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I'm planning to build a floating deck over some patio stones in my backyard, but I would like a second opinion since I have no previous experience on this.

My idea is to set it directly on the ground(patio stones) and it's a 12x10 floating deck.

So I would buy:

  • 8x - 2x8x12 spf dimension lumber
  • 2x - 2x8x10 spf dimension lumber
  • 25x - 5/4 x 6 x 10" treated wood decking

I live in southern Ontario(Zone 6) so we do get snow here, will snow damage it? If yes, what should I do to avoid it?

This is the frame that I have in mind:

enter image description here

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  • Be advised that your patio probably isn't level or flat. You need to deal with that. Even thin blocks under the corners of your frame may be adequate. – isherwood Apr 7 at 15:55
  • Thanks again. Yep, it's unlevel, that will probably be my biggest challenge here. I guess that my question is, will snow damage the deck? – Roberto Apr 7 at 16:05
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    The answer, of course, is "yes". Moisture damages almost everything, which is why we replace decks every 20-30 years. I doubt that's what you really want to know. – isherwood Apr 7 at 16:07
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    Rain and snow come from above. Raising the deck won't change that. Are you concerned about the ground contact? Let's cut to the chase already. :) – isherwood Apr 7 at 16:12
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    Pressure treated lumber is often rated for ground contact, and you can always isolate it with metal brackets or nylon blocks, for example. The fact is that it'll rot at the joist connections (where moisture tends to be retained) about as quickly as it'll rot at the ground contact, given that it's a well-draining platform. – isherwood Apr 7 at 16:50
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A floating deck is typically, as I understand it, not "lumber on the ground". It is lumber on "Dek Blocks" or similar, with posts as needed (which could be anywhere from nothing (on the highest natural spots) to several inches to a few feet tall) sitting in the Dek Blocks to support the deck frame in a level manner.

A floating deck is definitely a DIY project. It is quite different from a traditional deck: No footings to pour, no critical connection to the house, often no permits required. Permits are almost always required for a traditional deck. For a floating deck, the rules vary by jurisdiction - e.g., in my area (at least when I built my floating deck), no permit required up to 200 square feet.

As far as snow & rain, there are two issues - top and bottom. For the top, make sure that there is enough of a gap between the decking boards for water/melting snow to drain. For the bottom, if the boards are pressure treated and on top of blocks, you should be fine, as the rain and melting snow won't collect underneath. But if you put the frame directly on the patio then you could easily have sections where water would collect and cause problems, even with pressure treated wood.

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If it can't dry, it will die

As the saying goes! Timber can last for centuries in buildings even if it does get wet every now and then. The important thing is its ability to dry out. In practice on a deck in a wet climate, that means:

  • Making sure water can drain off the deck. You'll want a slight slope on your deck (away from the house). 1:100 appears to be the recommended grade, i.e. every meter away from the house your deck goes down by 1cm.
  • Grooved decking boards are designed to help water runoff, but rather awkwardly will often be a nasty water trap as people assume the grooves are for grip and don't consider that orientation matters. If you're using grooved boards, orient them downhill - it's very important that water can run down the slope in the grooves.
  • Make sure you have adequate ventilation under and around the boards to help them 'breathe'. Wood is a natural material which will expand and move as it gets wet and dries. This means making sure air can easily get underneath the deck, as well as leaving little gaps between the boards, as air will help the board dry quicker and more evenly, preventing it from warping.
  • Snow is heavy! Make sure your deck is well supported so snow doesn't make it collapse.
  • Stone slabs are excellent at absorbing quite a suprising amount of water. A dry piece of wood touching a wet slab will act like a pipette and suck water upwards, ultimately meaning the wood is wet for longer. Consider adding some form of plastic barrier (such as damp proof course, or bits of roofing as Ed suggested) to help prevent this from happening, and position it such that it doesn't have the opposite effect and trap water around the wood.
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I use spare roofing shingles or tarpaper/felt so I don’t have a direct contact with posts.

An area this large I would probably get a roll of sill seal slice it up and staple it to the bottom of the pressure treated lumber. This is how we attach the bottom plate to a foundation at least it would provide some isolation.

A 6” strip of sill seal foam could be cut into 3 pieces tripling the useable length.

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