How to build a deck over a concrete patio

Check the slab and the height first

The slab has to be sound enough to carry the deck. Hairline cracks are fine; a slab that rocks, has sunk at one corner or has frost heave needs repair or a framed deck on footings instead. Then check the height you have to work with, measured from the slab to the bottom of the door sill.

• Keep the finished deck surface at least an inch below the door sill, so rain does not run under the door.
• Keep the decking clear of the house siding and weep holes.
• Note which way the slab falls. A patio is built with a slope away from the house, and the deck must not dam that water.

The three methods below need anywhere from about 1 inch to 4 inches or more of height, so the sill measurement often makes the choice for you.

Option 1: interlocking deck tiles

Snap-together tiles, usually 12 in square, lay straight on the slab with plastic bases that let water drain underneath. They add about an inch, need no tools beyond a saw for the edges, and can be lifted again. For a 12x12 patio the deck tiles calculator gives:

Tiles are the cheapest and fastest option, and they follow the slab exactly, which is also their limit: they cannot level an uneven slab.

Option 2: sleepers and deck boards

Sleepers are pressure-treated boards laid flat or on edge on the slab, with deck boards screwed across them. Laid flat, 2x4 sleepers plus 5/4 decking come to about 2-1/2 inches. Use lumber rated for ground contact, because the sleepers sit where water runs.

Using the board prices in the deck materials calculator, 343 linear feet of pressure-treated decking is about $824, and composite at about $5.10 a foot is about $1,750, plus the sleepers and fasteners. Count boards for other sizes with the deck board calculator.

The drainage rule for sleepers

Run the sleepers in the direction the slab falls, from the house toward the yard, so water flows down the channels between them. Sleepers laid across the slope act as little dams and hold water against the wood, which is how a deck over concrete rots from underneath.

• Never fasten sleepers flat to the slab with no gap; lift them on composite shims or rubber pads so water passes under them.
• Tape the tops of the sleepers before decking, as you would joists. The joist tape calculator counts the rolls.
• Leave the ends open or use a vented fascia so air moves through.

Option 3: adjustable pedestals

Adjustable pedestals are plastic or rubber supports, each with a screw height, that carry either pavers or a low frame of aluminium or timber joists. They level out a slab that slopes or dips, keep everything clear of the water, and are the usual choice on rooftops and balconies. They also cost the most of the three, and need the most height, often 2 inches before the decking.

If the slab is badly out of level or you want a deck higher than the door sill allows, stop here and build a low framed deck beside or over the patio on its own supports; the ground-level deck guide covers that.

Which way to go

For a full priced build, the deck cost calculator gives installed prices by material.