How to build low deck on rear landing (concrete)?

[shrewdly4957]

Hi Bunnings community, 

Amateur DIY-er here, I'm currently planning out the build of a deck over a concrete landing at the rear entrance of my house. First off, a few photos to show the current setup, as well as some of the dimensions. The main slab is 2.4m by 1.2m, with a thickness of approx 10cm. The top of the slab is 83cm from the (stamped) concrete patio below. For the house side, the slab to the bottom of the sliding door is 13cm, and the slab to the metal trim angle is 10cm.I also want to deck the stairs in the same format, but planning around the main landing area first. 

 

Questions as I get started: 

1. Should the deck go to the bottom of the metal trim/angle, or the bottom of the sliding door? I was going to go to the bottom of the sliding door, to reduce the height difference between the inside room floor and the finished deck. If to the bottom of the sliding door (13cm), I could use 90x45mm for the joists, then with 19mm decking boards have approx. 21mm+ spacing between the joists and the concrete. 

2. What are some options to raise the joists directly off the concrete? I saw the Klevaklip (https://shop.klevaklip.com.au/adjustable-joist-supports) supports, but could I also get away with angle brackets to fix the joists to the concrete and window packers as shims given the spacing height is not too much? 

3. The finished deck height (assuming using bottom of sliding door) to the concrete patio below would then be 96cm - I'd like to also put in a balustrade/guard rails around the landing and down the stairs. I saw the Peak Aluminium balustrade kit, however noted that the fixings for the post brackets had to be 70mm away from the edges of the concrete slab (if going directly to the concrete) - this would reduce the already narrow space. I was thinking I could fix some posts (merbau, if matching with the decking) to the concrete patio below, and then join them to the deck frame using bolts. The balustrade could then be built using these posts as supports, including a handrail down the stairs. For a 1m handrail, the posts would then be about 2m+ tall. Would bolting (chemset, dynabolt) the posts to the patio below be okay with the additional fixings to the deck frame to reduce the lateral strain? Or would I need to core-drill and cement the posts in? 

Hope this all makes sense, hoping I can get some guidance on this project! I've read a lot of threads on low decks on concrete, so fingers crossed I'm not making any glaring mistakes. 

Cheers

[JacobZ]

Hi @shrewdly4957,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Your deck should go no higher than the finished floor level inside your home; otherwise, you may have issues with water getting inside. Based on what I can see, I would say this is at 130mm above the concrete landing.

With this limited space, none of our pedestal options would work, so you would have to use packers with angle brackets to attach them to the landing, or another solution that is capable of supporting your joists that low to the concrete.

The National Construction Code, Part 3.9.2 Barriers and handrails is the main document you can look at for your handrail. At 96cm high, you are just under the 1m limit where you are legally required to have a handrail, but it is obviously a good idea to install one anyway. 

Unfortunately, I can't advise on the design of the handrail with 100% certainty, but I imagine you would be able to get by without needing to core drill then concrete in posts. For this part of the project, I'd suggest you speak with a carpenter or builder for their assistance, as there is a fair bit of complexity involved with the installation of handrails, and I couldn't in good conscience provide you with advice because I am not experienced in this area.

Allow me to tag @Nailbag, who is an experienced handyman and home renovator to see if he has any thoughts. 

Let me know if you have any further questions.

Jacob

[Nailbag]

Hi @shrewdly4957 

1. I would go bottom of the sliding door to reduce the height difference too. Being able to then use a standard 90mm x 45mm joist will make life much easier and conventional.

2. Personally with small projects like this, I like to keep it as simple and easy as possible. I would be using galvanised right angles brackets fastened to the concrete slab with suitable masonry anchors and gal bracing brads into the joists. Fastened to the slab first, then level the joists and brad them into place.

3. I would consider fastening the balustrade posts to the outside wall of the existing landing. Rebate the first 25mm of the pilot holes into 90mmx 90mm dressed Cyprus posts with a spade bit allowing a socket to still access the head of the hex screws. Take the posts either to the ground or finish them short with a 45deg chamfer would look nice. This way the balustrade wouldn't encroach into your landing space.

Hope this makes sense.

Nailbag

[shrewdly4957]

Thanks @JacobZ  and @Nailbag ! Appreciate your time in responding.

Few clarifying questions: 

1. I've added a clearer picture with the door open. To match the inside flooring, it's probably more at the 15-16cm mark. Would it make much difference to go either to the 13 or 15cm (besides the additional clearance between the joists and concrete)? 

2. The joists spanning along the short edge would be approx 1020mm span (subtracting 2x 90mm bearers on the long edge, which I can fasten with joist hangers?) and probably 400mm spacings to make it even - I'll try draw up a proper plan later today. With this span, is a bracket on either end sufficient, or should I go for one in the middle as well? i.e. one at 0, 510 and 1020? And then use the shims in between the angle and the joist to level. Any reason to use nails over screws? 

3. For the posts, attaching to the wall sounds good - however I forgot to mention the slab overhangs by about 55mm. Could I attach some blocking to fill the gap between the brick wall and the post, such that the post fits flush against the slab? I figured notching out 55mm out of a 90x90 post would be too much. What screws/bolts would be suitable for this? 

Thanks again

[MitchellMc]

For the deck height, going from 13cm to 15–16cm isn’t a huge difference @shrewdly4957. The main consideration is the clearance between the joists and the concrete. At 15–16cm you’ll have slightly more room underneath for airflow, drainage, and any minor leveling adjustments, which is generally a good thing. The height difference inside versus the deck will only increase slightly, but it will still look neat and be easier to work with structurally.

For your joist spanning 1020mm with 400mm spacing, brackets at each end are typically fine for short spans like this. Adding a bracket in the middle isn’t strictly necessary unless you want extra stability or you notice the joists flexing when walked on. Using shims to level the joists with angle brackets is the right approach. Screws are preferable to nails here because they allow for better adjustment and provide a stronger hold over time, especially when you’re using shims to fine-tune the level. Nails could loosen more easily under movement.

Regarding the posts and the 55mm slab overhang, yes, you can use blocking to fill the gap so the post sits flush with the slab. You can fix the blocking to the wall or slab with masonry anchors, then fasten the post to the blocking using screws suitable for timber-to-timber connections. Make sure whatever fixings you use for the posts can handle lateral forces, particularly if they will be supporting a balustrade and handrail. Avoid trying to notch a 90x90 post by 55mm, as it will significantly weaken it.

Please let me know if you have any questions.

Mitchell