· Michael Blair
The temptation is to build on the patio that is already there. Patio slabs are usually thin, unreinforced and shallow, and Middle Tennessee clay moves enough to make that a problem.
Why the patio slab is usually inadequate
A patio is typically four inches of unreinforced concrete on minimal base, poured above frost depth. A sunroom applies concentrated wall and roof loads it was never designed for.
On clay that moves seasonally, the result is differential settlement, and a glazed structure shows that as racked frames and cracked glass long before a timber wall would.
What is needed instead
Footings below frost depth carrying the wall loads, and a slab or floor system designed for the structure. Where an existing slab is retained, it usually needs perimeter footings added around or beneath it.
This is a permitted and inspected element for good reason.
- Footings below frost depth at the load points
- Slab designed for the structure, not inherited from a patio
- Proper drainage away from the new footprint
- Insulation under the slab for a four-season room
- Flashing where the new roof meets the house
Insulating the floor
A four-season room over an uninsulated slab is cold underfoot for much of the winter, which people notice more than wall performance.
Under-slab insulation is inexpensive at construction and impossible afterwards, so it is worth specifying.
Where it meets the house
The junction between the new roof and the existing wall is where sunrooms leak, exactly as decks fail at the ledger. It needs proper flashing integrated with the wall’s weather barrier.
Ask specifically how that junction is detailed. Sealant alone is a temporary answer.