How do I pick the right screws for my wall?
July 31, 2007 6:59 AM   Subscribe

How do I tell what type of wall I have?

This is so elementary, I'm sure, for the handy people on AskMe. I want to hang up IKEA shelves which do not come with any sort of screws. They recommend me to use the correct type for my wall. I rent; I have no idea what my wall is made of. How can I tell? Can I tell after I start drilling holes? Also how long do the screws need to be? Thank you!
posted by pinksoftsoap to Home & Garden (7 answers total) 4 users marked this as a favorite
How old is your building? Even a rough guess can narrow it down.

Older buildings are generally going to be plaster.
New builds are often drywall.

I don't know as much about drywall, but plaster is hard as a freaking rock. Wrong screw and you won't be able to get it into the wall, it'll just spin in the starter hole.
posted by Kellydamnit at 7:11 AM on July 31, 2007

Drill a hole in your wall, about 1/16th of an inch that will eventually covered by your shelves. If there is a hollow space after about a 1/4", and the wall itself is chalky and covered by paper in the back, that's drywall. You will want to use toggle bolts, maybe wall anchors if there will not be alot of weight on the shelves. If you can space the shelf supports such that they are hitting studs, you may not need to use any kind of special wall support at all. You can find studs with the use of a studfinder which generally will use magnets to find the screws used to mount the drywall to the framing.

If you have plaster walls, you'll probably want to use the wall anchors, which involve drilling a larger hole than needed for the screw, tapping in an insert, and then screwing into the insert. There are directions on the package for drill bit size.
posted by Roger Dodger at 7:13 AM on July 31, 2007

You need to find out where your studs are too, so you can anchor the screws (molly or toggle bolts are what you should get) into them. You can't just expect drywall or plaster to hold up a shelf with things on it. So get or borrow a studfinder.

Drywall (also called gypsum board and wallboard and sheetrock) and plaster and the two most common types of wall. If the shelf is going in your bathroom, it probably has greenboard (also called limewall) in it, which is basically moisture-resistant drywall (unless it's really old and has never been renovated). Plaster is very rarely used these days, and most newish constructions is drywall. Our walls are plaster, and when you drive a screw or nail in you have to put some tape on the wall first, to prevent chips and chunks of wall from falling off; our house is 100 years old and the walls are a little brittle, but hard as a rock. With drywall, which is basically prefab sheets of wall made of a sandwich of paper, gypsum and paper, you don't need to tape. If where you live was built after the 70s you most likely have drywall.

If you knock on drywall and plaster you'll get back two noticably different sounds, but it's hard to describe. Why not call the people you rent from and ask to be sure though?
posted by iconomy at 7:24 AM on July 31, 2007

Do you rent an apartment? If so, you may not even have hollow walls as the previous posters have described. However, solid walls (plaster over brick) are treated the same as plater-over-lathe walls: get plastic anchors. You drill a hole, put in the anchor, the screw goes in the anchor. Don't tighten it too much or you'll just strip the plastic in the anchor. Just hand-tight.
posted by GuyZero at 8:00 AM on July 31, 2007

Drywall is about 3/4" of plaster held together with paper on both sides. It's hung off of studs (lengths of 2 x 4 lumber running vertically) that are spaced every 16" (in some interior walls it's every 24"). That means that for the majority of the wall the wall is just 3/4" thick. If you screw something to that 3/4" of plaster and put any weight on it it's liable to pull down a great big chunk of drywall.

Hollow wall anchors, which spread the weight from the screw, can help, but I only use them for small stuff -- it makes me too nervous to have bookshelves resting on them. You want to get a studfinder, which tell you when you're driving screws into studs and not drywall. Or you can just learn to tap with a knuckle, slowly, horizontally along a wall. You'll quickly learn to hear when it sounds hollow and when you're tapping over a stud. Or, as suggested above, you can drill. You'll definitely know when you've drilled into a hollow cavity vs. drilling into a stud.

As noted by others, if you live in an old house you might not have drywall -- you'll have lath-and-plaster, in which case finding the stud is less critical.

As for screws, I recommend — the irony! — drywall screws for general use. They're self-drilling and strong, although not usually beautiful. I also like the screws sold by Simpson to go with their Strong-Ties. Find the Simpson Strong-Ties at the hardware store and you'll find a small pack of them. They have flat heads, they're self-drilling and they're also very strong. You can't go too wrong with either kind of screw.
posted by argybarg at 8:10 AM on July 31, 2007

As for screws, I recommend — the irony! — drywall screws for general use.

The black ones? No way. They're designed to be low strength so that the heads pop off if you over torque them. They also have very poor sheer strength-they'll snap if hit on the side hard enough. Also, most drywall screws use philips/cross heads.

A much better screw type for general use that has the same narrow profile and aggressive screw is the deck screw. They're made to be much stronger than the black drywall screws, are much less likely to rust and can be removed much more easily than drywall screws (whose heads tend to shear off rather than backing out). Also, deck screws are widely available with Robertson/square-drive heads, a much better screw drive system.

Sorry for the rant, but that needed to be said. Drywall screws are great for hanging drywall, but they absolutely should not be used for places where screw strength, particularly shear strength matters.
posted by bonehead at 8:29 AM on July 31, 2007 [1 favorite]

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