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Deck block question

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[OP]
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Oct 6, 2007
290 posts
49 upvotes
Mississauga

Deck block question

Hello all,

I’ll be building a low level floating deck (18” height) with deck blocks. Presently there are some old patio slabs that measure 30” x 24”. Deck will be 12’ x 20’. Question is, should I remove the present patio slabs and put land scape fabric, then cover with stone? Or other option is to lay deck blocks on top of stone?

I’d like the first option but requires more work and money to buy gravel and fabric. Second option, I’d have to relevel the slabs( heavy suckers) , and maybe secure the blocks to the slab so they won’t shift?

Like to hear any opinions, advice please
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7 replies
Deal Expert
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Oct 6, 2010
15502 posts
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Toronto
I typically dig a hole, say 1ft, put in some substrate and put the block on that. You can do whatever you want to be honest and you shouldn't have any issues with shifting. If you want to throw it on them patio stones, go for it. Personally I'd won't but that's just me.

I note that I have sheds and gazebos in this method for years and there has been zero shifting, ect...

In fact, just built one over the weekend, 8x10 platform for an outdoor gym.
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Feb 25, 2004
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Longueuil
Do you know how those slabs were installed (how deep soil was removed under them)?

If the base is good, I would only level the slabs that will have a deck block on them. Having a base of slabs under you deck is even better than not having them. No animal will dig under.
Try not! Do or do not, there is no try...
[OP]
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Oct 6, 2007
290 posts
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Mississauga
A thin layer of limestone screenings underneath them all. Some have shifted since then.

I am concerned that building on the slabs Just as they are will cause then to shift. In that case I’d have to either remove them ( stick with only deck blocks +smaller slab underneath+screenings) or keep slabs but add more screenings, tamp the screenings down and then place the deck block on the slabs. The slabs are a real pia cause of their size to relevel when they’re off-kilter.

Plus anyone think that being a low deck (opened on the sides+ rodent screen barrier) would be prone to mold on these slabs vs gravel?
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Feb 25, 2004
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Longueuil
IMO a thin layer of limestone screenings is not enough, I would build a better base (I think it might have been overkill but I dug 2 feet for my patio slabs... but they have never moved).

I don't think it would be prone to mold either way. Water will simply run off between the slabs and in fact, the top of the slabs will dry quickly.
Try not! Do or do not, there is no try...
Sr. Member
Oct 21, 2005
807 posts
747 upvotes
I'd build it on top of the slabs. Don't even need deck blocks, just use ground contact 2x8 pressure treated lumber. Build the frame (beams and end joists), square it up by measuring corner to corner, level it out with temporary shims under the frame, nail on the rest of the joists, nail several cutoff pieces of 2x8 along each joist to prop it up (one per patio slab - make sure to use cut-n-seal on cutoff pieces of lumber) so that the shims can be removed. Then add your decking on top of the joists. Quick, easy, cheap, solid.
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Jun 24, 2005
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Richmond Hill
IMO, it's still best to use deck blocks so that you keep the water away from the wood even though it's pressure treated.
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Oct 21, 2005
807 posts
747 upvotes
rngun wrote: IMO, it's still best to use deck blocks so that you keep the water away from the wood even though it's pressure treated.
Floating docks made of PT lumber sit in a lake half the year and can last decades. The amount of water on top of pre-existing patio slabs will be negligible. You can save money on the deck blocks, and not have to waste time/money removing and disposing the patio slabs. You can pour out a few bags of gravel on top of the patio which will keep it off the ground a bit more, and allow for more drainage. But that's really only necessary if you're building directly on the ground.

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