The lanai slab question nobody asks: will it be enclosed?
Florida's residential code requires a 6-mil polyethylene vapor retarder under floor slabs, then exempts patios and other flatwork “not likely to be enclosed and heated at a later date.” A covered lanai is a prime candidate for that later enclosure, as a Florida room or an extra bedroom. Pouring it without a retarder is cheaper today and the one thing that can't be fixed later without breaking it out.
| Decision | Florida Building Code, Residential | For a Tampa lanai |
|---|---|---|
| Vapor retarder | 6-mil, laps 6 in (R506.2.3); patios not likely to be enclosed are exempt | Use one if enclosure is even possible |
| Slope | Impervious surfaces within 10 ft slope 2% away (R401.3) | Fall away from the sliding doors |
| Thickness | 3.5 in minimum for a floor slab (R506.1) | 4 to 6 in, the Tampa residential range (HomeBlue) |
| Joints | May be omitted with fiber or 6x6 W1.4xW1.4 mesh (R506.2.4) | Otherwise cut panels close to square |
| Post footings | Exterior footings 12 in below finished grade (R403.1.4) | Under each roof post or beam |
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Rain under the lanai roof
A roof keeps the sun off but not the sideways rain of a Tampa summer storm. The slab falls away from the house so that water runs out to the yard, and the step down from the interior floor stays where it is. Where the roof line drips onto the slab edge, a gutter or a slightly lower edge keeps water from pooling at the posts.
- Keep the lanai a step below the interior floor
- Slope 2 percent or more away from the house
- Footings sized for the roof posts or beam before the slab is poured
- Conduit for fans, lights and outlets under the slab
- A vapor retarder if the space might ever be enclosed
Extending an old lanai
Adding on to an existing slab means a joint where old meets new and matching the old surface height. If the old slab has settled or cracked badly, breaking it out ($1.10 to $1.80 per square foot on HomeBlue's Tampa page) and pouring one new slab is often cleaner than patching two together.
Lanai slab cost by size
| Lanai | Area | At $5.55 to $11.09 per sq ft |
|---|---|---|
| 12x16 | 192 sq ft | $1,066 to $2,129 |
| 12x20 | 240 sq ft | $1,332 to $2,662 |
| 14x24 | 336 sq ft | $1,865 to $3,726 |
| 16x30 | 480 sq ft | $2,664 to $5,323 |
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Computed from projectcosted, Tampa, August 2026. Footings, a vapor retarder, tear-out and pump access add to these.
Uncovered and out in the yard? See concrete patios. Pour timing: pouring concrete in Tampa rain. Steel and retarder detail: the thickness and vapor barrier guide.