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

[TonyandKhim]

Thanks for that Mitchell, all makes sense.  Mine is by your definition a gazebo then, being free standing over an existing slab in the garden.  15mm gap under the frame makes sense, drainage and airflow.  One quick follow up question though, instead of using 90x 45 vertically, is there any reason I cant use say a 70x45 horizontally ? so that the elevation will be 15 + 45 + 19mm (being the decking) so totalling 79mm.  The reason is that I'm just trying to limit the "step up" as with local council and max 2.4m with 2.1m at the rear facing fence side, by the time you take off beams and rafters you lose an additional 300mm of head room (roughly) and as I assume the max heights are above mean ground level  a large "step up" will reduce internal headroom.  So the question is whether 45mm height of frame is enough.  I'm planning 400m between framework.

Oh and is it best to seal the concrete first, so that water runs off rather than soaks in and stays damp ?

[MitchellMc]

@TonyandKhim,

You can place your joist on the side, but it's not the preferred way to orientate it. If you are to turn the timber on the side, I would advise going with 90 x 45mm instead of 70 x 45mm.

Mitchell

[TonyandKhim]

Thanks, I added a quick question to the last post but I think you answered quickly enough that you missed it....  is it best to seal the concrete first, so that water runs off rather than soaks in ? or not ?

[MitchellMc · Accepted solution]

@TonyandKhim,

If the slab has an adequate grade to it, then a sealer would help shed the water quickly. However, if it doesn't, then the water will pool on the surface and stay there longer than if it was just absorbed. As long as there is a reasonable grade, I would advise sealing the concrete.

Mitchell

[TonyandKhim]

Flat as a pancake I think, used to be an old shed base, so I'll leave as is.  Thanks for your help