How to install backyard pergola?

prakashkalai

Hello!

I'd like to get your valuable advice on building a pergola in the area shown in the picture. Plan is to have 4 posts(100x100) with post stirrups for support, 190x45 H3 treated pine for beams (2 x 4.5m length and 2 x 3.5m length) and 140x45 H3 treated pine for rafters (4 or 5 x 3.5m length) going across left to right in the picture.

I'd like to know whether these 4 posts are sufficient or should I add any additional posts to support the rafters? I'd feel safe if its a free standing pergola as I am a beginner diy'er and dont want anything to to be attached to the main house (okay to attach the waling plate to the brick wall of the garage, if needed though). Roof would be polycarbonate sheets.

Plan is to mount / attach the beams on top of the posts and rafters on top of the beams.

Thanks !!

JacobZ

Hi @prakashkalai,

Unfortunately, roofed structures are not the simplest thing to design and will often require engineered drawings for construction. This is because roofed structures are subject to uplift, which is the force applied when wind gets underneath a roof and pulls upwards. This force can lift and throw structures like this, so there is oftentimes fairly stringent controls on what can and can't be constructed by someone without a building licence.

The first thing you'll have to do is confirm with your local council what requirements they have in terms of planning. For example, in Brisbane, where I live, my local council requires building approval for a pergola that is over 10 square metres or 2.4 metres tall, which yours would be. It's best to confirm whether you'll need to submit construction plans to your local council before getting too deep in the planning stage, as it may require the assistance of a draftsperson, engineer or licenced builder.

In saying this, there are span tables available that you can use as a guide for calculating materials. If you Google "softwood pergola span table", you should be able to find one that relates to the MGP10 framing members you mentioned. You'll first have to work out the beam spans, followed by the rafter spans.

You'll note that span tables for roofed structures are all relative to the wind classification in your area and the roof load width. Depending on the wind classification of your area, a 190x45 pergola beam's span maxes out at about 3.5m and a 140x45 rafter at 5.1m when spaced 600mm apart. So, for the 4.5m side, you would need to add additional posts.

I hope this helps to clarify a few aspects and gives you some information to start with. I'd suggest you speak with your local council from here as there is a good chance you will need to provide engineered drawings as part of this build.

Let me know if you have any further questions.

Jacob