In Coconut Grove I watched a beautiful mahogany window surround go into a CBS wall with the jamb screwed straight to the block. Two rainy seasons later the back of that jamb was black. The joinery was perfect. The interface was not.
Masonry is a moisture reservoir
Concrete block, brick and stucco all hold water and release it slowly. That is fine for masonry. It is not fine for wood in direct contact with it, and in South Florida — where the wall may be wet for days after a storm and the ambient humidity rarely drops far — anything cellulose against a damp wall stays above the moisture content where decay fungi are active.
Pressure-treated lumber resists that. Untreated hardwood, even a durable species, does not indefinitely. And the finish carpentry people actually want to see is almost never treated.
So the rule is simple and it holds everywhere: no finish timber touches masonry directly. Something goes between.
The buck, and why it exists
A buck is the intermediate frame set into a masonry opening that the window, door or millwork then fastens to. There are two kinds.
Bucks cast or built into the wall — pressure-treated lumber anchored to the block with masonry screws, or a precast concrete buck. This is standard on impact-rated windows here, where the anchorage has to carry a real wind load into the structure.
Applied bucks set into a finished opening, shimmed and anchored, then flashed. More common on retrofit work.
Either way the buck is the sacrificial, replaceable, moisture-tolerant layer. Finish millwork lands on the buck.
The details that keep it dry
- A capillary break. A membrane, a bituminous strip, or a closed-cell gasket between masonry and any wood. Water moves between two materials in tight contact by capillary action even when neither is soaking wet.
- Shims that do not absorb. Composite or plastic shims at fastener points. Cedar shims wick.
- Back-priming. Every face of every piece of millwork, including the ends and the hidden back, sealed before installation. Water gets to the back; the back is where rot starts.
- A drainage path. A sloped sill, a drip at the head, and an air gap behind wide trim so anything that gets in can leave.
- Fasteners that survive. Stainless in this climate, not zinc-plated. Galvanic corrosion and salt air eat cheap screws in a few seasons and the trim comes loose.
Acclimatization and the schedule
Wood delivered from an air-conditioned shop to a house that is not yet closed in will pick up several percentage points of moisture content in a week and then shrink back once the AC runs. Joints open, panels cup, and everyone blames the joiner.
The sequence that works: masonry complete and dry, building closed and conditioned, millwork delivered and left in place for a week, then installed. On a project schedule that is an inconvenience. On a warranty claim it is the whole argument.
Typical costs: pressure-treated bucks $12 to $28 per linear foot installed, flashing and membrane $6 to $15 per opening, back-priming a small percentage of the millwork cost. The interface details are cheap relative to the joinery, which is exactly why it is worth asking whoever supplies custom millwork in Miami what they expect at the masonry junction and who is responsible for it — that gap between trades is where most of these failures live.
Frequently asked questions
Can I fasten trim directly to block with masonry screws? Structurally yes, and it is common. The problem is not the fastening, it is the contact. If the trim has to sit on masonry, put a membrane or gasket between them and back-prime the wood, or use a treated buck as the intermediate layer.
Is PVC or composite trim a better answer in this climate? For exterior trim in contact with masonry, often yes — it does not rot and it does not need the same back-priming discipline. It moves more with temperature than wood does, so fastening and expansion gaps have to be detailed for it.
How do I know if existing millwork is rotting from behind? Probe with a small awl at the bottom corners and where trim meets the wall. Sound wood resists; punky wood takes the point with light pressure. Peeling paint in a localized patch, particularly at a lower corner, is the usual first visible sign.