Most cracked plaster in an older Philadelphia house can be repaired, and repair is usually less work and less mess than tearing it out for drywall. Hairline and seasonal cracks can be filled and taped. Plaster that has come loose from the lath can often be screwed back with plaster washers. Holes can be patched over metal lath. Tearing out a wall makes sense mainly when the plaster is rotted by long-term moisture, crumbling, or bowed across most of the wall. Before anyone opens a wall, plan for two things: old paint on the plaster may contain lead, and the plaster itself or old patching compound may contain asbestos.
This is general information, not an inspection. The most detailed guide to old plaster is the National Park Service's Preservation Brief 21, Repairing Historic Flat Plaster: Walls and Ceilings. Much of what follows comes from it.
Why so many Philadelphia walls are plaster
According to the Park Service brief, plaster was the wall surface chosen for nearly all buildings until the 1930s or 1940s. That covers a large share of the city's rowhomes and twins and many older houses in the suburbs. Until about 1900, plasterers used lime plaster, often with animal hair as a binder. After that, gypsum plaster took over. Behind the plaster you'll usually find one of three bases:
- Wood lath: thin wooden strips with gaps. Wet plaster squeezed through the gaps and hardened into "keys" that hold the wall on.
- Metal lath: expanded steel mesh, which grips plaster better than wood.
- Rock lath: paper-faced gypsum panels, in use from about 1900. By the late 1930s, the brief says, rock lath was used almost exclusively in residential plastering.
On brick walls, including rowhome party walls, older lime plaster was often applied straight onto the masonry with no lath at all. Knowing which you have matters, because it changes the repair.
Read the crack before you fix it
The Park Service brief sorts plaster problems by cause. The shape and location of a crack tell you a lot:
- Hairline cracks in an otherwise solid wall are cosmetic.
- Horizontal cracks that come back every year often come from the wood lath moving. Wood lath absorbs moisture from the air, so it swells in humid Philadelphia summers and shrinks once the heat is on. That's why some cracks open up again every heating season.
- Diagonal cracks starting at the corner of a door or window are stress cracks. In old houses they often trace back to an opening cut in later without a proper header, or to framing that has sagged over time.
- Diagonal cracks running in opposite directions, or cracks that keep getting wider, can point to settlement or a structural problem.
- Bulging or bowed plaster, or plaster that moves when you press it, has lost its keys and come loose from the lath. On a ceiling, this is the one to deal with soon. If it isn't repaired, the brief warns, gravity will eventually bring chunks down.
- Bubbling, powdery or salty white deposits, usually on a brick wall, mean water is coming through the masonry.
Water is the cause you have to fix first. The brief lists the usual culprits: plumbing leaks, roof leaks, gutters and downspouts dumping water next to the foundation, dampness wicking up from the foundation, and rain splashing up off paving onto a brick wall. Repairs made over a wet base will fail again, and coatings on the inside won't stop water coming through the wall. If the damage is under a flat roof or a bathroom, find the leak before you patch. Our guide to flat roof maintenance covers the roof side.
Repairs that usually save the wall
Filling and taping cracks
According to the brief, fine cracks can be filled with all-purpose joint compound. For cracks that reopen with seasonal humidity changes, the crack is widened slightly with a pointed tool first and then filled. For stubborn cracks, fiberglass mesh tape is pressed into a setting-type (quick-setting) joint compound. A second coat covers the tape, a third evens out the surface, and then it gets a light sanding. Setting compounds are hard to sand, so the brief recommends all-purpose compound for the final coat.
Reattaching loose plaster
If plaster has pulled away from the lath but the area around it is sound, it can be screwed back to the lath with flat-head wood screws and plaster washers (thin perforated discs). Then the washers and cracks are taped and skim-coated. This is the standard fix for a sagging ceiling that's still in one piece.
Patching holes
The brief recommends patching small holes in two coats rather than one, because one-coat patches tend to dry concave and show through paint. For larger holes that go down to the wood lath, the loose material is cleaned out and the lath re-nailed. Then expanded metal lath is fastened over the wood lath, and the plaster goes on in layers, each one stepped to overlap the old plaster.
Covering instead of removing
When a ceiling is too cracked to patch but isn't water-damaged, the brief describes plasterers keeping it and building new lath and plaster over it on furring strips, which makes much less dust than demolition. Laminating new drywall over a sound old wall or ceiling works the same way. Either way, trim, outlets and casings have to be adjusted for the extra thickness.
When removal makes sense, and drywall vs veneer plaster
Partial or full removal is usually justified when:
- long-term moisture has left the plaster soft, crumbling or rotted;
- the base coat is so weak that it disintegrates (the brief shows an example of over-sanded plaster that "will most likely need to be totally removed");
- most of a wall or ceiling has lost its keys;
- the wall is being opened anyway for new wiring, plumbing or insulation.
Once it's out, you have three main ways to put a surface back:
- Traditional plaster on new metal lath, rock lath or the old wood lath. It's the most durable and the closest match to the rest of the house, and it's the most skilled and the slowest to install.
- Veneer plaster: drywall-sized gypsum "blueboard" panels covered with one or two thin coats of hard plaster. The brief describes it as less expensive than two- or three-coat plaster and only slightly more expensive than drywall. It's ready to paint almost immediately and looks more like traditional plaster than drywall does.
- Drywall: the most widely available option. Most crews install it, and patching it later is easy. It doesn't match the hardness or sound-deadening of three-coat plaster. Because it's thinner than old plaster on wood lath, it can leave door and window trim sticking out more than before unless the installer shims or builds it out. The brief calls this the trim "reveal."
On a brick party wall, new drywall or veneer board usually goes on furring strips or metal channels. That leaves room for insulation on an exterior wall, which the brief notes can keep the new wall about as thick as the old plaster.
Lead and asbestos: the part not to skip
Lead. If the house was built before 1978, assume the paint on the plaster contains lead until a certified tester shows otherwise. Under the federal Renovation, Repair and Painting rule, a paid contractor who disturbs more than 6 square feet of painted surface per room generally has to be an EPA-certified firm and follow lead-safe work practices. The small-job exemption doesn't apply at all when the work involves demolishing painted surfaces, which is what tearing out plaster is. Our post on lead paint in older Philadelphia homes explains what that means in practice and what to ask a contractor.
Asbestos. The Park Service brief warns that asbestos, used in the mid-twentieth century as an insulating and fireproofing additive, may be present in plaster. Older patching compounds are a second source: in 1977 the U.S. Consumer Product Safety Commission moved to ban patching compounds containing asbestos because fibers can be released during mixing and sanding. EPA's homeowner guidance on asbestos says you can't tell whether a material contains asbestos just by looking at it, and recommends leaving material in good condition alone. A trained, accredited professional should take samples for lab analysis. If you're planning to remove plaster, or to sand old patched areas heavily, test first.
The brief also recommends respirators and protective clothing for anyone removing plaster, and prying it off rather than pounding, so the walls in the next room don't crack.
Permits in Philadelphia
L&I's building permit page says that, as long as a property isn't on the Philadelphia Historic Register, you often don't need a permit for repairs that are regular maintenance, or for interior finish work. That includes painting, papering and similar finishes, and, in one- and two-family homes, paneling or gypsum wallboard installed over existing wall surfaces. The same page says major repairs that aren't regular maintenance and work that changes the interior of a structure need a permit. Moving or removing walls, structural fixes and party wall work fall in that group. Any new wiring or plumbing done while the walls are open has its own permit rules (see our post on old wiring in Philadelphia homes).
Patching cracks and reattaching plaster is typically maintenance. A full gut to the studs with new wiring is not. If you're unsure where your project falls, or the house is on the historic register, check with L&I before work starts. In Bucks and Montgomery counties, permits are usually handled by the township or borough, so check with yours.
A homeowner's checklist
- Note where each crack is, which way it runs, and whether it's new or comes back every season. Take dated photos.
- Press gently on cracked areas, especially ceilings. Movement or a hollow, crunchy feel means the plaster has lost its keys.
- Look for water: stains, bubbling, white salt deposits, and the roof, gutter, bathroom or plumbing above or outside the damage.
- Measure any crack that worries you, and check again in a few weeks. A crack that keeps growing calls for a structural look.
- Find out when the house was built (property.phila.gov lists a year built). If it's before 1978, treat the paint as lead paint.
- Before demolition or heavy sanding, have suspect plaster or compound tested for asbestos.
- Decide what you want: a repair that keeps the plaster, a layer of drywall over it, or removal and replacement.
- Check permit needs with L&I or your township.
When to call a pro
Small hairline cracks and a hole or two are fair DIY projects. Call a plaster or drywall pro when a ceiling is sagging or moves when pressed, when plaster has come loose over a large area, when there's water damage, when you want a patch that disappears into old plaster, or when the job means removing plaster in a pre-1978 house. Bring in a structural engineer if a crack keeps widening, if cracks cluster around a sagging floor or a doorway that's out of square, or if a party wall is involved.
When you ask for estimates, ask each pro whether they'd repair, laminate over or replace, and why. Ask how they'll handle the trim reveal, whether they'll use drywall or veneer plaster, how they'll contain lead dust, and whether they'll test for asbestos first. Our guide on how to check a contractor in Philadelphia covers the state registration and the written contract.
On HomeKept you can describe the job once, with photos of the cracks and a note on when the house was built, and compare up to three estimates from local drywall and plaster pros in Philadelphia. It's free for homeowners.
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