The quick answer
Prehung and slab are the two ways a replacement door is sold. Understanding the difference takes about two minutes. Picking the wrong one costs you a callback, a leaking threshold, or a door that won't close right in January.
What a prehung door is
A prehung door comes as a complete unit: the door slab, the frame (two side jambs and a head jamb), and usually the hinges already mortised in. You're buying the whole assembly. When we install a prehung door, we remove the existing frame down to the rough opening, set the new unit in, shim and level it, and build out the interior trim to match.
The advantage is that everything is factory-fitted and squared before it arrives on your job site. The door and the frame were made for each other. You know the reveal is even and the door swings true before the unit ever goes in the opening.
The disadvantage is labor. Removing and disposing of the existing frame takes time, shimming and leveling a prehung unit in an out-of-plumb rough opening takes skill, and the interior trim almost always needs to be rebuilt or matched. A prehung exterior door install typically runs 3–5 hours on a straightforward job.
What a slab door is
A slab door is just the door panel — no frame, no hinges attached, no hardware. You're swapping out the door itself while keeping the existing frame. The hinges are transferred from the old door or mortised fresh into the new slab. The same frame stays in the opening.
Slab installs are faster and cheaper when the existing frame is in good shape. A clean slab swap on an interior door takes under two hours. On an exterior door it takes longer because the threshold, weatherstripping, and hardware need to be re-set — but you're still well under the labor time of a prehung unit.
The risk is that a slab install lives or dies by the condition of the existing frame. If the frame is out of square, the new door will show the same binding or gap the old door had. If the frame is soft from moisture, the new door will have the same threshold leak before long.
Why Long Island homes lean toward prehung
Most of the homes we work on were built between 1950 and 1985. That's 40 to 75 years of salt air, freeze-thaw cycling, and — on the South Shore — ground moisture from the water table sitting high. The frames on those homes show it.
We inspect the existing frame before quoting any exterior door job. More than half the time, what looks like a door problem from inside is actually a frame problem. Signs we look for:
- Soft wood at the sill or threshold — press your thumb into it; if it gives, the frame has rot
- Paint bridging the door to the frame — usually means the door has swollen and someone painted it shut rather than fixing the real cause
- Daylight visible around the weatherstripping — often means the frame has racked
- More than 3/16 inch reveal variation — uneven gap around the door panel usually means the frame has moved
When we find any of these, we recommend prehung. Installing a new slab in a bad frame is a short-term fix that comes back as a callback in 12–18 months. The frame causes the leak, the binding, or the draft — not the door.
When slab makes sense
Slab is the right choice when the existing frame is plumb, square, dry, and solid. That usually means one of three scenarios:
Interior door replacements. Interior frames are not exposed to moisture or temperature swings. If the rough opening is square and the existing jamb is in good shape, a slab swap is almost always the right call. It's faster, cleaner, and there's nothing to gain by pulling a perfectly good frame.
Recent construction. If the home was built after 2000 and the frame has never been wet, a slab swap is reasonable for an exterior door replacement. We'd still inspect the frame first.
Budget constraints with a healthy frame. If money is a hard limit and the frame inspection comes back clean, a slab install stretches the budget. We'll say so — we're not in the business of upselling prehung units when the frame doesn't need it.
What the price difference actually looks like
On Long Island, the realistic gap between a slab and a prehung install looks like this for a standard 3/0 x 6/8 exterior door:
| Job type | Typical installed cost (Nassau/Suffolk) |
|---|---|
| Slab swap, existing frame in good condition | $800–$1,400 |
| Prehung unit, standard rough opening | $1,400–$2,600 |
| Prehung unit with frame rot repair | $1,800–$3,200 |
The gap narrows when you factor in that a slab install on a marginal frame often needs to be redone within a few years. We've replaced dozens of doors that a previous contractor installed as a slab swap when the frame needed to come out. Those homeowners paid twice.
The cost of a prehung unit includes the new frame, which comes with a factory warranty and is sealed against moisture before installation. On a coastal Long Island home, that's not an upgrade — it's the baseline.
What we check before we quote
Before we quote any exterior door job, we do a frame inspection. It takes about 15 minutes and it determines whether we're recommending a slab or a prehung unit. We don't lead with prehung to pad the invoice — on interior doors or newer homes with solid frames, we quote slab every time.
What we check: sill condition (moisture probe if the paint looks suspect), jamb plumb (we set a level against both sides), head jamb levelness, and reveal consistency around the existing door panel. If everything passes, you get a slab quote. If anything fails, you get a prehung quote with a clear explanation of what we found and why it matters.
If you're trying to figure out which situation you're in before calling, look at the bottom of your door on a cold or rainy day. Drafts at the threshold, visible daylight, or water staining on the sill plate are the three signs that point toward prehung — and toward fixing the problem correctly the first time.
Still have questions?
This guide was written by James Caruso. If your situation has a wrinkle we did not cover, call us direct. Most questions we answer by phone take five minutes.