TechNote#017 Canopies: Flat Roof Carports, Verandahs, Pergolas and Patios

Posted on by Conrad Harrison
Various structural forms (SF) of canopies, typically constructed for use as carports, verandahs, pergolas and patios. {NB: Pergola and patio have otherwise been redefined by marketing: thus lost traditional meaning in terms of sales, but not in terms of regulations and codes.}
If a carport, or verandah is made from cold-formed c-section and it matches the types described below, then manufacturers traditionally provided span tables. These span tables however were often used to size members and then buy materials from suppliers who hadn’t incurred the expense of developing such tables. The larger suppliers and manufacturers now use software: any quote they provide is likely to avoid mentioning member sizes. Secondly many of the manufacturers have also moved away from c-sections and adopted proprietary box sections: so any member sizes they do provide in a quote is not suitable with any other supplier. For that matter c-sections are not generic, and do differ from one supplier to another: whilst the c-sections may be available in common depths and thicknesses, they otherwise differ in the breadth of their flanges and that makes a difference to their structural capacity.
If has a owner-builder you specify one of the types of canopy below and make proper reference to the manufacturers span tables, then no “engineering” should be required, and here in South Australia no regulation 88 certificate of an independent technical expert.
If design varies from the following, for example two adjacent rectangles of different sizes with shared beams and columns, then additional calculations are required for the shared members. In such situation it is not legitimately possible to get a regulation 88 certificate, as independence from involvement in design is an important part of the qualification requirements. It may however be possible to get a certificate of structural adequacy , such certificate becomes the first assessment of structural adequacy by a person able to defend such assertion. Councils technical expert then independently reviews that assertion and if they agree issues the certificate of an independent technical expert. In the main, development application fees cover the cost of a regulation 88 certificate and neither council nor a private certifier should be sending you off to get one to submit: unless you have a unusual and/or exotic project: for example a large scale radio telescope dish.
 The following only shows the basic structural form, the canopies can be made from any suitable material.
FREE STANDING ATTACHED
SF01F
SF01F

The maximum span of this type of canopy is controlled by the capabilities of the roof cladding. Published span tables for roof cladding are based on weather proofing requirements for habitable buildings like houses. Manufacturers don’t typically publish extended span tables for roof cladding when used for canopies. Extended spans are typically only quoted in carport and verandah span tables.

Using normal cladding tables maximum spans around 2400 mm, with extended span tables, spans around 3600 mm.
SF01A
SF01A

Similar to SF01F, but with supporting columns on one side replaced by support by another building. If the building already exists then the strengthening of the existing building to support the additional wind load can be problematic.

In particular it is difficult to access the wall frame of a house, especially the bottom plate to slab connections, and the wall stud to bottom plate connections.

Wind class N1 and N3 are likely most problematic as connections selected using AS1684.2 for the house, will have little to no reserve capacity for additional wind loading. Wind class N2 is most likely to have some reserve capacity in the connections.

The main problems with attached canopies is accessing the resistance of the house to additional wind loads, and then the practicality of strengthening. The larger the canopy the greater the problems.

It is generally better to opt for a freestanding canopy with columns and piers adjacent to the house.

To be clear many of the manufacturers and installers of carports and verandahs have fine print which indicate the house owner is responsible for ensuring the house is strong enough to attach the canopy to. Any strengthening they do install, is done so without assessing its adequacy for the specific project.

The primary purpose of hockey stick brackets, is to distribute bolt forces in the connection so that the forces do not split the timber. If the house construction was from steel sections with plates greater than 3mm thick, then 2 bolts is all that would typically be needed. The number of bolts in the hockey stick bracket is not due to the strength of the bolts but the weakness of the timber.

The hockey stick connection bracket may serve a secondary function of strengthening the rafter. It may not however provide adequate strength, and additional stiffeners may need to be used.

Strengthening the house can incur a significant cost, far greater than cost of opting for extra columns. Not so much because of the extra materials, but because of the additional labour, and the cumbersome nature of the work. {NB: Ignoring the strengthening is the short term cheap solution. The future damage will be to the house, and its contents, not just the verandah.}
SF02F
SF02F

Similar to SF01F, except the cladding is permitted to over hang the beams. It allows slightly larger spans.
SF02A
SF02A

Similar to SF02F, but with supporting columns on one side replaced by support by another building. If the building already exists then the strengthening of the existing building to support the additional wind load can be problematic.
SF03F
SF03F

Cladding rails (B2) span between fascia beams (FB1). The width is dependent on the structural capacity of the beams rather than the cladding. The spacing of the cladding rails is dependent on the span of the cladding. Stormwater drainage is along the length: this limits the maximum length of the canopy.
SF03A
SF03A

Similar to SF03F, but with supporting columns on one side replaced by support by another building. If the building already exists then the strengthening of the existing building to support the additional wind load can be problematic.
SF04F
SF04F

Cladding rails (B2) span between rafters  (B1). The rafters span between fascia beams (FB1). The width of the canopy is limit by the structural capacity of the rafters. The spacing of the cladding rails is limited by the span capabilities of the cladding.

The cladding rails, can be either placed between the rafters and set down flush with the top face, or they can span over the top of the rafters.

Cold-formed C-sections are available in lengths upto 12m. However large flat surfaces tend to highlight distortions, and lack of flatness. Monoslope spans typically require heavier structural sections than a gable roof. Monoslope spans also produce greater roof rise than a gable roof. Unless there is a necessary need for a monoslope roof, a gable roof is likely a better option.
SF04A
SF04A

Similar to SF04F, but with supporting columns on one side replaced by support by another building. If the building already exists then the strengthening of the existing building to support the additional wind load can be problematic.

Related Goods & services:

If you need design (calculations and/or drawings), or a certificate of structural adequacy then contact us. For common structures we have some standard fees as set out below. Roughly each definable rectangle on the plan drawing represents a single span. So if have two rectangles then fee is 2 x the standard fee. If attached to the house then additional fee is incurred for assessing the suitability of the house and its required strengthening.

Timber
  1. Certificate of Structural Adequacy – monoslope canopy- timber– single span
  2. Certificate of Structural Adequacy – gable canopy – timber – single span
  3. Specification: Timber Canopies, carports, pergolas and verandahs
  4. Technical Notes: Timber Canopies, carports, pergolas and verandahs
  5. Design Guide: Timber Canopies, carports, pergolas and verandahs
Steel
  1. Certificate of Structural Adequacy – monoslope canopy- cold-formed steel – single span
  2. Certificate of Structural Adequacy – gable canopy- cold-formed steel – single span
  3. Structural Design Freestanding Canopy – cold-formed steel – span less than 12m

Similar fees if made from other structural steel sections. (eg. SHS/RHS/CHS, PFC, UB/UC)

Attached
  1. Certificate of Structural Adequacy – House Strengthening for Attached Canopy

Revisions:

  1. [05/12/2017] : Original
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