View Full Version : Deck on concrete - list of questions!
16th Sep 2007, 11:59 AM
I've searched this forum, and generated a list of questions!
I've got a job for a client ( I do garden renovating and drought proofing) which includes laying a deck over a concrete patio. The patio isn't very big, about 4m along each side, but also isn't particularly level - there's a raised drain access point in one corner which is about 25mm high.
The client's first idea was to buy deck squares from Bunnies for about $30/m. My first thought is to lay 75x35 bearers over the concrete which would allow me to pack it to get the deck level, then lay decking over the top.
Then I had the thought of extending the decking out to the fence line, which would be about 6m, and putting a couple of short stumps in the end to support the bearers, but then I assume I need larger bearers as the deck would need to be off the ground.
Of course the client's budget is way smaller than realistic!
1. sustainable decking timber? Its in full sun.
2. Is 75x35 treated pine bearers enough on top of concrete and what spacing?
3. what about a waterproof membrane?
4. If I go for the big deck, how big do I need the bearers?
5. And how high off the ground?
20th Sep 2007, 08:49 AM
Go on... someone give me some help....???
20th Sep 2007, 10:58 AM
Having the verandah not level will work in your favour because you need it to be able to drain under the decking.
I have laid a deck over concrete by fixing 1" thick battens to the slab with galvanised masonry anchors and then screwing the decking to that. It required some packing to get it level, but I don't think it is mandatory for a deck to be perfectly level, as long as it looks OK.
Make sure you allow for drainage. Run the battens so that water getting under the deck can flow away easily. I don't think there's any need for a membrane.
If you're going to extend the deck out, you'll need to decide on stump spacing, then we can look up the bearer size you need.
A photo or two would help.
I reckon the main problem with this set up is that you wont be able to access the underside of the deck once it's laid, so anything dropped in the gap will stay there. If it's located under trees, that might cause problems over time, I don't know.
20th Sep 2007, 11:46 AM
Just lay 70 x 35mm (or 70 x 45) traeted pine battens down spaced at 450mm centres and pack them out if needed and dyna bolt them down at 750mm spacing. If the slab is only slightly off level i wouldnt worry about packing the battens too much. as long as the deck looks level to the eye it should be fine. Under each dyna bolt put a square of malamine (i think thats its name.) , which you get from hardware store and looks like tar and comes in a roll, to stop moisture. Also use 75mm x 8mm dyna bolts counter sunk into the battens. Not sure which way you will be running the decking from your pic but if you have clearance best (iie cheapest) way is to use two 90 x 45 (treated pine) lenghs nailed together to make 90 x 90mm bearers and use 90 x 45mm joists and 90 x 90mm stumps on sole plated and concreted in. Max spacing will be 1500mm between stumps, 1500mm joist span and joist spaced at 450mm centres. Keep in mind this is max spans and is better to bring them in a bit to stop bounce. Hope this helps.:U
20th Oct 2007, 03:19 PM
Ok, I decided that the client wants to do it properly, so here's the plan.
Lay 100mm concrete to even up whole area
Fix 75x35 TP bearers (marked in red on plan) fixed with concrete screws from Sachys Robertson (http://www.sachys-robertson.com.au/specialised.html)
Lay Yellow Billau 90X19 on top with Sachys Robertson #10 50mm screws.I have 2 questions though, the concrete is about .5m2, and has to go through the garage and laundry and across the courtyard, so I figure it'll have to go in barrows. Should I get it delivered or mix it myself? Budget is tight, but so is time! Target price is $100, but I think that's optimistic, and I don't want to spend all day mixing concrete. I don't think I can get hold of a free concrete mixer, but I could hire one if its worth it. Will Minimix hang around waiting while I barrow loads into the yard? (I'm a one man band, although my client will put a pair of gloves on if I'm nice)
Also, how do I make sure the decking boards stay tight together - I dont' want small children falling between the boards, as we won't be able to access them (easily at any rate). Will the billau shrink?
21st Oct 2007, 08:19 AM
I am currently building a deck on top of an existing concrete area.
You will find that the slab isn't as 'level' as expected (well mine wasn't anyway!) This worked in my favour as I need to pack it out, and allowed for airflow under the bearers.
I'm using 70x35 T/P for bearers and 90x45 joists. The bearers are dynabolted (10 x 100) into the slab.
Just pack it out where needed ensuring that it is level and there isn't any bounce on the joists.
I test the bounce by walking on them - if it bounces more packing and/or gal brackets.
21st Oct 2007, 10:53 AM
Half a cubic metre is about ten barrows. It would take about five/ten minutes to mix each barrow, and you'll get a good workout. You'd have to inquire about waiting time for a minimix, but they usually give you 15/20 minutes free. Depending on how far you're wheeling, it shouldn't take you that long.
Of course if it's only half a square metre, as you've written, then that's nothing and I'd mix it by hand.
Shrinkage depends on the moisture content of the boards. I've found that TP shrinks the most (if it's not KD), but if your boards are dry, then they could expand when they get wet and you'll be in trouble if you lay them tight.
21st Oct 2007, 03:15 PM
You will have to allow time for the concrete to dry before decking over
wouldn't it be easier to do like below ...i gather your situation must be similar.... this one was a body corp job and couldn't lay concrete even if i wanted ....there is an under ground carpark below so concrete was under the dirt as was a membrane so the only allowable fixing for this one was perimeter only for the dirt section....
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