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How to build a DIY shed?

Hi Bunnings Team,

I’m planning a small shed floor using deck blocks and wanted to confirm a few things before purchasing materials.

The base will be 1.5m between each deck block, with approximately a 300mm overhang on each side, as shown in the Deck Block brochure. Would this span be structurally adequate, or would you recommend adding two additional deck blocks for extra support (as shown in the lower example image in the brochure)?

I’m also planning to use H3 treated timber for the framing. Would 45mm x 90mm be required for this span, or could I use a smaller section such as 45mm x 50mm to keep the floor height lower to the ground?

I’d appreciate your advice on the most suitable setup to ensure the floor is strong and compliant.

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JacobZ

Hi @Samc1234,

This is an exciting project that I'd be happy to help out with. Being a decking project, it is worth having a look at the following articles so you can get your head around the basics of deck building.

How to build a low-level deck

How To Build A Deck

How to understand deck components

How to understand deck span tables.

It is fine to have a short overhang with your joists, but this is only possible when the joists have a continuous span, meaning they run over multiple supports. In your design, if we assume the framing members running left to right are continuous, then the short framing lengths running vertically, cannot be continuous, so the overhang at the top and bottom of the design is not possible. What this means, in a single tiered deck, is that you need to have foundation blocks running along the perimeter of the frame on at least two sides.

90x45 H3 framing is the smallest calibre of timber that our supplier for H3 framing, Hyne Timber, would recommend is used as joists. While acting as a joist, as per the Hyne Timber joist span tables, a 90x45 can span 1400mm between supports. This is important as it tells us how many supports each joist needs.

The next thing to note is that decking boards, need to be supported every 450mm along their full length and this means the joists need to be spaced no more than 450mm apart. This means the direction of the decking boards have a pretty big effect on the layout of the deck. If your joists run left to right, this means the decking has to run top to bottom. If the joists run top to bottom, then the decking has to run left to right.

Based on your design, I would have to assume the decking boards are going to run top to bottom, so here is a quick design that would work.

We can certainly make the design more efficient by using bearers as well as joists, but this is the closest to your current design I could get while staying within the span limitations.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

Samc1234

Thank you Jacob,

This is great and reassuring thank you. Just double checking if I build the design above it will be suffice for an outdoor shed?

So my decking would be 1.5 m long running up and down?

In an alternate view, I would like to use plywood for the floor and interiror walls. Is there a plywood that has an exterior grade?

JacobZ

Hi @Samc1234,

Yes, this design will suffice as a base for an outdoor shed as long as you use H3 treated framing.

Also, yes, the decking would be 1.5m long running up and down.

Alternatively, you can use plywood for the floor as long as it is relatively thick. This 2700 x 1200 x 19mm Flooring F11 H3 Plywood is treated for outdoor use and would be suitable.

Let me know if you have any further questions.

Jacob

Samc1234

Thanks Jacob,

In addition I am looking at a stilted roof, wondering if there are any sites or tips you could give around blueprints, measuring for framing

JacobZ

Hi @Samc1234,

Roofed structures generally require engineering plans to ensure they are built to the required standard and are safely anchored to the ground.

Unfortunately, I'm not qualified to assist with this kind of planning. That is something you'd likely need to speak with an engineer, draftsperson or licenced builder for assistance with.

Let me know if you have any further questions.

Jacob

Samc1234

Thanks Jacob, once I get my framing designs sorted, could you recommend a timber to build the frames with?

JacobZ

Hi @Samc1234,

As the walls will be holding up the roof, the calibre of timber would be defined by your roof plan.

Typically, load bearing walls are constructed from 90x45 framing pine with a minimum timber rating of MGP10, but your designer could call for higher.

You can use H2 treated timber if the exterior is clad in a way that makes it weatherproof. This typically requires a layer of house wrap, followed by an exterior cladding.

If it is not going to be weatherproof, H3 timber is the better option.

Let me know if you have any further questions.

Jacob

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