Decking on a patio concrete slab

[Nathan88]

Hi All,

just got to know about posting our topic as I have commented on other topic and the admin team directed me to post a new topic.

I’m planning to build a merbaur decking on our patio concrete slab as in the photo. The slab area is 3.6 x 9 m and it is sloped with low of 150 mm and high of 300 mm to the indoor floor. I would like to have the decking same level as indoor which leaves me 150 mm spacing from concrete slab near brick wall and 300 mm spacing on the far end of the patio area.

the other side of the patio opens to our rumpus room, which is same level as concrete floor.

Can someone help me to choose the framing and connections to concrete slab? Also, I would like to hear your opinions to manage the deck level to the indoor, while providing access to the rumpus room. 

[MitchellMc]

Hi @Nathan88,

I've included an example of a deck layout below working with Treated Pine Outdoor Timber Framing H3 120 x 45mm for the bearers and Treated Pine Outdoor Timber Framing H3 90 x 45mm for your joists. The 120mm H3 will need to be special ordered into our stores for you.

As @Neo19 mentioned in their thread, they used Dunnings 65 x 130 x 65 x 5mm M12 Galvanised Angle Bracket and these would be suitable for fixing your bearers to the concrete slab.

I'll need a little more information on "150 mm spacing from concrete slab near brick wall and 300 mm spacing on the far end of the patio area.". Is the concrete slab sloped? You might like to consider using Builders Edge pedestal feet to adjust for the difference in height.

If you are increasing the floor height with a deck on the patio then you are going to need to create a step down into the rumpus room area. It's probably not a great idea to bring the deck right up to the sliding door and you might like to consider keeping the ground height even directly outside the sliding down then have a step.

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

Mitchell

[Nathan88]

Dear @MitchellMc ,

Thank you for providing detailed info. Yes, the concrete slab is sloped since it was a patio area previously. The drop of indoor floor level to concrete slab is 150 mm near the brickwall and if the decking is leveled, this leaves me a drop of 300 mm at the far outside of the slab. 

If I use bearer+ joist, this will rise the floor by 120+90 ( minimum) and this will be higher than the drop near the house level. Please advise.

[MitchellMc]

@Nathan88,

The height of the deck will consist of 120mm (bearer) and 19mm (decking board) giving 139mm total height which will allow 11mm for drainage under the bearers. The joist in your situation gets mounted to the sides of the bearers and not on top of them.

Mitchell

[Nathan88]

Hi @MitchellMc ,

Do you have an image showing how to mount joist side of the bearer. given that there are four bearer and the joist running across the bearer, I am confused how to mount them on side. Also, If i am using the angle/ Builders Edge pedestal feet , what would be the recommended spacing? Thanks

[MitchellMc · Accepted solution]

@Nathan88,

You might like to have a read through the article How to build a low-level deck by @Adam_W. Within it, you'll find details which will benefit you in your project. He also illustrates the correct mounting procedure for a single level deck.

I've also included below an example image showing spacing for supporting the bearers. 

Mitchell

[Nathan88]

Thanks @MitchellMc ,  That make things lot more sense. I am planning to run the decking board parallel to the house (which means parallel to the bearer). Would it be possible with this design?

[MitchellMc]

@Nathan88,

It's certainly possible but the bearer/joists orientation needs to be changed also. This is to reduce cost and because you need to nail into the joists.

Here's an updated plan.

Mitchell

[Nathan88]

@MitchellMc , I have a question on this design? What’s the contribution of joists here? I believe the decking boards are directly supported by bearers and bearer is supported by concrete slab.

[HandyAndy]

the bearers "bear" the weight of the deck, or transfer the weight to the footing, joists are for load spreading and fixing the decking boards.

bearers are generally beefier than joists.

joist spacing/decking board fixing is generally 450mm, some hardwoods can go upto max. 600mm spacing.

I think in your case

~7 x bearers with 4 supports each = 28 supports.

you could just fix your joists (@450mm centres) directly to the concrete, but you'd be looking at 9m x 9 joists = 81m of timber with supports every 1400mm = 60ish supports, fixed to the slab

not to mention the levelling it all.

you could possibly increase the size of the joists to 120x45, which would allow bigger spacing = less supports

using bearers will be much simpler as they run parallel to your slope, levelling is far easier. then your inline joists are fixed on the inside of the bearers with the same top height as the bearers.

hope this helps