small deck / retaining wall design??

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nickreno

28 posts

Geek
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#248783 9-Apr-2019 21:43

Hi - looking for some hive mind (or single expert) advice around deck/retaining wall combinations.

I am looking at building a small deck, approx 10m long and 1m wide, around 0.5m off the ground - just below the height of two outdoor sliding doors.

One beam is fine; the other beam I would like to attach to posts that are serving double duty as part of a retaining wall; the deck height would be approx 600mm off the ground, and the retaining wall would be approx 900mm off the ground - so minimally higher.

The retaining wall posts are 125x125 h5 posts sunk approx 0.9m into the ground, every 1200mm. I am not clear on how to attached the beam to the same posts - would this be using 1x M12+washer bolt per post for the beam? And should the beam be 2x 90x45 H3.2 timber? Joists would be every 400mm. I have really struggled to find any standard guides to this, but I've seen a lot of photos of decks adjoing retaining walls that retain above the deck level - has anyone built one or have any insight :) ?

Aredwood

3885 posts

Uber Geek
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#2214017 9-Apr-2019 22:48

Just support the deck independently of the retaining wall. As it will be quite close to the ground once you allow for the thickness of the joists and bearers.

I highly doubt that 2x 90x45 would be suitable as bearers, especially on a 1.2m span.

A retaining wall that also supports a deck would probably be in the category of engineer design required.

Bung

6733 posts

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#2214018 9-Apr-2019 22:57

Not so much advice as a few questions.

The beam described as "fine", is that stand alone or will it be attached to the house?

What is the wall retaining?

I had something similar with a deck that stopped short of a low wall. When the deck planking needed replacing I took it up to the wall with a bearer bolted to the wall posts. I used longer joists but just sat them on the wall bearer. If there's any wall movement didn't want the joists shoved against the house.

Wellingtondave

160 posts

Master Geek
+1 received by user: 88

#2214029 9-Apr-2019 23:57

Actual "Engineers" who will sign off on anything compliant are in short supply.

In the ridiculous world we live in it's worth consulting a "professional" just for the liability aspect. Even it it takes you a year [money doesn't seem to be a motivator for these people] get a design done and cosigned by a council person.

Plan B, Build the sh1t out of it, then argue the point later.

nickreno

28 posts

Geek
+1 received by user: 3

#2214343 10-Apr-2019 12:45

thank you for the replies, good suggestion to simply support the deck independently of the retaining wall.

I had been trying to follow the guidance in this BRANZ document around attached stringers to the house, but attaching them to the posts instead - which seems to suggest 2x M12 bolts would suffice. The retaining wall is supporting some clay-based soil - it would likely support itself, but it would crumble if people stood on it, so preference is to put in a higher retaining wall alongside it to avoid this.

But, if it all results in something that is outside the scope of standard & needing an engineered design, then likely cheaper to just put in a parallel set of piles. It just seems like the retaining wall posts would be far stronger!

https://www.buildmagazine.org.nz/assets/Uploads/Build-154-36-Design-Right-No-Strings-Attached.pdf

nickreno

28 posts

Geek
+1 received by user: 3

#2214344 10-Apr-2019 12:47

newby question - but how does a veranda work - are the posts that support the roof independent of the beam that supports the deck? or is the deck beam attached to the same posts, or sitting on its own set of piles?