Floor advice for a storage shed

G'day guys,

I'm gathering mental strength to embark on building a 2x3m storage shed in my backyard - a properly insulated one that will stay condensation free and not too cooked under Melbourne sun.

I really like this simple design: https://www.youtube.com/watch?v=FD0_2aO_3so
So I would like to take it as a basis and add sarking, insulation, and colorbond roof/cladding, plus put the beams on posts instead of ground.

Where I'm stuck is the flooring material. The guy in the video uses treated OSB which we don't have here in AU.

What would you recommend?- The shed base is essentially the same the one for a deck. Colorbond cladding will overhang below the floor level all the way to cover the beams, so the floor won't be exposed to direct rain - only(?) to moisture coming off the ground beneath it, which is my main concern.

Here are the options I've researched so far.

1. Structaflor H2 Treated Red/Yellow tongue
2. Same as 1 but to paint the bottom with a couple of layers of bitumen paint. My concern is that edges of the joints won't be fully sealed and moisture will get in overtime, and start swelling.
3. Same as 2 but also put a layer of sarking on top of joists before layering the yellow-tongue. My concern is that sarking will sag overtime.
4. Use compressed cement flooring board. Never worked with those before. Quite expensive - will be more than double the cost of the yellow/red tongue. My concern is that it will week moisture overtime, unless I seal it at the bottom which brings me back to square one

Your advice would be highly appreciated.

A couple of points to consider.
H2 is designed for well ventilated areas and helps protect against insect attack https://www.atpine.com.au/treated-pi...aIm4pnA7qEevEv

If you are building this close to the ground you need moisture protection H2 is no advantage. Likewise the compressed cement offers no advantage unless the framing is moisture resistant as well.

Another point is that most garden sheds are not designed to stop all water ingress. The bottom rail on the wall sheets is even both sides as water builds in the channel it can spill out both sides, this can be resolved for the walls with a flashing that comes up the inside however this flashing is exposed at the door opening making a trip hazard and will mean lifting things over the flashing.

I have a shed on a raised platform using C channel and sealed yellow tongue, I consider that the floor is limited in lifespan and will need replacing long term.

I consider there are two long term options, concrete slab with a recessed edge, or don’t start with a “garden shed” to start with.

Thanks for the quick response.

Quote:

---

Originally Posted by droog

If you are building this close to the ground you need moisture protection H2 is no advantage. Likewise the compressed cement offers no advantage unless the framing is moisture resistant as well.

---

Yes, I plan to use H3 treated pine for the floor joists.

Quote:

---

Originally Posted by droog

Another point is that most garden sheds are not designed to stop all water ingress. The bottom rail on the wall sheets is even both sides as water builds in the channel it can spill out both sides, this can be resolved for the walls with a flashing that comes up the inside however this flashing is exposed at the door opening making a trip hazard and will mean lifting things over the flashing.

---

I don't intend to buy a green shed kit (e.g. from Bunning), if that's what you mean. I'd like to frame one and clad with colorbond, properly. E.g. like here.

Quote:

---

Originally Posted by droog

I have a shed on a raised platform using C channel and sealed yellow tongue, I consider that the floor is limited in lifespan and will need replacing long term.

---

How did you seal yellow tongue?

Quote:

---

Originally Posted by aeroden

Thanks for the quick response.


Yes, I plan to use H3 treated pine for the floor joists.


I don't intend to buy a green shed kit (e.g. from Bunning), if that's what you mean. I'd like to frame one and clad with colorbond, properly. E.g. like here.


How did you seal yellow tongue?

---

H3 still expects to be clear of the ground but exposed to some weather, I would be looking at H4 or better if its going to be close to the ground and with limited ventilation.
A proper framed and clad shed you will be able to keep the weather out.
I used a drum of left over concrete sealer on the yellowtongue.

For what you are proposing I would get a slab poured with a vapour barrier underneath, even if you just batten the slab and lay a timber floor on that.

I can't find any H4 lvl structural timber - only stuff like pine sleepers or posts (i.e. for ground-contact use). I thought using H3 KD structured timber and slapping a layer of bitumen paint on top (once the foundation frame is assembled)

Slab out of question due to cost and the uneven ground (i.e. will require a small retention wall which is even more cost).

Quote:

---

Originally Posted by aeroden

Slab out of question.

---

Put it on posts. Perfect for uneven ground. And the dog will love being under there in summer.

I do shed floors this way. On posts. The substructure is treated pine sleepers. The posts mount to front and back bearers and the joists are hung on joist hangers, except the outside two which are coach screwed into the bearers. The floor is either H4 envelope treated 12mm ply or H3 ACQ treated 12mm ply if ventilation is good. The ACQ treatment is done on veneers before ply is made so is full thickness treatment. If I'm using a prefab shed I make the floor slightly bigger by about 10mm and install a two bend flashing all around to fully waterproof. Across the door I put a timber wedge behind the flashing to support it if things are being dragged in and out.
Attachment 537845

Quote:

---

Originally Posted by mic-d

I do shed floors this way. On posts. The substructure is treated pine sleepers. The posts mount to front and back bearers and the joists are hung on joist hangers, except the outside two which are coach screwed into the bearers. The floor is either H4 envelope treated 12mm ply or H3 ACQ treated 12mm ply if ventilation is good. The ACQ treatment is done on veneers before ply is made so is full thickness treatment. If I'm using a prefab shed I make the floor slightly bigger by about 10mm and install a two bend flashing all around to fully waterproof. Across the door I put a timber wedge behind the flashing to support it if things are being dragged in and out.
Attachment 537845

---

That's helpful, thank you! A couple of more questions please:

* How do you coupe with the fact that pine sleepers are usually wet and bend/cup as they dry out?
* I'm surprised you find 12mm thick enough for flooring. How wide do you space your joists?
* What do you typically use for wall cladding? (getting off-topic here, I know)

Quote:

---

Originally Posted by aeroden

* I'm surprised you find 12mm thick enough for flooring. How wide do you space your joists?

---

I have a bunch of 12mm ply in my roof over the 600mm joists for storage.

while it has a small amount of flex or bounce there's no way the average person jumping on it will break it once its all nailed down

Not sure any longer re cost comparisons, but form ply is another option. I've had 3 large pieces "temporarily" straight on the ground, fully exposed, for years and it's still flat. But be careful, "slippery when wet"

Quote:

---

Originally Posted by r3nov8or

Not sure any longer re cost comparisons, but form ply is another option. I've had 3 large pieces "temporarily" straight on the ground, fully exposed, for years and it's still flat. But be careful, "slippery when wet"

---

yep also need to remember there is a difference to form ply and "film faced ply", both are sold at the big green shed but the film faced stuff isn't actually rated for outdoors

Quote:

---

Originally Posted by aeroden

That's helpful, thank you! A couple of more questions please:

* How do you coupe with the fact that pine sleepers are usually wet and bend/cup as they dry out?
* I'm surprised you find 12mm thick enough for flooring. How wide do you space your joists?
* What do you typically use for wall cladding? (getting off-topic here, I know)

---

I get the lightest straightest sleepers I can, not the sodden ones, check with different suppliers. Also, once the sleepers are assembled and covered they will dry slowly and evenly without much distortion but you can put some trimmers through the middle of the joists if you're worried. I forgot to mention too when I build, I assemble the sleeper frame first, get it square and level by propping it on something temporary like bricks then dig the holes for the posts, install them on the bearers and concrete them in then remove the props - makes set out simple.

I install joists around 450-480 centres and 12mm is plenty strong enough. I make sure all ply edges are fully supported. You can run joists front to back or side to side too. In the one I pictured the shed was 2400 deep so I ran the joists side to side at 480 ctrs and had full sheets of ply running front to back. In that shed base I also set the posts on the joist one back from the front to make it look like it was floating. Edit also for clarity, in that photo I uploaded the sleeper frame you can see at the sides and rear is not part of the shed base, that is a small retaining wall. I partly cut the site so the front would not be as high as it otherwise would.

So typically I install a prefab garden shed but I have also framed up and built bespoke sheds clad with CCA treated pine weatherboards. It is an economical cladding