How I hang stuff on old plaster and lath walls

Have finally been hanging a few things on the wall, and this is a note-to-self about how best to do that in our current home.

I’m more used to either plaster over masonry (UK) or more modern drywall and studs (US), but our apartment in an old Brooklyn brownstone is mainly plaster and wood lath over masonry. Very crumbly old plaster, surprisingly dense lath, and very little void space between the plaster+lath and the masonry. Hanging something is more art than science and requires a good understanding of what you’re working with.

The first thing to consider is the “skin” of an old plaster-and-wood-lath-over-masonry wall which behaves very differently from modern drywall. It’s made up of horizontally-arranged long, thin wood* slats (a.k.a. lath) screwed to vertical studs with thin gaps (a.k.a. keys) between each lath. The plaster is applied on top of the lath and pushed into the keys to give a smooth surface. The older the plaster, the more crumbly and brittle it’s likely to be. Beneath the plaster, the quality of the wood lath could vary significantly. The original builders would have used whatever cheap, long slats were easily available. This was usually softwood of some sort (pine, spruce, fir, etc.), but could also be hardwood. In my place, it seems to be softwood but I’m used to modern softwood, as in working with it is not a big deal unless you hit a hard knot or something. With old softwood, results may vary. Old wood often has a lot of resin since it is more likely to include more heartwood (the inner, older part of the tree) and more likely to be old growth (grown more slowly, so rings are closer together). More resin usually means more density since resin oxidizes and hardens over time. Moving on from the wall surface, the next consideration is the gap behind it before you hit masonry. There’s often not a lot of space to work with, and the gap varies. This puts a hefty constraint on the type of anchor you can use for heavier hangs.

When hanging things on these walls, the first two things to iron out are the qualities of the object you want to hang and the quality of the wall exactly where you want to hang it.

First, roughly figure out the load will you be putting on each fastening point. Remember: object weight ≠ load. The total load on any one fastener depends on the object’s weight, how that weight is distributed, and how far the center of gravity is from the wall. So let’s say you have a 60lbs mirror. A 60lbs mirror hung by two level D-rings will put a lot less load at each point than the same mirror on a wire and single hook. Along similar lines, a 60lbs cabinet will have very different load considerations since it juts out from the wall, to say nothing of the objects you might put inside it.

Next, based on your estimated fastener load, determine your most likely fastening method. For something hanging flush to a plaster and wood lath wall, my rule of thumb is:

• Under 2 lbs load: Picture hook
• 2-10 lbs load: Pilot hole and screw in to lath
• 10-30 lbs load: DuoPower wall anchors or a similar fastener that will spread the load behind the lath, rated to at least 3× the load
• 30+ lbs: French cleat or screw directly in to stud

This is relatively conservative, but keep in mind that many variables might incline me to be even more careful. Shelves or cabinets, TV mounts, anything that a kid might interact with, stuff above beds or couches, earthquake-prone areas, or particularly crumbly walls would all require more caution.

Once we have our fastener hypothesis, it’s time to investigate the wall. First consideration: Are you trying to drill in to a stud? Stud finders are pretty useless on plaster and lath, so you’ll have to be more intuitive. This Instructables article** has some pretty good ideas.

Once you’ve identified where you want to drill, put some painter’s tape over the spot (reduce plaster cracking) and a DIY envelope dust-catcher taped underneath it (reduce mess), then use a sharp multipurpose bit to make an investigatory pilot hole. Use the normal drill setting, no hammer mode. Take a look at the quality of the debris you pull out with the drill bit. Is it just plaster? You probably hit a key and need to move up or down by about ½ inch if you want lath. Did you get plaster and wood, then clearly hit a void? Fine if you’re ok with lath, not so good if you wanted a stud. Make small further pilot holes as necessary until you know exactly where you can hang your object.

So now you know where you can hang your object. Next it’s time to figure out how you can hang it. Picture hooks are straightforward. If you’re screwing in to lath or in to a stud, just make sure you do an appropriate pilot hole so you don’t crack the plaster and use wood screws that are long enough. For studs, that means at least 2½ inches long.

If you’re using a toggle bolt or similar anchor, you need to make sure there’s enough space for it to “spread” behind the lath. Stick a wire in to the hole until it stops against the masonry, then mark it at the surface of the wall and pull it out. If you subtract about 1 inch, that is roughly how much space is behind the plaster and lath. If you want to be exact, put a small 90° bend at the base of the wire and “hook” that to the reverse of the plaster + lath to find out how thick it is.

I really like SNAPTOGGLE anchors, but in my walls there’s rarely enough space to get the metal end all the way in and expanded, I end up struggling with it and chewing up the plaster. Molly bolts are generally out because they require a specific wall thickness. Toggler’s Alligator Anchors are pretty good, but I’m planning to try nylon DuoPower anchors the next time I need an expansion anchor. They should account for the variable wall thickness, should spread the load, and should be short enough to deal with narrow wall voids. According to Lea Valley, the 5mm × 25mm anchors come with 35mm (1⅜”) screws, the 6mm × 30mm anchors come with 40mm (1½”) screws, and the 8mm × 40mm anchors come with 60mm (2⅜”) screws.

So now you know what you’re hanging, where you’re hanging it, and (most likely) how you’re hanging it. Regardless of what method you’re using, cover your hole destination with painter’s tape to reduce plaster cracking, make a DIY dust catcher with a business envelope taped beneath the tread, then have at it. Don’t forget: sharp multipurpose bit, no hammer mode! For screws in to lath or studs, the pilot hole should be about the same as the solid shaft of the screw. For anchors, follow the directions on the anchor package. If you drill your pilot hole and find that one part of it seems “looser” than another side, you might be too close to a key. Move accordingly. When tapping in an anchor, be gentle so as not to crack the plaster. When screwing something in, don’t over tighten for the same reason.

Congratulations, you have hung your thing. Now try to forget the first-try second-try third-times-a-charm-oh-wait holes behind your mirror, or dig out your filler and paint.

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* You can also get gypsum lath strips from builds and renovations done in the 30s and 40s I believe, but I haven’t encountered that in our place.

** To summarize in case that article on finding studs in plaster and lath houses is ever taken down: Check near electrical boxes, if you find one stud then there may be another to the left or right about 16 or 24 inches away, knock gently on the wall, or use a magnet.