Cost of Building a Pergola
The cost of building a pergola depends on the materials used. The structure can be timber, steel, reinforced concrete or a combination of these. However, regardless of which materials are used, the construction cost is not going to break your bank because this is a simple structure and small in size. A few materials are needed and one man can finish building the structure in a day.
The traditional Italian pergola was an ornamental outdoor structure situated in the garden. It was a shade with four posts, and roof made of beams and rafters to allow for climbing plants. As the plants grew, they would cover the roof, providing a natural shade. Free standing pergolas are built in the garden. However, you can also have pergolas which are attached to the house, occupying the patio area.
What is the purpose of a pergola? As you may have guessed, it provides a shaded outdoor area to relax and rest. It’s a good place to sit during hot mornings and afternoons. It can be a quiet place to rewind if it’s located in the garden or backyard.
Modern pergolas are not necessarily square or rectangular in aerial plan. They can also be triangular, circular, hexagonal or octagonal in shape. It’s important to note that no roof covering is required for a traditional pergola, although some modern pergola designs may feature roofs with thatch, aluminium, tin, fibreglass or polycarbonate sheets. The structural frame may be vinyl, aluminium or architectural grade laminated saligna timber beams.
A free-standing pergola will have four posts, either timber, steel or reinforced concrete columns. Beams and rafters are going to be timber or steel. The floor can either be natural ground with lawn, timber board/deck, precast concrete roadstone paving, clay brick pavers, natural stone or cast-in-situ concrete pad.
How to Build a Timber Pergola
A timber pergola is very easy to set up. Timber will be used for the structure except the floor. Before you start digging holes on the site, you have to find out the location of underground service cables, water and drainage pipes. Choose a site that is free from underground cables and pipes. DO NOT build on top of the pipes, remove, damage or divert their direction. If you need to divert the direction of the pipes, you have to consult the local authorities first.
Now that you have a suitable site which does not interfere with underground pipes, you can begin your construction activity.
Clear the site that you want to build on. Remove grass, rubbish, shrubs etc. The area to be cleared is roughly equal to or slightly bigger than the floor area of the pergola – Area: 4100 x 3600mm
If you intend to build on a fairly level lawn field, you won’t need to remove grass or clear anything.
After clearing the site, scarify the top soil to a depth of 150mm. Level and compact the ground to 95 % Mod AASHTO density.
Now that you have level, prepared ground, you can start digging holes. Before you dig holes, you must mark the site and set up pegging lines. Mark the rectangular floor base, as well as the position of the foundation holes.
Dig four holes, size 400x400x500mm deep. Compact bottom of holes to the required density. Apply anti-termite/ant soil poisoning and weed killer on the bottoms and sides of the holes. Finally, spread a layer of 50mm sand blinding.
Concrete Foundation Bases and Posts
Before you pour in concrete foundations, you must set up the posts straight and level inside the holes. The timber posts (100x100mm thick square posts) are going to be embedded inside the concrete.
Set up the posts on the bottom of holes and hold them in position using braces. Prepare wet concrete, grade 25 or 30MPa and pour inside the holes, around the posts. Tamp or vibrate the concrete to prevent air bubbles.
The overall length of the timber posts is going to include the foundation depth – (2200 + 500mm) = 2700mm high.
Allow the concrete to set. This takes at least 24 hours. When the concrete has set, remove the pegging lines and braces.
Precast Concrete Paving Blocks
The pergola floor is going to have precast concrete paving blocks. On prepared ground, add a 25mm thick sand bed. On top of the sand bed, start laying precast concrete pavers. The blocks, size 200x100x60mm thick, 25MPa strength, will be jointed with a cement/sand mortar mix.
The approximate area covered by the blocks will be (3,500 x 3,000) mm.
Once you have the posts set up, you must install the beams. The beams are used for supporting the rafters and carrying away the load to the ground via the posts (columns). For this project, beams of size 200x45mm thick will be used.
Overall length of beams – 4200mm long, including 300mm overhang at both ends.
To achieve a good looking elevation, the beams are sloped at the ends.
- Horizontal distance from end – 300mm
- Vertical dimension – 35mm
To fix the beams in place, you must first cut out a rebate joint (bearer) on top of the posts. The rebate joint will be 200mm high x 100mm wide x 45mm deep. Lift up the beam and rest it horizontally on the rebate joint. Drill two holes on each joint, and fix the beam to the post using bolts and nuts.
You will need two beams (4100mm long), one on each side.
Timber rafters, size 200x45mm thick will be fixed on top of beams. The overall length of rafters for this project will be 3700mm, including a 300mm overhang at both ends. Using a hammer or nail gun, fix the rafters on top of beams with long nails hammered diagonally (45 degrees or less) to the vertical line. Hammer in a nail on both sides of the joint.
Number of rafters required – 8 No.
Rafter spacing – 461.43mm end to end spacing.
Finally, to finish your roof structure, you must lay softwood battens across the rafters. Fix the battens on top of rafters using nails.
Batten size – 38x25mm thick.
Batten spacing – 300mm end to end spacing.
Overall length of battens will be 3700mm, including a 50mm overhang at both ends.
Posts, overall height (2200+500)=2700mm.
Size: 100x100mm thick, H5 treated square posts.
Post spacing – 3100mm (pole to pole/corner to corner measurement.)
Timber Treatment and Protection
Posts must be treated with H5 solution.
Beams, rafters and battens are treated with H3.2 solution.
In your cost estimate for building a pergola, you can include the cost of climbing plants. Allow a sum for maintaining the plant beds up to a year (watering, removing weeds, pruning and replacing dead plants etc.) You will need to plant four vines on each corner, close to the posts. A variety of climbing vines is available. This includes:
- Bignonia (Cross vine)
- Morning glory
Climbing vines and flowers are sold at the nursery in containers, measured in gallons or litres. They come in 2L and 4L containers.