Deck-block foundation plans, sized to your space

Parts

Cut list

Kerf 1/8″ included. Every cut runs edge to edge.

Steps

• Tape measure
• Speed square
• Pencil
• Drill/driver
• Clamps
• Safety glasses
• Hearing protection
• Dust mask
• Circular saw or miter saw
• Line level
• 4′ level
• Flat spade and a post-hole digger or trenching shovel
• Hand tamper
• Rubber mallet
• Small sledgehammer
• Plumb bob (optional)
• Wheelbarrow
• Sander, 120 and 180 grit

Underlined words have a short explanation: hover or tap them. All of them are on the glossary page.

1. 1Before you start

   Read every step once, start to finish, before you buy or cut anything. Each step names the parts it uses by letter; the parts list gives their sizes.

   Set up somewhere flat with good light. A pair of sawhorses with a sheet of plywood on top makes a fine work table.

   Wear safety glasses whenever a tool is running, hearing protection for saws, and a dust mask for sanding and for cutting treated wood or sheet goods. Tie back long hair and loose sleeves. Keep children and pets out of the work area.

   • Tip At the store, pick straight boards: hold each one up to your eye and look down its length. Skip boards that bow, twist or have big knots near the ends.
   • Tip Measure twice, cut once. Most mistakes are a misread tape, not a bad cut.

2. 2Cut and label the parts

   Cut every part on the cut list. Cut the longest parts first: if one goes wrong, it can still become a shorter part.

   To mark a cut, hook your tape on the end of the board, make a small V with its point exactly at the measurement, and draw a square line through the point with a speed square. Put an X on the waste side.

   Cut on the waste side of the line, so the blade's kerf (about 1/8″) comes out of the scrap, not your part.

   Write each part's letter in pencil on a face that won't show, and stack the parts by letter.

   Sand the faces before assembly while they're flat and easy to reach: 120 grit, then 180.

   ✓ Lay the parts out and count them against the parts list. Measure a few: if one is off by more than 1/16″, recut it now.

   STOP. Let the saw reach full speed before it touches the wood, keep both hands clear of the blade's path, and never reach under a board you're cutting. Support long boards on both sides of the cut so the blade doesn't pinch.

3. 3Call 811 and check the spot

   At least 3 working days before you dig, call 811 (or use call811.com). It's free. They send people to paint or flag buried gas, electric, water and cable lines near where the blocks will go.

   Check your property line on your survey or plot plan, and ask your town about setbacks, easements and whether this needs a permit.

   Pick ground that drains: not a low spot where puddles sit after rain, and not over a septic field.

   STOP. Don't dig until the marks are down. Hitting a gas or power line can kill you and is expensive to repair.

4. 4Mark out a square rectangle

   Hammer a stake at one corner of the platform. Measure 96″ along the long side and hammer a second stake. Tie mason's line tight between them, a few inches off the ground.

   For the first square corner, use the 3-4-5 rule: from the first stake, mark 3 ft along the string. Run a tape 4 ft out roughly at 90°. Swing it until the 3 ft mark and the 4 ft point are exactly 5 ft apart. That direction is square: set a stake 72″ out along it.

   Measure the last corner from the other two (96″ and 72″) and set the fourth stake. Run string around all four.

   Measure both diagonals, corner to corner. Nudge the stakes until the two numbers match.

   • Tip For bigger layouts use 6-8-10 instead of 3-4-5: the bigger the triangle, the more exact the corner.
   • Tip Batter boards (two stakes with a crosspiece set 2 ft outside each corner) let you take the strings down to dig and put them back in exactly the same place.

   ✓ Sides are 96″ and 72″ and the diagonals match within 1/2″. Spray marking paint along the strings, then pull the stakes you're about to dig around.

5. 5Build batter boards and set level strings

   At each corner, 24″ outside it, hammer three stakes (A) in an L and screw a crosspiece (B) across each pair with 2 1/2″ screws, so each corner has two crosspieces at right angles. 12 stakes and 8 crosspieces in all.

   Find the lowest block spot (usually the corner the ground slopes toward). The block tops will be 8″ above the ground there, so the strings go 9″ above it: 1″ higher than the blocks, so the blocks never push the strings.

   Mark that height on the crosspieces nearest the low corner. Run a string from there along one side and hang a line level in the middle. Raise or lower the far end until the bubble is centred, mark it and screw the crosspiece at that height. Work round all four sides the same way.

   Tie the strings so they cross exactly over the corners you staked. Re-measure: sides 96″ and 72″, diagonals equal. Slide the strings along the crosspieces until they are.

   • Tip Nick each crosspiece with a saw cut where the string goes. If a string is knocked off, it goes straight back in the same place.

   ✓ All four strings read level on the line level, cross over the corners, and the diagonals match within 1/4″. Pluck a string: it's tight, with no sag.

6. 6Mark the block centres

   There are 3 rows of 3 blocks: 9 in all. Along the length, block centres are 42 1/2″ apart; across, 30 1/2″ apart. The outer blocks sit with their outside edge on the strings, so their centres are 5 1/2″ inside.

   Measure along the strings and mark each centre with a dot of marking paint on the ground, dropping a plumb line or a level held upright from the string to the ground.

   For the middle blocks, stretch a temporary string across between marks on opposite sides and measure along it.

   ✓ Count 9 marks. Spot-check that neighbouring marks are 42 1/2″ and 30 1/2″ apart and the rows look straight from each end.

7. 7Dig the holes

   At each mark dig a hole 20″ square with flat sides. Measure every hole's depth down from the string, not from the ground: the bottom of every hole is 15″ below the string.

   Because the ground slopes, that makes the lowest hole about 6″ deep and the highest about 7 3/4″ deep. Dig the high side down; never build the low side up with loose soil, which settles.

   Scrape each bottom flat and tamp it with the end of a 4×4 or a hand tamper until it's hard.

   • Tip Set the strings aside while digging (they stay marked on the crosspieces) and put them back to measure.
   • Tip Don't dig in soaking-wet or frozen soil; it won't pack.

   ✓ Every hole bottom measures 15″ below the string, within 1/2″, and is firm, flat and free of roots.

8. 8Fill 4″ of gravel in each hole

   Pour crushed gravel into each hole in two layers of 2″, misting each until damp and tamping it hard. About 3 bags per hole, 20 bags in all.

   Finish each gravel bed 11″ below the string and level. Lay a level across it both ways.

   • Tip Use crushed gravel, not pea gravel. Round stones roll and the block will rock.

   ✓ Every gravel bed is 11″ below the string, level both ways, and doesn't dent under a heel.

9. 9Set a patio stone in each hole

   Set a 16″ patio stone in the middle of each gravel bed. Twist it down and tap it with a rubber mallet until it sits firm.

   Check with a level both ways. A high corner: lift the stone, scrape the gravel, reset. A low corner: add a handful of gravel and tamp.

   ✓ Every stone is level both ways, doesn't rock when you stand on it, and sits 9″ below the string.

10. 10Set and level each block

    Centre a deck block on each stone, slots lined up with the direction your beams or skids will run.

    Measure from the string down to the block top: 1″ at every block. Lay a level on top both ways.

    Too high: lift the block and scrape away a little gravel under the stone. Too low: add a little and re-tamp. Never shim with wood or stack stones; they rot or slide.

    Then lay the straightedge (C) across neighbouring blocks with the level on it, row by row. The level reads level on every span.

    • Tip Work from the corner blocks first, then the middles. Recheck the corners at the end: tamping nearby can shift them.

    ✓ Every block top is within 1/8″ of the others (1″ below the strings), level both ways, and doesn't rock when you step on it.

11. 11Backfill and do a final check

    Shovel soil back around each block up to the ground line and tamp it with your heel, so the blocks are held on every side.

    Measure the block-centre diagonals of the whole grid one last time: corner to corner, they match within 1/4″.

    Take down the strings but leave the batter boards until the frame is built, so you can re-run them to check.

    Build your frame so it sits in or on the blocks without forcing them. Blocks sit on the surface, so the platform can be moved later, and it rides a little with frost.

    • Tip Rain is the enemy of a block foundation. Make sure roof water and downspouts don't pour onto the blocks.

    ✓ Every block is firmly packed in, level, and a straight board across any two neighbours shows no gap.

12. 12Finish

    Ease every sharp edge and corner with 120-grit sandpaper so nobody gets a splinter.

    Sand all over with 180 grit, always along the grain (the direction the wood fibres run). Scratches across the grain show through stain.

    Wipe off the dust with a barely damp rag and let it dry.

    Paint, stain or a clear coat (water-based polyurethane is the easiest). Apply thin coats with the grain, let each dry fully, and sand lightly with 220 grit between coats.

    Let it dry for the time on the can before using it.

    • Tip Try your finish on a scrap of the same wood first: stain looks different on every board.