Patios poured to a measured fall

How a patio slab gets poured

The first day is layout, and demolition if there is something failed sitting there already. We set the form boards to the line you want and to the fall we need, stake them off, and check the height against the door threshold and the slab that is already there. Then the base gets graded and compacted. Nothing goes inside the forms until the string line and the grade both read right, because once the truck is on site the shape of that patio is already decided.

A patio that came out right sits a step below the door, runs away from the wall the whole way across, and holds a straight edge when you lay a level on it. The finish is a broom texture pulled in one direction, because a smooth trowel finish outside turns slick the first time an afternoon storm comes through. Edges are tooled and the corners are square. You should not be able to feel a lip where the new slab meets the old one.

Skip the compaction and the patio will tell on you inside a season or two. Loose sand under a slab does not stay put in Tampa, where the rain comes most summer afternoons and moves fine material through the ground beneath the concrete. The slab bridges the void for a while, then a corner drops, a crack opens across the middle, and the water that used to run off sits in a low spot against the house. By then it is a repour.

• Grade shot against the door and existing slab
• Base graded and compacted before forms go in
• Wire mesh or rebar set on chairs
• Broom finish with tooled edges
• Site swept and washed before we leave

Slope, and where the water ends up

Every patio we pour runs away from the house. That sounds obvious until you start looking at slabs around here, sitting dead flat or tipped back toward the wall, so every storm pushes water at the bottom course of the block and along the edge of the house slab. We set the fall at about a quarter inch to the foot, decide where the water is meant to end up, and shoot the grade before a single form board is nailed off.

Where it goes matters as much as the fall itself. A patio that drains into a fence line, a neighbor's yard, or a low corner that never dries out has only moved the problem. So we walk the lot first and look at where the downspouts land and where the yard already ponds after a storm. Sometimes that means a step down at the outside edge, so sheet water leaves the slab instead of creeping back under the threshold.

When the fall is wrong, you find out in June. Water stands in the middle of the slab, algae goes green where the shade is, and the sliding door track collects grit and standing water every time it rains hard. On a block house the damp works into the bottom course and the paint blisters off the stucco above it. On a wood framed room addition, and there are plenty of those bolted onto older houses here, it goes after the sill plate, and by the time it shows up inside, the fix is no longer concrete work.

What the ground under Tampa does

The ground here is mostly sand, and sand behaves differently than clay. It drains fast, which helps, and it moves fast, which does not. Under an old patio you find fill nobody ever compacted, or a soft pocket where a stump got buried when the lot was cleared. So we dig, we look at what is actually down there, and then we either compact it properly or pull the soft material out and bring in base that will hold.

Then there is the water table. Out on the flat ground east of the shop it sits high, and after a wet week the subgrade is still saturated when we get there. Pouring on soupy base is how a slab ends up with a soft corner that shows itself a year later. If it needs a day to drain, it gets a day, and where a spot never drains we build the base up and give the water a way out from under the slab instead of trapping it there.

Curing is the other half of it, and in a Tampa summer the sun pulls water out of the top of a fresh slab faster than the concrete can use it. That is where shallow map cracks come from, the ones that look like crazed glass across a patio barely a year old. Our day starts at 6:00 AM for that reason, so the pour and the finish happen before the worst of the heat, and the surface stays damp or covered while it sets. Nobody chases a finish on a slab that has already begun to go off.

Walkways and the joints that control cracking

A walkway is a narrow slab, and narrow slabs crack for reasons a patio does not. The run from the driveway to the front door usually crosses a sprinkler line and comes close to whatever tree went in when the house was built, and both of those are reasons the ground under it moves. We keep the thickness consistent the whole way, set the width wide enough for two people where there is room for it, and cut the joints at a spacing that suits that width.

Concrete cracks. Everything we do about it is aimed at deciding where. Control joints get cut into the slab at a planned spacing and a real depth, so the slab breaks along that line instead of wherever the stress finds a weak spot on its own. On a walk the spacing follows the width, and on a patio it follows the shape, with a joint pulled off any inside corner, because an inside corner is where a crack starts every time if nothing is cut there.

Roots are the other thing, and the older neighborhoods here are full of big oaks. A root running under a walk lifts one section at a time until there is a lip standing proud at the joint, and that lip is what a person catches a toe on. Where the tree is staying, we can shift the run or thicken the slab over the root and leave it room to grow. Where the walk is already broken up over a root, taking that section out and pouring it again beats grinding a patch that will lift again next year.

Tying new work into an old house

Most of the patios we pour are extensions. There is a small original slab under the back door, put in when the house was built, and the family has outgrown it. The new pour has to meet the old one at the same height, carry the same fall or better, and not be locked to it so rigidly that the old slab drags the new one along when it moves. So we saw a clean edge on the existing concrete, dowel it where that makes sense, and put a joint right on the tie line.

Around here that old slab is usually attached to a concrete block house with a stucco skin, and often there is a screen enclosure sitting on it or coming off the back of it. That changes the work. The footprint of the cage has to be in the plan before the pour, the anchors need something solid to bite into, and the finished slab cannot come up higher than the sliding door sill or the bottom course of block, because that is exactly how water ends up inside a room.

New concrete never matches old concrete on color, and anybody who tells you it will has not poured much of it. A fresh slab starts out lighter and comes down over the months as it weathers, so the seam shows for a while and shows less later. What we can match is the texture and the line, the same broom direction across both slabs and an even joint where they meet. That is the part you feel underfoot and see from the door.