How to build deck over sloped concrete?

Hi all, I have a strange spot next to my house I'm trying to find a use for. I'm thinking about putting a deck there so I have a level area to put a small storage/bike shed on to free up room in my garage.

The issue is, what is the best way to attach the posts to the angled concrete?

I was thinking about using precast concrete footers or tuff blocks for the bottom/flat part, but can I do the same on the angled concrete if I cut the bottom of the post on an angle so it's vertical? I'm thinking tuff blocks for the angled part but was worried about them sliding down the concrete but a friend said that the structure of the deck will keep them still and the load would be straight down vertically so it should be ok. He also said I could put urethane caulking (or similar) around it to seal it and keep it still.

There's bush right across the road from my place so I need to be as termite-concious as possible. Anybody got any pro tips? I've quickly drawn the area up in SketchUp so there's some dimensions. Hopefully this is clear, if I have something wrong please let me know, I'd rather be wrong now than later. Thanks 🙂

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Comments

EricL

Hello @OopsOhBoy

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your decking project.

I believe it will be necessary to build a flat concrete footing for the post to sit on. However, in order for the footing to be properly built I recommend engaging the services of a draftsperson or engineer to design the footing so that it will be properly anchored to the angled concrete floor. This would be a good opportunity to have them design your decking frame as well.

Cutting into the existing concrete, assembling form work, imbedding and adding rebar, plus steel supporting mesh will probably be necessary to build a sound footing.

Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/58671#Comment_58671

Good Morning @OopsOhBoy

That is an awkward slope for sure tho I do think its doable.

That slope is the issue, depending on how steep it is you could use a stirrup post support and pack out the low side with steel washers to make the footing level. (I would not recommend attaching the footing and then shaping the timber to fit an angled stirrup)

Pryda 65x90mm M10 Full Stirrup Post Anchor is what I am thinking. The holes for the bolts will have to be vertical in line with the packed oyt stirrup. And as you will be drilling just note you may have a higher chance to hit any reo that is in the concrete slab/path.

I dont like the idea of attaching any part of the deck to a barge board/the house as it adds complexity to the build. I figure it should be able to stand alone. With the small area you have in may be worthwhile to investigate it tho to help stabilize the deck.

If you are concerned about termites there are a few composite timbers that would work for you, But figure out your spacings and how your footings will work, next is posts, then crossbeams and finally joists. Keeping in mind which way you want the slats to run. I like the idea that they run perpendicular to the house so the boards will be 2600mm long.

Dave

OopsOhBoy

https://www.workshop.bunnings.com.au/discussion/comment/91705#Comment_91705

Thanks for the advice! I was thinking about something similar to the pic below, but didn't know if it was overkill (would pack underneath with non shrink grout too so it's super-not-going-anywhere). There's only going to be a small shed for stuff like a lawn mower etc on it to free up space in the garage, so nothing super heavy but also I prefer to overdo things so they're done forever so maybe the extra time is worth it. Would some kind of threaded insert into the concrete with some kind of sealant around it work? Then use the nuts to level and pack with grout?

Pic doesn't seem to be attaching, here's the link to the video. The thumbnail shows what I'm talking about

https://youtu.be/nGa1244hK9Y?si=QBShVDjEuH4afAw2

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/111855#Comment_111855

Hi @OopsOhBoy,

If you were keen, you could likely drill into the concrete and glue in threaded rods. From there, you'd create a timber formwork to build up a pad of structural grout around the threaded rod. Your post support could then be affixed to the top with nuts. I've created the rendering below to illustrate.

Mitchell

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/111855#Comment_111855

Good Morning @OopsOhBoy

The threaded rod into a ramset base could work, tho I still prefer the mechanical packing of the low side of the stirrup over any grout. I wouldnt actually be using grout other then to maybe make it look pretty after you have it bolted down. Even tho its only a smallish deck there will be a lot of force wanting to take it towards the front yard with that slope.

Dave

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/123419#Comment_123419

Just a FYI @Dave-1 (though you might already know this), structural grout is very different from the standard grout used for tiles. Unlike tile grout, structural grout is designed for heavy-duty construction applications. It’s commonly used to create solid bases for extremely heavy structures, like a chimney stack weighing thousands of kilos, or to pack out under large steel columns, stirrups, and other structural elements where precise levelling and load support are critical.

In this situation, it's important to use it in conjunction with threaded rods or reo bars into the concrete to stop it from separating and slipping down the slope. It doesn't have much adhesion to flat or sloped surfaces, but it has incredible compressive strength.

I’d also agree that mechanical packers can be a practical alternative in some situations.

Mitchell

OopsOhBoy

https://www.workshop.bunnings.com.au/discussion/comment/123418#Comment_123418

Thanks! This is exactly what I was thinking about doing. I figure that'd be worth of a "that's not going anywhere" when it's done hahaha

OopsOhBoy

https://www.workshop.bunnings.com.au/discussion/comment/123419#Comment_123419

Thanks for the tip @Dave-1! What would I pack it out with? Just a bunch of nuts/washers? How will they sit against the angled ground vs the flat top?

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/131001#Comment_131001

Good Afternoon @OopsOhBoy and @MitchellMc

@OopsOhBoy Washers is what I would have used 🙂

and

@MitchellMc theres my "new thing Ive learned for the day" 🙂 Will be having a stickybeak next time im in the bunnings aisle for sure 🙂

Dave

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