Building a large low-level deck over sandy soil

[Ralph1958]

Hi, my partner and I are just about to move into a new home that construction is finally coming to an end. I am planning to build a deck area out the back using composite decking and framing design with a plank in the middle to avoid butt joints. I have researched up and down the web and think I have covered most basis but would still value the input, feedback and advice of community members. I have attached a picture below showing design, measurements and materials. I also have a series of questions that I need help with.

1. Our home is in Mindarie so the sand is pretty much beach sand, and as access to the backyard is limited I will be digging it out by hand and wheel barrowing it to the front skip bin. I was planning therefore on having a 50mm clearance below the bearers and a total clearance to the top of the deck of 255 mm, is this sufficient or is the 300mm clearance that is referred to in the literature a hard and fast rule?

2. The distance between some joists will be 452mm CtC (others will be 440mm), 2mm over the 400mm to 450mm range that is suggested, should I be including another joist to bring the distance under 450mm or will the 2mm not matter.

3. I will be using ledgers attached to the house and surrounding limestone wall, should I be using spacers between the walls and ledgers to allow water to run away and what should be the spacing between the ledger fittings?

4. The strip at the side of the house is only 1550mm wide so can I just use ledgers and 3 sets of bearers to support the decking or do I still need joists? Note: the side path steps down 100mm from the main deck area which is essentially the thickness of the joists (90mm)

5. I have heard mixed reports regarding the use of weed mats etc under the deck, do they work, are they wort the bother is there a better solution to controlling weeds under a deck that wont have any ready access?

6. Have I overlooked any critical issues?

Thank you for any help or advice you can offer.

[Akanksha]

A warm welcome to the Bunnings Workshop community, @Ralph1958. It's a pleasure having you join us and we trust you'll find our site a great resource for handy D.I.Y. advice and inspiring ideas.

Thanks for sharing your deck plans and providing those details. I'm sure our resident Bunnings D.I.Y. expert @EricL will be happy to assist you with your questions when he's back online later today. Let me also tag our experienced renovators @Adam_W, @wooshka, @Jimi and @ProjectPete in case they have any thoughts. 

In the meantime, can I request you to provide photos of your backyard and the area around where you wish to construct your deck? This will help Eric and our other members provide more tailored advice suited to your space.   

Deck projects are very popular among our community. While waiting, you might like to read through Adam's guide on How to build a low-level deck and How to build a deck by D.I.Y. expert @MitchellMc in case they help. You can also check out our Top 10 most popular decking projects if you're after some project inspiration.

Please do keep us updated as your project progresses. We'd love to see the final result.

Akanksha

[MitchellMc]

Welcome to the Bunnings Workshop communtiy @Ralph1958.

I apologise for the delay in my reply.

1. 300mm is not a hard rule, but the greater the distance under the decking timbers, the more airflow in the structure, reducing the early onset of decay. The closer you go to the ground, the more moisture will be retained in the structure. Moisture is the enemy of timber.

2. 2mm difference on your joists is negligible; there is no need for another joist, as that's fine.

3. Is this home brick veneer? Ledger boards can not be attached to brick veneer as they are not a weight-bearing structure. The course of bricks is only designed to bear the weight of itself. On a similar note, although somewhat convenient, retaining walls are not designed to be used as deck attachments. Instead, I'd advise creating the deck as a free-floating structure. Also, given the deck size, you'd likely need to approach your local council for a permit. They'll be able to advise on your soil type and what type of footing will be required in your area. An engineering plan might be requested by them also. 

4. For the side area, your "bearers" will become the joists, so there are no issues there. In regards to the number you'll need, see above, as I have concerns about you using both the house and wall as mounting options.

5. In my opinion, weed matting or drainage matting is a requirement for a low-level deck. They help suppress weeds and prevent water dripping through the deck from eroding footing while assisting with water drainage. I personally haven't heard any negative reports about using weedmatting and know from personal experience that it suppressed the weeds under my deck.

6. No critical issues are jumping out at me, and you've thoroughly planned your deck. The only issue I see is the attachment to the house and retaining wall and what footing you'll need in your sand soil. I'd encourage you to enlist the service of an engineer to assist in those hurdles.

Please let me know if you have any questions.

Mitchell

[Ralph1958]

Hi Mitchell,

Thanks for your detailed reply and the clarification you have provided.

The house is double brick but I think I will still steer clear of using ledger boards.

I will also engage the services of an engineer to check everything , define tolerances etc and prepare a set of engineer drawings for the local authority.

Thanks again.

Regards

Ralph

[MitchellMc]

Keep us updated, @Ralph1958, as this is going to be a fantastic project! I can't wait to assist further and see your results.

Mitchell