Need advice for deck construction

kaitolin

Hi there, wondering if anyone could give me some advice

For context, my deck build will not be attached to the house and be about 600mm high. The below rough sketch shows where my bearers and posts would go (the left bearers are placed to avoid pipes along the house wall). I was gifted sixty 140 x 45mm joist hangers, so I am thinking I will use 140 x 45mm joists and double these up for bearers on 100x100mm H4 posts. There is a garden bed/ retaining wall to the right and I want to add a privacy screen along the right side of the deck, as tall as possible, likely utilising 1800x902mm merbau alternating slat fence, but horizontally.

My main questions are:

1. I have seen some decks where instead of the joists sitting on top of the bearers they hang between them. When is this okay to do? As I have free hangers, would it be worth doing this? For example for more air circulation/ a bigger crawl space.

2. I have seen 101 different ways of putting in posts/stumps. What's best practice for longevity? (Preferably without breaking the bank). The two posts on the far left are situated on an existing concrete pad that is sloped away from the house, what should I use here to have a level foundation rather than digging holes?

3. How should I attach the bearers to the posts? I have seen them be strapped directly on top (but if my bearers are 90mm and my posts 100 - assuming this won't work?), notched (this wouldn't work with my 90 x 140mm bearers though), bolted in from the side, etc.

4. How best to make and attach a privacy screen on the deck? I have seen people attach things directly to the top of the deck or to existing gazebo posts that come through the deck (unsure if these posts are cemented into the ground or attached to the frame of the deck). Either way, would my posts on this side of the deck need to be stronger to support this?

Any advice would be greatly appreciated!

JacobZ

Hi @kaitolin,

Thank you for your questions about your upcoming decking project.

You've got a great plan in place already that will make a fantastic deck that will be extremely strong.

As with any larger project like this, the first step I'd advise is to contact your local council to check whether you need building approval and the assistance of a draftsperson or engineer. A quick phone call can help direct your next steps.

I'll answer your questions individually and then offer some general advice and ask a few questions.

Joist hangers are fine to use in almost all situations and with careful planning of the frame, I can't say I've come across anywhere you couldn't use joist hangers. Using joist hangers in your circumstance would be perfectly ok and considering you've got some for free, it makes sense to design the deck accordingly.
Where new posts are being installed, I'd always recommend full stirrup post anchors that are wet set into the concrete. They elevate the posts slightly so water can run beneath them, minimising the potential for your posts to rot. In high wind environments, use Pryda High Wind Post Anchors.

Where there is existing concrete, like the pad near your back door, use Pryda Bolt Down Post Anchors w Raised Mount to achieve the same slightly raised post. You can use packers underneath the post anchors to level them before bolting them in place.
With your design, there is no real reason to double up your bearers and with the posts you are using, realistically it's best not to.

Following the relevant Bearer Span Table, 140 x 45mm H3 Treated Pine Framing can span 1400mm between posts, which assuming your scale is relatively accurate, your bearers are within.

With this being said, you can cut a 45mm wide and 140mm deep notch on one side of your posts for the bearer to sit on. You can then bolt the bearer to the post.
The best way to attach a privacy screen at the far right of your deck, would be to continue the posts up above the height of the finished deck. The bearer can still sit in a notch on these posts without terminating the post at that height. Continuing the posts up and above this height will allow you to run horizontal rails between the posts to attach your merbau screening. Your posts shouldn't need to be stronger to support this.

Now for some general advice and questions for you.

As I mentioned above, there's no real need to double up your bearers as all your spans are well within the guidance of the relevant Timber Span Tables. This will reduce your material costs without reducing your deck's strength.

At 600mm above ground, you are not legally required to install a handrail or balustrade, but it is worth considering, especially if you'll have young children on the deck.

Have you decided which direction you'd like your decking to run? Have you thought of how you'd like to finish the edges of your decking?

These things can affect how you build your frame, so thinking about these things in the planning stage is worth doing so you can factor them in early.

All in all, you already have a fantastic plan in place, and I'm excited to see it develop further and eventually become a reality.

Let me know if you have any further questions or info to add.

Jacob

kaitolin

Hi @JacobZ , thanks so much for your fast and thorough reply!

I just have one last (probably silly) question about joist hangers, if I'm attaching them to the same bearer from opposite sides - won't the screws clash? Do I have to stagger the joists? I'm a total deck newbie so forgive my ignorance.

I'll follow your advice and use the full stirrup post anchors, but how do I find out how deep my concrete footing needs to be? I'm pretty sure because the council cosiders this low risk work I don't need a permit, but I'm lost on how to find out the correct information per the concrete depth. I will give them a call - or will I have to ask a local builder for this information? I've heard something about a rule of thumb is 1/3 or 1/2 the depth of how high the post is above ground? Would this mean my privacy screen footers would have to be significantly deeper?

I am tight for time to do this project so I'm thinking of adding my railings in the future if possible. I intend for the deck boards to run away from the house so that I can have full lengths by using the 5.7m merbau lengths cut in half. I was thinking of adding a picture board frame to it all so I know that will mean I need to do some extra joists around the edge for support.

I'm also thinking of using this to waterproofing the framing and extend the life of the deck Protectadeck 45mm x 25m PVC Joist.

Do you think that's overkill or worthwhile? Considering I'm using 90 x 19mm merbau decking, what do you think would fail first? The decking or the framing? I ideally want to build it to last!

Thanks so much for all your help

EricL

Hello @kaitolin

Please allow me to try and answer some of your questions. Thank you for sharing your decking plans, can I please request for a more detailed drawing where the joists are included and if you are putting in a rim joist. I just want to be sure we are referring to the same parts of the deck you're referring to. In regards to the joist hangers, if there is also a joist hanger on the opposite side then it would be best to stagger them so that the screws or nails will not run into each other.

In regards to your footing, I agree with your idea of asking a local builder as they will know how deep your footing needs to be, this includes the footing for your privacy screen as well. As to the Protectadeck 45mm x 25m PVC Joist this is a great way to protect your deck framing from the elements. If you prepare the ground and footing properly, there is no reason why your deck should not last a long time.

If you need further assistance, please let us know.

Eric

JacobZ

Hi @kaitolin,

Upon review, I've found that Pryda, the manufacturer of the steel post anchors, does not recommend the use of their post anchors in decking applications without the consultation of a structural engineer. If you'd like to use them, you would need to get in touch with a structural engineer to ensure they are fine for your deck.

With this, I would change my recommendation to directly installing the posts in your pier holes with 2-3 coats of bitumen membrane on the post to protect it from rotting.

As you've mentioned, the rule of thumb when setting posts is that one third the height above ground should be below ground. As an example, if you have 1.8m of post above ground, you should have 600mm below ground.

Your privacy screen footers will definitely need to be deeper.

Apologies for the mix-up with the post anchors.

Jacob

kaitolin

Hi @EricL

Here’s a slightly more detailed drawing - I plan to make a better one soon.

The pink is my bearers, the green are my joists, the picture frame boarder framing is not exactly how it’d be, and I know I also would still have to add noggins between the joists. I’m currently trying to figure out how I’d go about dealing with the angled section of the deck - I’ve done it green for joist but I’m realising it should be pink for bearer, if I add a post half way along it to support it (as it currently spans more than 1400mm) would that work?

I’ve only really seen people use joist hangers when their deck has just the two outside bearers, is that because most people wouldn’t want to have to stagger their joists and the extra expense of hangers? I would prefer for my decking screws to be able to line up along the deck, so maybe I need to rethink using joist hangers and simply sit the joists on top of the bearers? I think I had initially wanted all my joists to be attached with joist hangers for more height under the deck and to more easily avoid pipes along the house wall, but now I’m not as sure. What do you think I should do?

Thanks for your advice

kaitolin

Hi @JacobZ

That is a shame about the post anchors. Unless for some reason my preliminary planning advice comes back saying I need to consult a structural engineer for the deck or the privacy screen, I’ll probably just do the posts in the ground covered in waterproofing to save the expense.

Thanks for your help

EricL

Hello @kaitolin

Thank you for sharing your updated decking frame plan. One of the reasons single level, low-level deck works is because of the amount of footing it has. A standard 90x45 can be supported every meter by an angle bracket or a pedestal foot. This provides a large amount of support and keeps the timber steady. The noggins are placed as extra support intermittently to stiffen the frame. The same can't be said for your decking frame plan as you are using larger timber pieces, and you are a fair distance away from the ground.

In regards to the angled section, I agree it should be pink with a footing to support it. I propose re-evaluating your decking frame plan and engaging the services of a deck builder or a draftsperson. These trades people will be familiar with the rules and regulations of your local council. They can therefore draw your decking frame to code and will comply with the rules of the council. I also recommend checking the council rules on deck size just to make sure that your deck is exempt from a permit.

The best part of having your plans done by a professional is that it conforms to the rules and regulations of the council. Plus, you don't have to second guess yourself while you are assembling it. Should it ever get inspected your plans will be your saving grace as it was done by a professional.

If you need further assistance, please let us know.

Eric