Pergola and Deck over concrete slab

TonyandKhim

Hi, about to start a project covering an old slab in the garden with Merbau decking and a pergola/Gazebo (not sure what the difference is). I'm thinking of using 90x45 H3 frame over the concrete for the decking.  2 quick questions spring to mind.... does the frame need to be lifted slightly to allow run off and ventillation (if so how much) and I see the decking at Bunnings says around 4mm gap between planks, whereas most I've seen seem to have a nails width between planks.

Thanks

MitchellMc

Welcome to the Bunnings Workshop community @TonyandKhim. It's terrific to have you join us and many thanks for your questions.

There are a few differences between a pergola and a gazebo. Commonly in Australia, a gazebo is a freestanding structure, and a pergola is connected to a house. You should find this step-by-step guide helpful: How To Build A Pergola.

H3 timber is perfect for the frame. The more ventilation you can leave, the better. The finished height is governed by your door sill, as you obviously can't have the decking boards above that. If there is 200mm from the doorsill down to the exterior surface then 19mm will be taken up by the boards, 90mm from the joist/frame and 91mm will be left open for ventilation under the deck. Over a concrete surface, you would need at least 15mm for drainage, and over soil, it is best to leave at least 150mm.

Boards swell when they get wet. To prevent the boards from touching each other, it is best to leave at least a 4mm gap when laying them dry. When you see a deck that has a nail width between boards, it is likely that the deck was laid with a larger gap in the first place. You'll also find if the gap is too small, then debris will get stuck in it.

We look forward to hearing more about your project and following along as you build it.

Please let me know if you need further information or had questions.

Mitchell