How to build a low-level deck over concrete?

Hi team,

Looking to tackle a DIY deck at my parents backyard.

Some details of the space:

- roughly 24m2 (7mx4m)

- Working area is concrete slabbed. Level is variable in certain parts

- Clearance from the internal space to the concrete slab is around 100mm only

- The planned deckspace is covered with an awning - still yet to decide if I'll work around the 4 posts or work right up to it

My thoughts:

- From what I can understand the internal to slab clearance available would be a ideal for 'low profile' deck

- I'm thinking of using adjustable pedestal feet that will accept the joist frame directly (no bearers). I saw that there's also pedestal feet that will accept blockers

Questions:

- Are gal nails or framing screws better?

- is there any significant feature advantage with pedestal feet that will accept blocker pieces? does it even matter?

- H3 treated wood is reccommended for above ground/outdoor use but what size timber pieces should I be considering and how far apart are the spacing between joists at their centre? I'm sure there's a best practice/building code to this question I'm not familiar with where to start looking

- How do I know the spacing requirements for the pedestal feet?

- Can anyone improve on what I'm thinking of doing or provide other ideas?

thanks in advance!

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Comments

EricL

Hello @Halima

Welcome to the Bunnings Workshop community. It's sensational to have you join us and thanks for sharing your question about your plan on building a deck.

Please allow me to try and answer some of your questions.

100mm is not a lot space to work with, if you were to use 70x35 in combination with standard Merbau decking you would be using 89mm of space leaving you with 11mm as an air gap. There would not be enough space for a pedestal, but you can use angle brackets to hold and support the timber joist.
In my opinion screws are better at framing as they don't easily pull out.
In regards to timber size the space available will dictate the size you can use. You can also look at this guide - How to understand deck span tables. It will give you a general understanding of timber spacing and how certain timber sizes can be used with less support.

I also suggest checking the rules and regulations of your local council regarding low-level decks. There might be rules in regards to size limitations and permits.

If you need further assistance, please let us know.

Eric

Jason

Hi @Halima,

Let me also extend a very warm welcome to the Bunnings Workshop community. We are looking forward to seeing your deck project come to life.

In addition to Eric's helpful advice, you should find the guide How to build a low-level deck useful.

Jason

Halima

https://www.workshop.bunnings.com.au/discussion/comment/46100#Comment_46100

Thanks Eric,

After doing some more reading (guides on here actually!) I noticed there was a low level project that used L brackets - would you be able to suggest any specific bracket? Ive also noted that the short side of the doorway is actually only 90mm. It qickly extends out to 100mm though so would an L bracket in this scenario be appropriate? Im thinking the real challenge is picking a lower profile timber to use as joists.

Would me options become lower profile again if I were to use a hardwood?

Halima

https://www.workshop.bunnings.com.au/discussion/comment/46106#Comment_46106

Appreciate the link! I came across it shortly after posting and have bookmarked for reading again 🙂

EricL

https://www.workshop.bunnings.com.au/discussion/comment/85236#Comment_85236

Hello @Halima

There are several sizes of angle brackets you can use depending on your need. For example, the Dunnings 60 x 100 x 35mm M10 Galvanised Angle Bracket allows you to position the timber at varying heights and can easily be anchored with a nut and bolt configuration. There are single hole variants along with double hole mounting styles. I suggest visiting your local store and having a look at what you'll be needing.

If you need further assistance, please let me know.

Eric

Halima

https://www.workshop.bunnings.com.au/discussion/comment/46100#Comment_46100

Hi Eric,

What kind of approach would you suggest if the clearance were between 60-65mm ? the 70x35 wouldn't suit the job in that case.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/85242#Comment_85242

Hello @Halima

The first thing I would do is to double check the available space. If you only have 60mm to 65mm I'm afraid that is not enough space to install a low-level deck. Unless you can find extra space, I suggest thinking about an alternative cover to the area such as concrete and tiles.

If you need further assistance, please let us know.

Eric

Halima

https://www.workshop.bunnings.com.au/discussion/comment/107780#Comment_107780

Thanks Eric. The 60mm query was for my own place, I won't be looking at that anytime soon.

Back to my parents back deck. Here's an image of my plan so far drawn on cad. Top of the photo is north, House starts from the west side.

Pictured below (to scale)
Frame = 4430 x 2780

Picture Frame style

joists = 70 x 35

supports = L and Pedestals

West joist/bearer secured to brickwork of the doorway with concrete screws(dynabolts?)

Joist hangers for both ends

Deck boards planned for 140 merbau with 3mm gaps

450mm centre to centre spacings

deck lines account for 3mm spacings between boards

W E

questions-

- I've planned for 70x35 H3 timber for the frame. For span/support would a L bracket or pedestal support every 1.5metres be appropriate? span table linked above doesn't have anything for 70x35 timber.

- Any suggestions for how many rows of noggins?

- For the various fixings, where would concrete screws, dynabolts be most approrpriate? for timber/timber I'll use framing screws rather than nails as you've suggested

- What's the best practice regarding overhangs when provisioning for fascia plates? Would it be more appropriate to fix fascia plates under the top deck boards or flush to the top as I have imaged below?

- With the supporting pieces for the frame part of the top deck, should a joist run alongside directly along it? or will my noggings pictured below be enough?

anything to consider that I might've left off that'll dictate the build?

feel like I'm getting close to working out a BOM for the project

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/121150#Comment_121150

Hi @Halima,

You'd typically support 90 x 45mm timber joists every 1000mm, so for 70 x 34 (I'd go with 45mm thick), you'd need to support more readily. Something like every 700mm should be fine.

Noggins can be placed every 1000mm between joists and offset on the next row.

Dynabolts or Ankascrews are appropriate masonry fixings. For anything within 100mm of the edge of the slab so with Ankascrews as they are less likely to crack the concrete.

Your plan is looking good so far.

Please let me know if you have any further questions.

Mitchell

Halima

https://www.workshop.bunnings.com.au/discussion/comment/129491#Comment_129491

Thanks Mitchell, I like the idea of the 45mm thick for that extra bit of strength. I cant seem to find any joist hangers suited for this dimension wood however- what options can I use to fix these? I noticed in another thread you suggested a pryda 40x50 hanger however they dont seem to exist a product listing anymore

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134770#Comment_134770

Hi @Halima,

You can screw through the rim joist into the ends of the joists with 125mm bugle screws. This might prove to be a bit of an issue where the rim joist is against walls. Typically, you'd attach the joists to it and then move them all into position. That might require a helper, considering the size of the deck.

Mitchell

Halima

https://www.workshop.bunnings.com.au/discussion/comment/129491#Comment_129491

Thanks Mitchell

70x45 sounds like a goer. I cant see any joists specfically for this dimension timber however. Could you suggest a joist hanger and L bracket?

for the L bracket I found this from pryda

https://www.bunnings.com.au/pryda-pergola-angle-bracket-88-x-63-x-36mm_p1071032

Would this be appropriate for the job

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134771#Comment_134771

Hi @Halima,

70mm timber is typically not used as joists as generally 90mm is the smallest considered. You won't often find joist hangers or brackets to suit 70mm being used as joists. Does the above method of screwing through the rim joist into the ends of the 70mm joists to support them not sound like it would work for you?

Take a look at this guide on How to build a deck. In step three, you'll see how I've used screws to connect the rim frame in the corners and attach the joists to it. No brackets or hangers are used, just the bugle screws.

Mitchell

Halima

https://www.workshop.bunnings.com.au/discussion/comment/138207#Comment_138207

apologies just noticed my previous reply came through so doubled up there!

the west section of the rim joist will be hard against the house where the back door is located so perhaps pre screwing with bugle heads as suggested might be the only way, unfortunately at the lowest section I only have 90mm clearance so the larger joists are out of the question.

Plan is then to fix the west rim joist to the house via masonry screws (10mmx100mm) and after level have everything else referenced from that.

Does the pryda L bracket I suggest work as a supporting piece? unsure of what gauge I am expected to use as part of the support structure

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/134771#Comment_134771

I am unsure how you would use the L bracket to attach your joists @Halima. I would suggest creating your rim frame, installing the joists within it with bugle screws, then shifting the deck frame into position with the help of a few friends, lifting it to the correct height on the adjustable pedestal feet, levelling and then attaching it to the wall where needed with Dynabolts.

Mitchell

Halima

https://www.workshop.bunnings.com.au/discussion/comment/138208#Comment_138208

Thanks Mitchell. Sounds like thats the way I will go.

L/angled steel brackets are for support along the length of the joints - fixed to the concrete because clearance is quite low. ie. No room for pedestals

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/140508#Comment_140508

Thanks for clarifying @Halima. For brackets that support the joists and connect them to the concrete, our members typically go for a more heavy-duty bracket. The size of the bracket depends on how high the joist is above the concrete; you want the mounting hole to be about midway up the joist.

Mitchell

Halima

https://www.workshop.bunnings.com.au/discussion/comment/142192#Comment_142192

Thanks @MitchellMc

I was on the lookout for cheaper alternatives thanks for that bracket suggestion! and yes will keep positioning in mind. for fixing the bracket to concrete will m10 dynabolts around 50mm suffice or should I go for 100mm? and what about bracket to joist, will a red tip screw like I am going to use for the joist hanger suffice or should I opt for something else?

https://www.bunnings.com.au/pryda-timber-connector-screw-12g-x-35mm-hex-head-pack-50_p0084754?region_id=117342&gad_source=1

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/143388#Comment_143388

Since these brackets will essentially be bearing the entire weight of the deck, I'd suggest going with the thicker versions @Halima. The Pryda brackets are not designed for such use. M10 Dynabolts would be fine. I'd go with the 75mm as the 100mm might be a bit excessive.

Our members either bolt the bracket to the joist or use coach screws.

Mitchell

TaraL

https://www.workshop.bunnings.com.au/discussion/comment/121150#Comment_121150

This looks great @Halima! Can I ask what software you used for the drawing?

Jason

https://www.workshop.bunnings.com.au/discussion/comment/129585#Comment_129585

Hi @TaraL,

Just in case @Halima can't respond, I wanted to refer you to the article What software do you use for drawing plans?

Hope it helps.

Jason

TaraL

https://www.workshop.bunnings.com.au/discussion/comment/134833#Comment_134833

Thank you @Jason! Do you recognise the image as being from one of platforms mentioned?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/138252#Comment_138252

Hi @TaraL,

I'm not sure what program @Halima has used, but their drawings look similar to those @Jimmy2 has made with Adobe InDesign in his discussion What timber spans for single layer deck frame?

Hopefully this helps.

Jacob

Halima

https://www.workshop.bunnings.com.au/discussion/comment/129585#Comment_129585

Hi @TaraL . I used Fusion 360 for this one 🙂 apologies for the late reply

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