Second Story Addition on a Block House: What the Stucco Seam Needs

A frame second story on a one-story block house in St. Johns County needs a planned break in the stucco at the floor line, with flashing that drains out. What goes there and why a smooth coat across it fails.

When you put a second story addition on a block house, your new top floor is almost always wood frame with stucco over it. The floor below it is your old stucco over concrete block. Those two walls on your house move in different ways and handle water in different ways. So the stucco seam where they meet on your house needs a planned break, not one smooth coat across it. Your break is a joint or trim line at the floor line. It sits over a flashing that drains water out, with a weep at the bottom of the frame stucco. If you skim stucco straight across that line, nothing in it can take the movement.

Why do a block first floor and a frame second floor move differently?

Your block wall and your new frame wall are two different kinds of wall, even when both wear stucco. On your block floor, stucco goes straight onto the masonry. The Stucco Manufacturers Association calls clean concrete block like yours an excellent base. It says your stucco on block is a barrier system, where drainage is not part of the design. That is why the block wall on your first floor is not required to have a weep screed at its base.

The frame wall on your new floor works the other way. Your house would be the most common two-story type in central Florida, according to a 2005 Building Science Corporation report on Orlando-area homes after the 2004 hurricanes. That type is stucco over a frame second floor, on top of stucco over a block first floor. The report calls your kind of frame wall a drained wall. Your frame wall holds very little water, so it has to dry at least as fast as it gets wet. Your upper wall only dries that way if there is a drainage space behind the stucco and the water can get out at the bottom of the frame wall.

Your materials also move. The Masonry Advisory Council says the concrete block in your first floor shrinks as it loses water. A Sika technical bulletin on stucco cracks says the wood framing in your new floor twists and warps as it dries, and that your wood sheathing has to be gapped. Your new floor and your old floor will not move as one piece.

What goes behind the stucco on the new frame floor?

On your frame floor, the stucco sits on metal lath over a water barrier, not on the wood. Under the 2023 Florida Building Code, Residential, section R703.7.3, your wood sheathing needs a water-resistive barrier over it. Your barrier must equal at least two layers of Grade D paper. An exception lets you use one 60-minute Grade D layer with a non-absorbing layer or a designed drainage space. The same code asks for at least three coats of stucco over the lath on your new floor. Your block floor, where stucco goes directly on masonry, needs at least two coats.

The Building Science Corporation report adds two points that matter for your upper wall. Your wall needs a gap or drainage path between the stucco and the barrier. Paper-backed lath over your barrier acts as a bond break, so your stucco does not stick to the paper.

What does the seam between the block and the frame need?

Your seam needs a joint made for movement, and a way for water to drain out. ASTM C926, the national standard for how stucco is applied, covers your exact case: two different bases that meet and take one coat of stucco. For your seam it calls for an expansion joint: a two-piece joint, two casing beads set back to back, or a factory-made joint. The other choice it gives you is a 6-inch strip of self-furring lath that laps 3 inches onto each side.

Sika’s data sheet for stucco trim names the floor line of wood frame walls as a typical spot for an expansion joint. It names any place the base changes, too, which is exactly what happens at your floor line. For your seam, it calls for a zinc expansion joint or a similar trim piece. Sika also says these trim pieces are not flashing. On your house they must be fastened to the structure, and your design professional picks them and draws the detail.

Water needs its own path at the same line. ASTM C926 asks for a drip screed where your upper wall sits on a floor or foundation. That screed comes with through-wall flashing or weep holes, so water that gets behind your stucco can drain away. Once your house is more than one story tall, the standard also asks for a 3/8-inch gap between the stucco and the edge of a floor slab.

What did the Florida research find going wrong at the floor line?

The 2005 Building Science Corporation report found that water was often not getting out at the bottom of second-floor frame walls on houses like yours. Part of the cause was one detail at the same joint you are about to build. Code officials at the time were enforcing a rule from ASTM C1063 at this joint that was not meant for it. So builders ran the building paper down over the joint as a “bridge flashing,” then set the control joint trim on top of it. That made a dam. It sent water inward, and homes leaked in wind-driven rain. You do not want that detail on your house.

The fix the report shows for your seam is a flashing between the floors that drains water to the outside. The water barrier from your upper floor laps over the top of that flashing. Water running down the back of your upper wall lands on the flashing and leaves the wall.

Keep the date in mind as you read it. Your house is in St. Johns County, but the report is from 2005, it looked at Orlando-area homes, and it was written under the Florida code before 2007. The principle still holds for your house: water behind the upper wall has to drain out at the floor line, not get trapped there. The exact detail on your house is set by the current Florida code and by the inspector who signs off on your permit.

Can you stucco straight across the seam so it looks like one wall?

No. A smooth coat across your floor line has nothing in it to take the movement between the block and the frame. If you ask for just a groove or cut in your stucco, ASTM C926 says that alone is not a control joint or an expansion joint. A Walls & Ceilings magazine column makes the same point: a control joint cut with a trowel cannot bridge the structural break below your seam. Without a real joint, your seam is likely to show as a line or a crack over time.

The break does not have to look like an afterthought on your house. You can have it done as a joint, a band, or a trim line that runs level around your house at the floor line.

Do the walls on the new floor need their own joints?

Yes, the frame walls on your new floor usually need control joints of their own. Sika’s trim data sheet recommends a control joint at least every 144 square feet of your wall. It says no stretch of your stucco should run longer than 18 feet in either direction. And no panel on your wall should be more than two and a half times as long as it is wide. The joint at your floor line also counts as one of the lines that divides those panels. Sika says joint layout is the architect’s or engineer’s call, so you should expect it on the drawings rather than decided on the wall.

Will the new stucco on the second floor match the first floor?

New stucco next to your old stucco will not match on day one, even with the same texture. ASTM C926 says the color of your new stucco cannot be guaranteed by the maker or the applicator. Thickness, mixing, curing and how much your base soaks up all change the shade. If your old finish was troweled by hand, the standard says a different crew will find it hard to copy. It suggests you keep a sample panel on site to compare against.

Whether your whole house gets a new coat or paint at the end, or only the new floor, is a choice you should settle in writing before the job starts. The seam joint helps you here too. A level line at your floor line gives the eye a clean place for one finish to stop and another to start.

Can a one-story block house carry a second floor at all?

A structural engineer decides whether your house can carry a second floor, not the stucco crew and not a builder’s guess. Building up is mostly a question of whether your walls, footings and connections below can take the new load. The engineer looks at those, and your design and permit follow from what the engineer finds. If you are still deciding between going up and going out, building out vs. building up walks through how your lot and your house settle that choice.

Who checks the seam before the stucco goes on?

Your permit runs through St. Johns County or through the city your house sits in. That office’s inspector decides which floor-line piece your house gets: a weep screed, a drip, or a flashing. Confirm it with the office for your address. Before the stucco goes on, you can ask your builder for photos of the flashing, the water barrier lap and the joint at the floor line. Once the stucco covers them, you can never see them again.

When is this not a job for Wilson & Co?

If your stucco is cracked or needs repair and you are not adding on, you need a stucco repair company, not an addition builder. For that, call Stucco Home Repair, which Jay Wilson also owns; it handles cracks and repairs on houses with no addition project. If you only want an engineer’s report on whether your block house can take a second floor, with no build behind it yet, hire a structural engineer directly.

Planning a second story on a block house in St. Johns County

If you are planning to add a floor, put the seam in your first design talk, along with the engineering. That way your joint, flashing and finish are drawn before anyone orders lath. The home additions page lays out how your addition would run, from survey to final inspection. For outside finish choices on a coastal house, see siding and cladding choices for coastal Northeast Florida. For how long the work takes, see how long a home addition takes in Florida. You can call Jay Wilson at (904) 792-6175 to talk through your house.