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I want to build a lean to Gazebo covered with Polycarbonate roofing. Dimensions are 5 metre long by 3 metres wide. It will be attached to the back of my house. ( Brick ( veneer I think is the correct term. ) I have sketched out 3 100x100 posts. One on each corner. Rafter spacing.8 How close do my purlins need to be? How many for a 3.0 metres span?. I would attach a ribbon board or ledger to the side of the house. Would .120 by .40 be strong enough? The soffit is .6 do you think I need to put a flashing over the end by the ledger board? If I use 120x 40 ledger would I use dyna bolts to hold to the brick or are anka or Ramset fittings be enough and how far apart should I put these What size
? What dimension timber would I need for rafters and purlins? Would 1cm every meter give enough fall for pitch. Will be 2.1 at highest point.
Hi @Radar,
A warm welcome to the Bunnings Workshop community and thank you for your question about building and installing a lean-to structure in your home.
The first step in the process will be contacting your local council to see if building approval is required. With structures of this size that are subject to wind loads, it is fairly common for engineering plans to be required for approval. Giving your local council a call will determine whether this is required or not.
If your local council requires engineered plans, you'll have to get in touch with an engineer for assistance with designing the structure to ensure you are compliant with the NCC (National Construction Code), the relevant Australian and New Zealand Standards and construction is suitable for your area's wind classification (N1 - N6).
Attaching your pergola to the house via a ledger board, which is often referred to as a waling plate, makes this a complex structure that you will likely need the assistance of an engineer with as you will be transferring the wind load of the new structure into the brick wall, which was not originally designed for this purpose. An engineer will be able to assist you with the correct size of timber and a suitable anchoring method. Dynabolts or Ankascrews are likely good options, but you will need to confirm with an engineer.
Unfortunately, I can't make recommendations on specific timbers for your rafters and purlins without knowing the wind classification in your area, but these Woodhouse Span Tables show the spans for a number of timber profiles that are commonly used for pergolas.
As an example, using the Woodhouse Span Tables as a reference, if you are in an N1 or N2 wind area, at 760mm spacings, you can use 185 x 30mm F7 H3 Treated Pine Primed LOSP or 138 x 42mm F7 H3 Treated Pine Primed LOSP as your rafters.
Your purlins will have to be spaced depending on the requirements of the polycarbonate sheeting you install and the wind area you are in. For example, these Suntuf 3.0m Greca Roof Sheets have a technical guide that details purlin spans.
The minimum roof pitch is also relative to the sheet material you use. With polycarbonate sheeting a five-degree slope is often the minimum. To achieve a five-degree slope, your roof sheets will need to fall by 8.7cm for every metre.
Nothing is stopping us from getting a basic plan together, but you'll likely need the assistance of an engineer and council approval before starting this project.
Allow me to tag @Nailbag, @TedBear and @Jewelleryrescue for their thoughts.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
