Deck design onto widely spaced existing piles.

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ElectronicHaggis

2 posts

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#243624 21-Dec-2018 13:12

Hello,

I am planning a deck to be built under my house - currently there is a large cavity under one half of the house which would be ideal for a covered deck built onto the existing piles of the house. (Images below will help to explain better).

The problem is the spacing between the existing piles is slightly beyond the scope of NZS 3604:2011 (https://www.standards.govt.nz/assets/Publication-files/BSP/SNZHB3604-2011.pdf) and I am unsure how best to proceed. In practice, I am sure that larger beams/joists would be fine, but I still need to ensure I comply with the standard. I am wary about adding new piles as I need to make sure I don't cause any damage to the foundations of the house (it's possible I'm being overly cautious!). Any thoughts/suggestions would be greatly appreciated!

Under the house:

This image shows the house piles, the cladding/framing is not load-bearing and will be removed.

A very rough sketch showing an early concept (this shows a small room next to the deck - this will likely not be included at this stage).

Birds-eye view/scale:

Key:

Grey circles: existing house piles.
Grey squares: New piles which will be added to extend the deck beyond the house.
Dotted line: This area of the deck will be covered by the house above it.
Blue/red rectangles: Beams/Joists

Dimensions:

Existing piles: ~100mm diameter
Pile spacing: 2m front-to-back, up to 2.8m side-to-side.

Coil

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#2148821 21-Dec-2018 13:15

All doable, I would consult a registered builder when ever you are going near the foundations of a house. You will need to put in stand alone piles for that deck as far as I am aware. The spacing as you also confirmed is far too great.
Don't forget, decks are designed to have peoples standing on them shoulder to shoulder and most Kiwi Christmases will easily achieve that!

timmmay

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#2148823 21-Dec-2018 13:22

There's not going to be much of a view from there...

wellygary

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#2148832 21-Dec-2018 13:42

I don't know if its just the camera angle or the lens but those posts look awfully thin given their height and spacing, ..

What are the dimensions of the bearers that directly sit on the posts? also the one to the right of the image doesn't even seem to be sitting on the far post at all!! ... i

Also you are probably going to want to install some sort of insulation for the floor above, if you remove the fibre cement panels from around it to open that space up the amount of airflow is going to greatly increase...

DarthKermit

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#2148852 21-Dec-2018 14:15

If it was my house, I'd install the deck so it doesn't actually touch the house piles at all. Much like you'd leave a gap if you were building a deck around an existing tree.

If you put in some new piles a decent distance away from the house piles (I'm thinking 600 mm) it shouldn't interfere with them. But confirm that with a competent builder please.

Aredwood

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#2148855 21-Dec-2018 14:24

If it is beyond the scope of NZS3604, then you need to get it engineer designed.

Note also that driven pole type foundations are sometimes used when the upper ground levels are nor suitable for normal foundations. So you might have problems with independent foundations. And the cladding around the outside might still be used to provide bracing, if so then it does count as structural.

Why not just put down pavers? It would be far easier.

nickb800

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#2148858 21-Dec-2018 14:25

Our place was built last year subject to specific engineering design - piles are in a grid with 2-2.2m spacing, doubled 2x8s (possibly 2x10s) form the bearers, then 2x6 joists sit on top. Your 2.8 span is pretty long, but my point is that there might be a solution from a chat with a friendly engineer - increase the depth of the bearers/joists and/or double/tripling them.

Otherwise, intermediate piling should be fine, either proprietary ground screw like 'stop digging' to avoid ground disturbance or normal dug and concreted pile. If digging, maintain a distance from the existing piles equal to the depth of your hole (i.e. 45 degree line from the pole at ground level) plus a little bit to play safe. The piles won't need to be very deep as they're mostly weight bearing, not so much bracing (unlike your house piles) - but 3604 should guide you on that.

Iirc 3604 specifies a higher design loading for decks than inside a house, due to the potential for party crowding.

MikeAqua

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#2148892 21-Dec-2018 15:17

wellygary:

I don't know if its just the camera angle or the lens but those posts look awfully thin given their height and spacing, ..

~100mm according to OP.

Mike

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Bung

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#2148988 21-Dec-2018 17:47

MikeAqua:
wellygary:

I don't know if its just the camera angle or the lens but those posts look awfully thin given their height and spacing, ..

~100mm according to OP.

I think OP's eyeball estimator needs recalibration. The poles aren't ~100mm. Compare them with the nominal 100x50 sawn timber on the exterior. The galvanised flat bar on the diagonal braces is probably 50mm wide if it is the usual fixture.
They are more likely at least 150mm at top end, thicker at base.

ElectronicHaggis

2 posts

Wannabe Geek

#2149923 24-Dec-2018 10:35

There's not going to be much of a view from there...

It's hard to see in the photos - the property is on a moderate slope that the deck will look out over - an existing deck (partly visible in one of the photos) will be removed in the future, exposing a nice-enough view out over the rest of the property (mostly trees).

Also you are probably going to want to install some sort of insulation for the floor above, if you remove the fibre cement panels from around it to open that space up the amount of airflow is going to greatly increase...

Thanks for the suggestion - I had half considered this but will certainly add something in now.

And the cladding around the outside might still be used to provide bracing, if so then it does count as structural.

Why not just put down pavers? It would be far easier.

I have had the cladding/framing inspected by a builder who confirmed that it is non-structural (it isn't on the original plan for the house, so I suspect it was added as an after-thought at some stage.

The land is far from level (there will be a drop of around 500-1000mm from one side of the deck to the other), so pavers would not be suitable.

I don't know if its just the camera angle or the lens but those posts look awfully thin given their height and spacing, ..

~100mm according to OP.

I think OP's eyeball estimator needs recalibration. The poles aren't ~100mm. Compare them with the nominal 100x50 sawn timber on the exterior. The galvanised flat bar on the diagonal braces is probably 50mm wide if it is the usual fixture.
They are more likely at least 150mm at top end, thicker at base.

I didn't pay much attention to the size of these in the first instance - I have re-measured and they have a diameter of 130mm.

To proceed with this I think I really should get a builder in to help with the piles and beams (probably with some intermediate piles added in). Thanks again for all the advice.

driller2000

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#2149936 24-Dec-2018 11:08

If it doesn't meet 3604 you will need a specific design from a CPEng structural engineer.

They will need to look at not only the proposed structural members but also the foundations.

That said - this is likely pretty basic stuff - so your local consulting engineer should be able to prepare design, plans, spec and BC application for not too much $$$.

SATTV

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#2149937 24-Dec-2018 11:16

We are redoing our deck next year and have been looking and found what I think is the best system I have seen bu a company called Outdure.

They have a product called Quickbuild https://www.outdure.com/products/qwickbuild/

It is like a big aluminium meccano set, you can use piles or screwjacks or both, it can be built in a day as the prefab it.

I was told the cost is approx $200 M2 for tiles and $280 m2 for composite decking ( plus labour )

The great thing about this system it will be straight and true, Aluminium does not rot and you can use their hidden nail system if you want to use wood.

John

I know enough to be dangerous