to gradually turn a gray color over time. Climbing vines or hanging flower pots can be arranged as desired on the pergola.

Tips & Warnings

  • Assemble all tools and materials before beginning the project. Separate the various materials so they will be easy to identify and use when it is time.
  • Enlist the help of a friend to assist in supporting and balancing posts and beams to avoid injury.
  • Measure carefully and position the support posts as directed before the cement sets. If the posts are crooked the entire structure will be very difficult to work with.

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  • Photo Credit Thinkstock/Comstock/Getty Images
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