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

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