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Home Improver Guide 8
TIMBER PERGOLAS

DESIGN STEPS

1. SELECT TIMBER

There are four types of timber suitable for pergola construction. They are CYPRESS - HARDWOOD TREATED SOFTWOOD and OREGON.

CYPRESS is most readily available in F5.

TREATED SOFTWOOD is to be regarded as F5 unless branded otherwise. Treated pine is also available as F7 and F8. Treatment to be H3 hazard level. Posts in contact with the ground must be treated to H5 level.

HARDWOOD (unseasoned) is generally F14 and should be of a durability class 1 or 2 species.

Readily available hardwood species include: Mixed Queensland hardwoods, Blackbutt, Forest Red Gum, Spotted Gum, Ironbark, Kwila (Merbau), White Mahogany, Tallowwood, Turpentine.

OREGON used in weather exposed situations should be F8 or better, and must comply with the recommendations in Department of Primary Industries Forest Service Timber Note 38. "Use of Douglas fir (Oregon) in Weather Exposed Applications". The following should be noted:

• Posts must be fixed above the ground.
• Housed, checked or birdsmouth joints must not be used. (Lap joints  only)
• All ends and surface interfaces (joints) to be treated with a  preservative emulsion.
• Pergolas to be finished with paint or oil-based stain.

2. PLAN
Draw a true-to-scale plan of the pergola on graph paper to a scale (e.g. 1 m = 5 squares). For economy use a grid (i.e. post spacing and rafter span) of 3.6 m or less. Remember to allow in your design for the beam and rafters to overhang the amounts shown in the table notes.

3. FOOTINGS
Posts should be supported approximately 75 mm above ground on hot dipped galvanised steel brackets. Concrete pads 300 x 300 mm are required to support steel brackets at grid intersections. 300 mm deep footings would be sufficient for unroofed pergolas in firm clay soils. For roofed pergolas and/or poor soil conditions you should consult a soil engineer.

4. POSTS
The minimum size of posts up to 2.7 m high shall be:

• F5         100 X 100 mm
• F7/F8    90 x 90 mm (dressed)
              100 x 100mm (undressed)
• F14        75 x 75 mm

5. BEAMS
These members are attached to the posts, and support the rafters.
Their size shall be determined from TABLE - BEAMS as follows:

a) Select the type of timber to be used (stress grade).
b) Determine the post spacing (beam span).
c) Determine the rafter span, enter the table, and derive beam size.

NOTES:

1. Maximum overhang for beams is 900 mm.
2. Seasoned softwood sizes 70 mm or thicker can be made by vertically laminating two 35 mm or thicker timber pieces together using 75 x 3.75 mm galvanised nails every 450 mm along its length.

6. RAFTERS
These members are supported by the beam and they in turn support battens, shade cloth or lightweight roofing material. Their size shall be determined from TABLE - RAFTERS as follows.

a) Select the type of timber to be used (stress grade).
b) Determine the rafter spacing.
c) Determine the rafter span, enter the table, and derive rafter size.

7. BATTENS
The size of battens required to support shade cloth or lightweight sheet roofing material may be determined from TABLE - BATTENS.

CONSTRUCTION PROCEDURES

1. SETTING OUT
Set out on the ground with a string line tape and level. Locate post positions of pergola, measure diagonals and check that the structure is square. (Diagonals should be equal). Use a level and a string line to obtain the heights of the slab or concrete pad footing. If the area is to be paved allow a fall of 1 in 50 away from the house. Peg the positions and note from the forms the height to which concrete should be placed.

2. FOOTINGS
Dig holes 300 x 300 x 300 mm deep to take the footings illustrated . If necessary box the hole with scrap plywood or the like. Using lengths of timber and the string lines, position the hot dipped galvanisedbrackets. Check the height and allow for 12 mm clearance between the concrete and the posts. Proprietary brands of brackets are available, but if you prefer you can get the type shown made up.

3. POSTS TO FOOTING
Posts are bolted to the galvanised steel post brackets when the concrete is at least four days old. The bolts used should be M 12 hexagonal headed galvanised mild steel. Allow 12 mm clearance from the concrete to the underside of the post.

4. POSTS TO TIMBER DECK
Where a timber deck exists, the post should be halved and bolted to the bearer using two M 1 2 galvanised bolts or coach screws. A blocking piece nailed to the post will support the end of the decking. Unless the deck is designed to take the extra load, pergola posts should be directly over deck supports.

5. BEAMS
The top of the post should be halved to take the beam that supports the rafters. The beam is bolted to the post with 2 -M 1 2 galvanised cup head bolts. Cut top of post 12 mm lower than top of beam. If the pergola is not to be attached to a substantial structure, 1 00 x 38 mm knee braces should be bolted to the posts and beams to provide bracing. Ends of brace should be 600 mm from post/beam junction. Alternatively, bracing can be achieved by infill screens such as diagonal lattice.

6. FIXING TO HOUSE
Where an adjacent structure is sufficiently substantial to support the loading, a ledger can be fixed to it with masonry expanding bolts or coach screws. The bolts or coach screws should be galvanised and a minimum of 10 mm in diameter and fixed at 1200 mm centres.

IMPORTANT:
Pergola rafters up to 3000 mm long may be supported off timber fascias where the fascia has been adequately fixed to the ends of the house rafters with metal framing anchors.

For high wind areas (W41) plus), and where metal fascias are used, additional framing and connectors may be required to connect the pergola rafters to the fascia. Refer local authority.

7. RAFTERS AND BATTENS
Rafters to unroofed pergolas should be skew nailed to the beams or ledger. If the roof is to be sheeted, one galvanised steel framing anchor at each joint should also be provided. If battens are used at close intervals they may be nailed to the rafters. If battens are to support sheet roofing they should be secured with one 75 mm long 4.5 mm diameter screw at each crossing. 

8. FINISHING
Finishes are important to preserve the life of the timber.
BEFORE FIXING:
For STAIN FINISHES, give all surfaces one coat of water repellent preservative, followed by the first coat of stain. However, it is important to note that some stains may not be compatible with water repellent preservatives. Seek advice from the stain manufacturer as to the product's compatibility before applying it. For PAINT FINISHES, give all surfaces one coat of quality oil base wood primer, followed by the first coat of the selected paint.
END SEALING:
All cut ends should be sealed with stain or primer prior to being fixed in position.

 9 .MAINTENANCE
The long term performance of a timber pergola, in weather exposed situations is dependent on regular and effective maintenance. The frequency of maintenance will depend on the type of finish and the degree of exposure of the pergola.

Information sourced from TRADAC Home Improvement Guide.
(Timber Research and Development Council)

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