Large Freestanding Deck

[HappyHammer]

Attached is a drawing of the structure I'm considering to support a freestanding deck below three large gum trees. I haven't completely laid out and measured the area yet so the placement of the trees is approximate at the moment.

The posts will be Ironbark or Red Gum and set in concrete straight into the ground. The remaining timber for the structure and the decking board will also be Ironbark or most likely red gum.

The size of the deck will be approximately 10m x 8m. Any feedback and suggestions welcome. I'll be going to the site in the next couple of weeks and want to finalise the structure first so that I can begin to mark it out on site and confirm dimensions and the exact location of the trees.

HH.

[echnidna]

should work

[namtrak]

Is there an issue with the timber being set in the concrete? When the concrete dries it shrinks and leaves a small gap around the stump which then allows for the ingress of water.

[silentC]

I think you are going to need more than 6 posts. You'll have a hard time finding bearers that will span 5 metres, let alone 8.

I reckon I would have 3 or 4 bearers running right to left with four posts under each, then run the joists top to bottom and block them around the trees, or maybe just a big hole where the trees are with a balustrade around.

[pawnhead]

I'd agree with silentC there, and I'd add that if you're going to cut out joists and trim up, you'll have to double up next to the openings:



edit: Even that's not enough in this case. The double joists in between are carrying the load of two and a half joists. Slightly overloaded, unless they are under maximum span in the first place.

[HappyHammer]

It does look more stable with a beam through the middle but it increases the height of the deck which I don't want. If possible I want to be able to step straight up on to it or at most have only 1 step.

HH.

[HappyHammer]

This might solve the height problem...

HH.

[silentC]

The simple fact is that you wont get joists to span 8 metres without intermediate support. A 240x35 F7 hardwood joist will span 4200 single span, so maybe you can hang the joists either side of a central bearer. What you have marked as 'Face board' becomes a load-bearing fascia beam.

But you are going to need four posts under each bearer because a hardwood bearer wont span 5 metres.

[HappyHammer]

OK I think I've got it.....will it not span 5 metres because they don't come that big or because the distance is too far for the board to support the weight?

The intention for the face boards is for them to be load bearing hence the ledger. All the face boards will be bolted to the posts.

How's this for a revised plan....

HH.

[silentC]

Put it this way, my tables only go up to 4200.

For the joist trimming exercise, my framing manual says it's OK to use the same material as the joist for the trimmer for openings up to 1000mm. Then for openings 1000mm to 3000mm you increase the trimmer width by one fifth for every 300mm over 1000mm. The trimming joists (the joists framing the opening) are the same size as the trimmer. So for an opening 1000m wide, you can use the same joist material and you don't need to double the joists, but as you get wider, you either need to upsize the trimming joists, or double them up. Doubling up will cost more but will be easier than working with different width materials.

I don't see any reason you can't hang the joists off a central beam. You can use a ledger as you have done there, or use joist hangers. I don't know if you need to double up on the bearer though. A single bearer would carry that load if it was underneath, so I'm not sure why hanging the joists would make any difference.

I also believe your bearers need to either be checked in to the posts, or some sort of connection used that transfers the weight to the top of the post. I don't think you can just bolt them to the side and have them hanging by the bolts.

[Dirty Doogie]

What is the distance between ground level and the underside of the bearer/faceboard?

Building a low deck under certain gum trees can turn into a bit of a disaster. Every year the gum trees shed their bark either in long strips (the worst) or flakes, which can be as much as few cubic metres for larger trees. This will need to be cleaned up or the space you leave for the trunks will soon become a pile of (very good ) bush compost and your sub framing and decking will soon be buried.

This has happened to a friends bush retreat where we built a low deck under and next to some large coastal scribbly gums. He didnt visit for a year and came back to find the deck completely invisible and buried. Luckily everything was treated pine so no serious damage was done.

I am of the view that low decks under 400mm high don't need stumps if they are made from treated timbers and aren't in a wind exposure zone. Landscape architects call them "stage decks" and joists and bearers sit on treated timber blocks or blobs of concrete.

Doog

[silentC]

I guess if it's not connected to any building, there's probably nothing wrong with just sitting it on the ground, although you'd want to make sure it's tied down so the wind can't get under it.

I think I would either steer well clear of the trees, or put a large opening around the lot.

[namtrak]

I guess if it's not connected to any building, there's probably nothing wrong with just sitting it on the ground, although you'd want to make sure it's tied down so the wind can't get under it.

I think I would either steer well clear of the trees, or put a large opening around the lot.

And that would make 10,000!!!!

[silentC]

Took me a minute to work out what the hell you were on about!

I sure talk a lot, don't I?

[Dirty Doogie]

Hi Silent,

I would suggest putting a quite large opening around the gum trees also.

There is a phenomenon called the ground level effect at play in windspeeds. Below a height of about 600mm the windspeed reduces considerably and as the wind speed increases the difference becomes greater. so for example in a gentle breeze at 10kph the speed of the air under 600mm from the ground might be only 8 kph, but if the windspeed is 150 kph the the ground effect speed could well be only 80 kph.

What happens is that the ground surface creates turbulence (friction) and the wind begins tumbling and swirling close to the ground and causes higher air pressure which keeps things stuck to the ground. You have probably seen sheets of corro iron lying on the ground and vibrating and flapping in strong breezes. It is the ground level effect at work. The sheets wont lift until there is drop in the wind gusts.

Well thats the theory...

Doog