Concrete patios that drain on flat, low lots

Start with where the water will go

On a hillside the answer is obvious. On a lot that is nearly level, as many are across the south shore, it takes deliberate planning. The patio surface needs a steady fall away from the house, commonly a small fraction of an inch per foot, so that rain moves off the slab instead of sitting on it. Too little fall and water lingers. Too much and chairs rock and tables tilt.

Just as important is what happens after the water leaves the edge. If the lawn beyond the patio is higher than the slab, the runoff has nowhere to go and will pool along the border. A thoughtful plan looks at the whole yard: downspouts, the low corner, any existing drain lines and whether a channel drain or a swale is needed to carry water toward the street or an approved outlet.

Patios beside raised houses and pile-supported slabs

Plenty of older homes in the city sit on piers a few feet above the yard, and many newer ones rest on pilings. A patio poured against either kind of house will move differently from the building. The soil under the patio settles over time, and the house does not, so a patio tied rigidly to a foundation or a porch can crack along that line or tip away.

The usual approach is an isolation joint between the slab and the house, porch posts or steps, so each can move without breaking the other. Ask where isolation joints will go and how the gap at the house will be kept from funneling water under the slab. If the patio will sit partly under a raised house or deck, ask how it will be kept dry and ventilated.

Preparing soft ground so the slab stays flat

A patio carries far less weight than a driveway, but it still needs even support. Sod, roots and the dark organic topsoil common in older yards should come out. A compacted granular base, usually crushed stone, gives the slab a firm, even bed and lets water that gets underneath drain away rather than soaking the clay.

Find out how far down the digging goes, which stone or granular fill forms the bed, and which machine packs it tight. Old flower beds, filled-in ponds, removed tree stumps and former pool sites are all places where loose fill hides, and they deserve extra attention. If you know about any of them, tell the contractor before the visit.

Live oaks, shade and the roots under a patio

The big oaks that shade so many New Orleans yards are a blessing on a July afternoon and a challenge for flatwork. Their roots spread wide and shallow, and over years they can lift slabs and sidewalks. Cutting major roots to make room for a patio can also harm the tree, so it is worth talking to an arborist before digging close to a large one.

Shade brings its own maintenance. A patio that rarely dries in direct sun is a welcome surface for mildew, moss and algae, which turn gray concrete green and make it slick. A slightly rougher finish, a good fall and a sealer suited to damp conditions all help. Ask the contractor what finish they would suggest for a shady spot.

Thickness, reinforcement and joints on a patio

Patios are commonly poured about four inches thick. If a hot tub, an outdoor kitchen, a masonry grill or a heavy planter will sit on part of the slab, that area may need to be thicker or have its own footing. Plan those features before the pour, since cutting in for gas lines, electrical conduit or plumbing afterward is messy.

Wire mesh, rebar or fibers help hold the slab together if it cracks, and control joints decide where those cracks show. For a patio of ordinary depth, joints often end up roughly eight to twelve feet apart. On a patio the joint layout is also part of the look, so ask to see where the lines will fall relative to doors, columns and furniture.

Finishes that stay comfortable in heat and rain

A light broom finish is the practical default. It gives grip when wet and hides minor wear. Smooth troweled finishes look sleek but can be slippery once rain or algae arrives. Stamped patterns and color are popular for patios and have their own upkeep, covered on the stamped concrete page.

Concrete absorbs heat, and a dark color gets hot under bare feet in summer. Lighter finishes stay cooler, which matters around a pool or a play area. Exposed aggregate, where small stones show at the surface, adds texture and traction. Whatever you choose, ask to see a sample or a finished job in similar light before deciding.

Curing and the first weeks

Heat and wind can dry the top of a fresh slab before the concrete below it has hardened, which weakens the surface. Early pours, a curing compound or a covering help. Keep sprinklers, pets and kids off the patio until the contractor says it is ready, and hold off on heavy furniture and grills for longer.

Some fine surface cracks are normal in the first months as the concrete finishes shrinking. Wide cracks that keep opening, or sections that tilt, point to a base or drainage problem and are worth reporting while the job is fresh in everyone's memory.

Questions to put to any patio quote

1. What fall will the surface have, and where will water go once it leaves the slab?
2. How will the patio meet the house, porch or steps, and where are the isolation joints?
3. How deep is the excavation, and what base material will be compacted underneath?
4. Where will control joints be placed, and what reinforcement is included?
5. What finish is proposed, and how will it handle shade, heat and wet weather?
6. Are drains, conduit or gas sleeves for future features part of the plan?

Concrete patios that drain on flat, low lots — common questions

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