TIMBER LENGTHS AND SIZES

DETERMINING WIND CLASSIFICATION

The force of the wind is a factor in choosing the type of timber to use for your pergola and its dimensions. In fact, the building code of Australia has a separate set of recommended timber lengths and sizes for areas with high wind classification and for low wind speed areas.

Determining the wind classification of your area requires identifying four variables: Region, Terrain Category, Shielding Factor and Topographics Classification. For a complete and detailed procedure on how to estimate these factors and your area’s wind classification, check out Stratco’s guide to “Determining Wind Speed”. It should be noted that for engineering purposes in Australia there are very few areas that are affected south of Brisbane.

ROOF LOAD WIDTH (RLW)

The other important consideration in determining timber lengths and sizes of your pergola is the size and weight of the roofing material the timber structure intends to hold up. The size of the roofing material is needed to calculate how much load the roof members and substructure need to support, and therefore what size the underlying timber parts need to be.

SINGLE SPAN AND CONTINUOUS SPAN

Finally, the type of support needs to be considered — single span or continuous span. Continuous span support allows the structure to carry heavier loads, and so requires longer timber lengths and larger sizes. Here’s a review of the math behind all these.

With these three factors in hand, all you need now is to consult a span table such as this “Pocket Span Table Book” from the Timber Promotion Council to find the right timber lengths and sizes for your pergola roofing project.

Source: