Deck posts in sandstone

[BushTurkey]

Hi all

I am new, I searched for an answer to this question but it looks like it hasnt been asked before. I am wanting to build a deck right at the bottom of my yard. I have wanted to do this since I bought the place 3 years ago. The main issue to getting started is that essentially it would be built on large sandstone boulders and Im not sure how I would go about getting stable footings. I realise I would have to drill and set steel posts in the stone but not sure how. I have attached a picture of the area I want to build on. Once I get my posts up I could tackle the rest using information from these forums and other sources but I want to gather some opinions on the drilling of the collars for the posts.

I would only be wanting to get 500mm to 1m off the ground at the fence line, which would taper back into the boulders on my side of the fence.

Any advice would be great.

[Dirty Doogie]

That looks like a great area to do some creative landscaping.

You may have to get council approval if you attempt to go very near the property line though. If that is the case the council will want to see engineers drawings as to the method of attachment.

I have seen 75mm round steel stumps (galvanised) set into 80 mm core drilled holes about 600 deep into sandstone and glued down into the hole with epoxy mortar. The Brisbane city council made a large deck this way in Brisbane forest park. But they were much higher stumps and engineered to carry greater load.

If you are not going to go through council I'd be thinking of setting steel post anchors with bolt down plates onto the sandstone with Dynabolts or some sort of chemical set. Much depends on how hard/brittle the sandstone is and how close to an edge you have to work. I think it would be fairly easy to level small surfaces on the rock with a larger angle grinder and a stone cutting disc.

Interesting problem - maybe some experimenting is the way to find out.

Doog

[BushTurkey]

Here's another pic from the reverse angle. you get an idea of why i want to do this, its got a great view but until i make the area trafficable i cant do much with it.

Thanks for the info doogie. So what kind of person would I call out to give me an initial assessment? I am currently clearing the loose dirt and rocks etc so we can see exactly what we are working with.

[Dirty Doogie]

HI BT,

I'm not sure who you would call out - sometimes the best place to start is with a local building certifer or maybe the council themselves - tell them the situation and ask them if they know of any local structural engineers/designers that could (or who have already) design suitable footings (although I'd be surprised if the footings have to be really elaborate).

Then ring the Engineer/designer and try and pick his brains over the phone - a lot of these people will just tell you what they think needs doing. A local person will know your situation immediately and should prove helpful. If they want to come out they'll start charging so try and put them off - at least until you've cleared the site and know where you need to put stumps etc.

As I remember that sandstone terrian changes from large boulders to gravelly pockets of shallow sandy soil. me thinks you may require a couple of different types of footings/attachments.

And Yeh!
I love that coastal bloodwood you got growing out from under the large boulder.

Doog

[Dirty Doogie]

it looks like you've got a cliff at your back fence?? Nice spot!

Doog

[BushTurkey]

Yeah its a 4m drop at the highest point, 1m at the other end of the fence line. It continues to descend away so the floor of the deck would be about canopy height. Thanks very much for your tips, I am going to speak with the council and my neighbour who might know someone in the industry. I think he does work for councils in national parks.

I have attached the picture which I should have sent the first time, It gives a better idea of what the ground looks like now. As you can see I need to get rid of the ground cover/loose dirt/rocks and mark where the posts will go. then I will know what kind of foundations/attachments I will need. The area in this photograph (where it sort of opens up) is actually going to be paved in sandstone, with some railway sleepers to level it out. The decking goes more along the fenceline (or as close as I am allowed to get to it)

I will post an update after this coming weekends work. Any more ideas are welcome.

[Dirty Doogie]

I went for walk through the national park here yesterday arvo and made note of how they put the timber walkways down over similiar terrain to yours.

It seems they used treated pine posts and jammed them down between smaller rocks then poured a large lump of concrete around them. the concrete flows out and forms itself to the shape of ground and seems very stable.

In other places they have dynabolted custom made brackets onto larger rock outdrops. The rock here though is mostly granite or schizt type which would have different properties to your sandstone. I wanted to take some pics but I discovered my camera had been nicked the previous night.

you know I'd be inclined to water blast the loose dirt away from your work area - but then I love making a mess!

Doog

[BushTurkey]

I went for walk through the national park here yesterday arvo and made note of how they put the timber walkways down over similiar terrain to yours.

It seems they used treated pine posts and jammed them down between smaller rocks then poured a large lump of concrete around them. the concrete flows out and forms itself to the shape of ground and seems very stable.

In other places they have dynabolted custom made brackets onto larger rock outdrops. The rock here though is mostly granite or schizt type which would have different properties to your sandstone. I wanted to take some pics but I discovered my camera had been nicked the previous night.

you know I'd be inclined to water blast the loose dirt away from your work area - but then I love making a mess!

Doog

haha! No its ok i will shovel it into the garden beds.

There may be a couple of posts I could do like you say, dig between two boulders and place the post directly in the ground surrounded by cement. But the ones out on the edge will need to be directly in rock, and as you say sandstone would be too brittle to dynabolt, cause the dynabolts place force outwards and could crack it when under load.

I spoke with a friend on the way home yeserday and he seemed unphased by the project, he said just get a 20mm masonary bit and use an impact drill to make 3 holes for each post, roughly 300mm deep. Then take my kango hammer and join the 3 holes to a single 60mm hole. Then insert the styrups and cement them in (non quick drying).

Then place the posts that are further away from the edge in dirt where possible or between boulders with lots of concrete. I was also thinking it wouldnt be a bad idea to place some sort of lateral attachment on the boulders further up the hill, to completely elimenate any movement and make it mega-safe.

p.s. would placing the posts directly in the ground cause it to wrot or get white ants etc? I was going to make it entirely out of hardwood, or is treated pine my only option for this sort of structure?

[BushTurkey]

I just had another idea
Since my boundary line is actually at the bottom of the rock face, what if i drilled into the face of the boulders to attach my posts. I think that would be a better idea?

[BushTurkey]

I just ordered the decking book by Allan Staines so I can get the rest of the deck planned. This weekend I will post pictures of the fully exposed rock and the intended post positions

[Dirty Doogie]

Me thinx treated pine posts will last longer than hardwood when set in concrete. I'm talking about H4 non taper builing logs about 150 mm dia.

There are a lot of options once you work out how to bed the posts. Square timber is easier to set than round. Best to have a play with drilling holes etc in the sandstone - see what works.

I'm off for a few days

cya Doog

[mr_john]

I'm just looking at a similar problem - a deck edging onto a huge sandstone bolder. I would love to know how you did. My bolder is house sized so I'm not worried about breaking the rock with dynabolts but I'm pretty sure that posts set in holes will be much stronger. Only thing - I'm a bit worried about how much work it will be to put big larger holes in the sandstone. I'm guessing the drilling holes then breaking away the rock inbetween is going to be a huge pain. Has anyone got any easier methods?

[Rough Sawn]

Has anyone got any easier methods?

Yes, hire one of these from Kennards!



http://www.kennards.com.au/page.asp?37=2326&E_Page=2294

[BushTurkey]

Due to council regulations I wont be building a deck, as it would require a DA. And since I can simply pave the area, thats what i have been working on. I will be making a metal deck though, to house a spa, so I will post some photos as the job progresses. This weekend I will be putting in the first retaining wall for the paving.

[mr_john]

It's a remote property, solar panels, batteries and all that.. with just a 1000Watt inverter so the 1700Watt drill is out for me I'm going to have a try the hard way so I'll put up any tips I have by the end of it.