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Decking on concrete slab

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Budding Contributor

Decking on concrete slab

First time post'er. Looking at installing a timber decking (90x19) on a concrete patio (red dotted line). The Concrete Slab also has ceramic tiles on it that i might need to drill through. 

 

The slab is 75 mm below the house floorboards (door on top left that opens outwards). I would like to have the decking just below the floorboards (if not flushed).

 

Decking is 4.2m L and 3.0 m W. There are steps going down (bottom left and top right) as shown in drawing that I would like to cover with decking (blue highlighter marks showing direction of laying).

 

Joists are 70x45, laying them as shown (green highlighter), 70 wide and 45 high. That way, 45 + 19 (decking) gives me about 65cm. I am still unsure if I need to anchor the joists onto the concrete slab (through the ceramic tiles). Need some clarity on this and whats the minimum number i can get away with?

I was also thinking of using those spacers/footing protectors attached under the joists at regular intervals all through so that there is about a 5 mm gap under the joists and it doesnt come in direct contact with the ceramic tiles and has space for any water to run off.

 

The slab is curved (as shown), but I am making the decking rectangular so it will jut out and plan to support it with 3 posts (90x90). The height of the decking where the posts are going down will be 600 from the ground. So, i plan on taking the posts 450 deep in the ground and the balance 600 above ground.

 

Plan on using 10G-50mm SS screws (countersunk), Zenith?

Area is exposed and is North facing, exposed to run/no cover.

 

Stuff i am confused/need clarification on:

Do I need to dyna bolt the joists, if so, whats the least i can get away with? I will be staggering joists supports as shown. 

Home floorboards are tassie oak (stained it a bit darker....) so, getting Merbau decking.... would look odd?

I was also going to pre-drill holes using smartbit countersunk and then use the decking screws.

 

No council permission is required as this is classified as regular maintenance.

 

Would love for you experts to cast your eyes on this post and the attached drawing and let me know if there are any gotcha's or things i might have missed (or validate some of my thoughts).

 

Thanks in advance...

 

 

 

 

 

 

 

 

Rough sketch of the proposed deckingRough sketch of the proposed decking

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Bunnings Team Member
Bunnings Team Member

Re: Decking on concrete slab....

Welcome to the Bunnings Workshop community @sverma. It's great to have you join us and many thanks for your questions.

 

Your total height of joist and board is 64mm high. The remaining 6mm will be essential in allowing water to drain under the deck and for air-flow. You'll need to pack out under the joists with shims to establish this gap. You can then Dynabolt the joists down and I'd suggest one for every meter of the joist.

 

If you are laying the timber on its side I would recommend using Treated Pine Outdoor Timber Framing 90 x 45mm as 70 x 45mm joists on their side might have some bounce to them and leave you with a spongy deck. I would also be concerned with using 70 x 45mm timber on its side on the overhanging section.

 

You can certainly pre-drill and countersink the screw holes. I've used these Zenith 12G x 50mm Stainless Steel Self Drilling Decking Screws and they saved me a lot of time and effort.

 

You should find this article helpful: How to build a low-level deck by @Adam_W

 

Please let me know if you need further assistance or had questions.

 

Mitchell

 

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Re: Decking on concrete slab....

Hi Mitchell,

 

Thanks for your reply.

 

1. Shims is a good idea, I will certainly use that.Cant believe i didn't think of that earlier !!!

2. I am a bit confused about 70 being more spongy than 90. With the same height (45), I would have thought the wider the timber, the more spongy/bounce it would have? Or is the 90x45 more dense compared to the 70x45?

3. The overhang section (the bottom side of the pic) is the area where the decking will be off the slab, but will be supported by 3 posts (the shaded squares along the bottom bearer). Wouldn't that be enough? Do you recommend that I add noggins along that span in the overhang section to give it additional strength.

4. Noted on the dyna-bolt at 1m spacing. Will I need to bolt down the joists/bearer around the periphery (section over the slab) as well? Also, what size dyna bolts would you recommend as I will be drilling through PORCELAIN tiles that are stuck on the concrete slab.

 

Many thanks

Sid

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Bunnings Team Member
Bunnings Team Member

Re: Decking on concrete slab....

Hi Sid (@sverma),

 

The 90mm timber has more mass and therefore can support more weight. This might not be an issue if it is properly supported by shims but my advice also relates to my other comment about the section overhanging the concrete. Please check with Building Code of Australia and relevant span tables to see if 70 x 45mm timber on its side is suitable for joists/bearers in this application.

 

The far-right joist seems to have an unsupported span of around 1.5m. This is too large for 70 x 45mm timber laid on its side.

 

Please bear in mind you are not building a deck to support its own weight, it needs to meet requirements in supporting a minimum kilogram per square meter. Your 70 x 45mm timber needs to support BBQ's, table settings, a mass gathering of people etc. As mentioned before the BCA and timber span tables tell you how timber needs to be supported to meet minimum requirements.

 

Ideally, you'd have all structural timber, excluding noggins, either supported or bolted down. I'd recommend Ramset 10 x 100mm Countersunk DynaBolt Plus Masonry Anchors. That would account for the 45mm of timber, 10mm of tile and leave 45mm of the bolt in the concrete. 

 

Mitchell

 

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Re: Decking on concrete slab....

Hi Mitchell,

 

Will switch to 90x45 - easy :smile:

 

The far right & the bottom (along the posts) bearers, i can easily fix them 90 high x 45 wide as they will be off the concrete. Infact, the more I think about it, that would definitely be a better option.

 

Noted on the 100mm dynabolts.

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Budding Contributor

Re: Decking on concrete slab....

Hi Mitchell,

 

Attaching joists:

1. Using a Galvanised angle and dynabolting that and then screwing that to the joists? OR Dynabolting through the joists to the slab directly?

2. Fixing the joists to the bearers (even though they are both 90x45), are there any joist hangers that I can use to get a good perpendicular fit?

3. How deep should the posts be in the ground that I was going to set in with some rapid set concrete. 400 deep is ok?

4. Can you point me to articles/link regarding framing the decking please. I understand that framing along the sides gives the decking a much better look, we overhang the framing by 20 so that the side decking (vertical) will fit under that overhang and finish nicely.

Thanks.

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Bunnings Team Member
Bunnings Team Member

Re: Decking on concrete slab....

@sverma,

 

I feel bolting straight through the joist is a more direct and positive fixing. There are no joist hangers that I am aware of to suit 90 or 70mm wide joists laid on their side as it is not a typical construction technique. Footer requirements are generally site-specific and your local council should be able to advise further.

 

You should find this step-by-step tutorial helpful: How to build your deck.

 

If you want the side trim decking pieces underneath the decking, I believe you would set the framing back in 19mm from the overall size and overhang the decking from it by 19mm. That way the side trim pieces of the deck will sit underneath the top boards.

 

Mitchell

 

 

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Mixed Decking (Steps & Deck)

IMG_4500.JPG

 

In the above plan, the proposed decking is for a 4.2m x 3.0m decking over a concrete slab. Slight changes to the drawing, the joists will be 90x45 instead and dyna-bolted to the concrete. While taking final measurements and finalising the drawings, I realised that the 2 steps, bottom left (5 risers & 4 treads) and top right (4 risers and 3 treads) are posing a bit of a challenge. After taking into account the 45mm joist fixed to the existing concrete steps, it will become tricky for me to match the 90mm wide boards. (Risers are 150 and Treads are 300 each, give /take a few mm). However, if I use the 140mm decking, then i can easily attach 1 board fo riser and 2 boards for each tread and workout the spacing accordingly.

 

Question is, can I use 90x19 decking for the top of the decking and 140x19 just for the steps? the 90x19 fits very well for every other part of the decking, except for the steps. Or is that a huge design no-no and I should stick with 90x19 all around.

 

Alternatively, Swap everything to 140x19, but then, is that too wide a board, and then do I use 3 screws/joist instead of 2 to attach the decking boards?

 

Thanks

Sid

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Bunnings Team Member
Bunnings Team Member

Re: Mixed Decking (Steps & Deck)

Hi Sid (@sverma), 

 

There is no reason why you can't use 140mm decking on the steps and 90mm on the deck. However, 140mm is generally thought to look better and you might want to consider switching over to it for the whole build.

 

140mm is not too wide a board and you can still fix it in place with two screws. You'd want those screws reasonably close to the edge of the board to prevent cupping. A distance of between 25 and 30mm from the edge on either side of the board would be suitable.

 

Please let me know if you need further information or had questions.

 

Mitchell

 

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Re: Mixed Decking (Steps & Deck)

Thanks Mitchell,

 

The 140 is seeming more attractive (Lesser number of boards to use as well) :smile:

1. Thickness... 19 or move up to 22?

2. Decking Screws .... i'm confused! 10G... 12G...14G...... suggestions welcome. I am definitely inclined toward Stainless Steel. With Joists 45thick, and decking 19 (or 22), what depth screws should I be using... 50mm countersunk ?

3. Also, for attaching the joists... and building the base frame.... 1G / SS / 65 mm?

 

Thanks.

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