A Huntington client sent me photos of the three-season room a previous contractor had built for her: a nice job, sitting on a slab poured on grade, with a two-inch step now opening between the room and the house. Nothing about the room was wrong. It had simply been built on something that moves in winter, attached to something that does not.
Frost heave, briefly
Water in the soil freezes and expands, and in silty soils it does more than that — it draws additional water up to the freezing front and forms ice lenses that can lift a structure by inches. The lift is rarely uniform, so what you get is racking, not a clean rise.
On Long Island the code frost depth is generally 36 inches. A footing bearing below that is in ground that does not freeze in a normal winter. A slab on grade or a shallow pier is not.
The house next to it is on a full foundation, well below frost. So every winter the addition moves and the house does not, and the joint between them takes all of it.
The three ways to support a sunroom
Full frost-depth foundation. A continuous footing and stem wall to 36 inches or more, exactly like the house. The most money and the fewest problems. $130 to $260 per linear foot of wall in this market.
Concrete piers or helical piles to below frost. A framed floor spans between them. Good on sloping sites and where excavation is awkward, and much less concrete. $400 to $1,200 per pier, typically 6 to 12 of them; helicals run $600 to $1,800 each installed.
Floating slab, deliberately detached. Legitimate for an unheated structure if — and only if — it is designed to move as a unit and is not rigidly attached to the house. The connection becomes a flashed, flexible joint rather than a bolted ledger. Cheapest at $9 to $16 per square foot, and the option most often done wrong.
The failure I photographed for that client was the third option built as though it were the first: a floating slab with the room bolted to the house wall.
Details that decide the outcome
- Bear on undisturbed soil. A footing on backfill from the original house excavation will settle regardless of depth. If you hit fill, go through it or specify a compacted engineered fill.
- Drain around it. Reducing water in the soil reduces heave. A footing drain and gutters that discharge away are worth more than an extra six inches of depth.
- Insulate a slab edge. On a heated room, two inches of rigid insulation at the slab perimeter cuts both heat loss and frost penetration under the edge.
- Detail the connection honestly. Either the addition is on the same foundation system as the house and can be tied rigidly, or it is not and needs a movement joint with proper flashing above it.
Where owners get caught is that the cheap foundation and the expensive foundation look identical once the room is finished. The difference shows up in year three. Anyone quoting Long Island sunrooms should be able to say in one sentence which of the three systems they are proposing and how the room attaches to the house — and if the answer is vague, that is the question to keep asking.
Frequently asked questions
Can a three-season room go on an existing deck? Sometimes, but deck footings are often shallower and lighter than a room requires, and deck framing is designed for a live load, not for walls and glass. It needs an engineer’s review, and upgrading footings under an existing deck is disruptive enough that rebuilding is often cheaper.
Does an unheated room still need frost-depth footings? Under most codes, yes for a permanent attached structure, unless it is specifically designed as a frost-protected shallow foundation or a detached floating slab. The exemptions are narrower than people assume — check with the building department before pouring.
My sunroom has already heaved. Can it be fixed? Usually. Helical piles can be installed under an existing slab or beam and the structure jacked back to level, at $1,000 to $2,500 per pile. It is not cheap, but it is far less than demolition, and it addresses the actual cause rather than repatching the joint every spring.