digest
Short Summary
Main recommendation: For a floating deck on clay soil, build a box beam frame, set each post on a leveled block with limestone screening, and cantilever the deckboards past the posts so precision is less critical. The video demonstrates this on a 12 by 24 foot two-tier deck with a wheelchair access ramp, and explains why the block-on-clay approach is preferred over digging and gravel-filling holes.
Key Points
- Use landscape cloth only on the ground areas closer to the surface (the elevated section, ~46-48 inches off the ground, doesn't need it) to keep hogweed and similar weeds from rooting under the deck, which also simplifies maintenance around the skirt.
- Box beams allow the deck to cantilever past the post loads, so you can be within a couple of inches of your target dimensions and still end up with a sound structure.
- Setting posts on blocks over clay, rather than digging out and filling with gravel, avoids a water-filled hole that could encourage post rot, and lets the whole deck lift and settle together through winter and spring.
- Lifting and leveling box beams is best done as a two-person job, with one person holding the beam level while the other fastens it.
Details Worth Noting
- Concrete instructions: Remove the existing stairs and any attached sections first (the video notes only a few screws were holding the stair section, so it pulls off with a firm tug).
- Concrete instructions: Mark the deck perimeter from your plan, and work out ramp length using a standard of 1 inch of rise for every 10 inches of run (for example, a 1 inch drop needs 10 inches of ramp).
- Concrete instructions: Lay landscape cloth on the ground areas, then level each block location with about half a bag of limestone screening per block, tamp it with a little water, and set the block on top.
- Concrete instructions: Find the deck centerline and build out left and right from it so the beams overhang evenly on each side.
- Concrete instructions: Set the middle post first, measuring from the top of the post to the target deck height minus the height of the 2x6 decking, and transfer that mark to the post to set the beam height. Target about 1 inch of drop from the sill plate to the deck surface for wheelchair accessibility.
- Concrete instructions: Fasten beams with half-inch galvanized bolts, nuts, and washers, using two bolts per beam connection, and tighten them after fine-tuning the level across a board.
- Concrete instructions: Cut off excess post height in one pass with a saw with a long blade, then frame the platform with 2x6s every 16 inches, building it in two sections from the middle beam outward plus the ramp extension.
- The presenter jokingly says the three main beams transfer roughly 6,000 pounds to the ground through each beam's support, adding that if a party ever exceeded that weight he'd want to be there himself.
- A 6 foot level was extended by clamping a straight board to the beams, turning it into part of the leveling system across posts that are farther apart than the level.
Bottom Line
Building on blocks over clay with box beams and cantilevered decking is a forgiving, practical method for a floating deck, provided you level each block carefully and work in pairs when setting the heavy beams.
