Spreadsheet for Structural Design of Timber Gable Canopy

Price Details


Product Description

Spreadsheet for the Structural Design of timber canopy with doubly pitched roof (gable), having tied rafters at 1/3rd height: free standing or attached. It provides for design of gable canopies beyond the 4.2m span limit of the TDA construction guide for carports, verandahs and pergolas. Connection design covers the use of nails, screws or bolts for each connection. Bolted connections can be designed for the use of standard washers rather than the fabricated washers specified in AS1720. At the ridge the forces are both horizontal and vertical, a timber collar is used to tie the rafters together. Note that the maximum span of canopy is dependent on the maximum length of timber which can be used for the rafter collar-tie. Larger canopies typically require steel posts, therefore additional calculations would be required for the post to either AS4100 or AS4600.

Attachment of canopy to a house largely assumes brick veneer construction on timber frame, with concrete floor slab. The eaves of the canopy is assumed attached to the eaves of the house. The assessment considers the bending moment in the rafter and the required strengthening, and tie-down of the rafter by passing the house wall frame. Simply fastening the rafter with additional strapping or framing brackets is not adequate, as the top plate to wall stud connections need strengthening, as does the bottom plate to wall stud connection, and the bottom plate to floor slab connection.

Calculations not covered for house strengthening are those for lintels, house fascia boards, hip rafters and jack trusses/rafters in the vicinity of the hip corner. NB: The timber framing code AS1684.2 requires that rafters have a backspan equal to twice the over hang, rafters at the hip corner do not comply. The fascia board is therefore an important part of the support structure for jack rafters. Light weight metal fascias should be avoided.

Attached canopies should in general be kept away from hip corners. The canopy fascia beam should not over hang the connection brackets used to attach to the house, doing so will magnify the load at the last bracket: therefore provide brackets to the end of the fascia beam, and strengthen the house fascia as necessary.

Attaching gable end of canopy to house roof should be avoided as it places much higher loads on the house roof structure. If the gable end frame is a simple triangular frame, then the reactions will be concentrated at the bottom corners of such triangle: it makes little difference how many fascia brackets are used to connect the fascia board to the house, all the force will be concentrated at the two ends. If the fascia is to be attached at say 1.2m centres, then the gable end frame  needs internal webbing with nodes at 1.2m centres.

Long span canopies, may require a strut to the gable end fascia to restrain against lateral buckling. Long collar-ties may also have lateral buckling problems if designed for central point load.

Future Issues

Will contain pdf notes covering the derivation of the frame formula, and AutoCAD drawing files covering construction of the canopies and associated connection details. More detailed assessment of attaching to  a house will be covered by a separate spreadsheet and drawing bundle.


Users should be familiar with the Australian wind loading code AS1170.2 and the timber structures code AS1720, and the wind classification system of AS4055. Users also need access to these codes as they are needed to provide meaning to variables and otherwise provide the values of parameters to the spreadsheet.

Recommended Minimum Education

A 2 year Associate Diploma in either mechanical or civil engineering, or the newer Australian Qualification Framework level 6 (AQF-6) award such as an Advanced Diploma in structural/mechanical design. At the very minimum studies in engineering mechanics (statics) and the mechanics of materials covering strength and stability. The occupational title would be Engineering Associate, Engineering Officer or Associate Technologist.

Notes to Builders, Manufacturers and other Suppliers

  1. The spreadsheet can be customised, with a more user friendly interface to allow sales agents to get the answers they and their customers need. Contact us for more information about custom software.
  2. Idealistically engineering design takes place at the frontiers of both science and technology, whilst technical design is concerned with an established body of scientific knowledge applied to established technologies. Buildings and structures in the scope of the building code are established technologies, therefore there is no engineering involved just structural design. Unfortunately gaining the necessary knowledge and experience about specific technology is dependent on industry not formal education. Consultants called on intermittently, and at the last minute, cannot develop optimum solutions, whilst the builders do not have the necessary knowledge in-house to be able to pass it on and sustain such know how. At MiScion Pty Ltd we are interested in getting businesses to employ Associate Technologists on staff, and the providing technical support so that such businesses can do more structural design in-house, and become more aware of the relationship between product design and process design, so that product and process can be better optimised. For more information about structural help desk contact us.
  3. It should be obvious there is no such thing as over-engineered as the first thing people do to resolve such issue is seek more engineering. A product will be found to be uneconomical due to lack of engineering. A product will be found not suitable for purpose and unfit for function, due to a lack of engineering. If a technology is established then the engineering should be complete, and the required technical science for optimum design should be established. If there is apparent variability and inconsistency in the approval process it is due to a failure to consolidate an established body of knowledge and sustain that knowledge from one generation to the next. Someone needs to be guardian and curator of the body of technical knowledge not just the fundamental sciences. For more information about capturing and documenting knowledge contact us.

Download: schGableCanopyTimber.zip


If unable to use the spreadsheet and need structural design or assessment of a canopy structure contact us about one of the following:

  1. Certificate of  an Independent Technical Expert
  2. Certificate of Structural Adequacy
  3. Structural Design Calculations and Drawings
  4. Specification: Timber Canopies, carports, pergolas and verandahs
  5. Design Guide: Timber Canopies, carports, pergolas and verandahs
  6. Technical Notes: Timber Canopies, carports, pergolas and verandahs {Contains derivation of the frame formula used in the spreadsheet.}