26 June, 2011

Hanging Stuff 0n Plaster Walls

Lath-and-plaster walls are common in houses constructed from the late 18th Century until the early 1950’s when drywall became the common building technique.  Whether it’s that oil portrait of Great-great Aunt Sadie or kitchen cabinets, hanging things onto older lath-and-plaster walls can present a bit of a dilemma.  So, if you don’t want to see old Sadie crashing down onto the piano in the parlor, you’ll need to learn how to attach things to the walls. 

First, let’s describe how lath-and-plaster walls are constructed.  Once the house was framed (generally with the studs at 16” on centers); the exterior sheathing, siding completed, and the doors, windows and mechanicals (plumbing, heating, electrical) installed, the plasterers would start covering the walls with wood strips called ‘lath’.  The lath would be about 3/8” thick, 1-1/2” wide and 48” long with a gap of 3/8” between them.  A thick layer of gypsum-based coarse plaster (gray or brown colored) was troweled onto the lath until it oozed through forming ‘keys’ that held the plaster to the lath.  A thin layer of white finish plaster was applied once the coarse layer had cured.

To attach lightweight items to the walls, drive a nail or screw into the wall.  If the nail goes through easily, pull it out and go up or down a 1/2” and try again to get into the lath.  But the hard, fast rule of hanging anything on a plaster wall--if it's more than a pound-or-so, find a stud and screw or nail your item to that.  Also, it’s best to attach switch and outlet workboxes directly to the studs so they won’t become loose and move about as you try to use them.  That all said, how do you find the studs?  Well, some of the old-timers used a powerful magnet to locate the lath nails (My grandfather kept a small horse-shoe shaped magnet inside the lid of his metal tool box just for that job).  Gramp would wrap a piece of wax paper around the magnet to keep it from marring the wall finish.  He would sweep the magnet horizontally across the wall until he felt the tug on the magnet over the nail. Then he would slide it vertically to see he could tell if there were more nails indicating the stud.
Many years ago, tool manufacturers came up with a swiveling magnetic stud finder in a clear plastic vial. This finder locates the lath nail in the plaster. Keeping with my family tradition, I keep some powerful rare-earth magnets inside the lid of my tool box that a friend had salvaged from some old computer hard-drives. 

In this our semiconductor era, someone had invented an electronic stud finder.  They have been on the market for quite a long time and they usually work well on drywall, though sometimes, they have a problem locating a stud in a plaster wall. I have a few different brands, but all of them measure the density of the wall to locate the stud. The uneven distribution of plaster behind the lath can give you false readings. So, I continue use my magnet (as Gramp did) to find the nails.