HOW MASONRY ANCHORS WORK

• There are three basic types of masonry anchors: 1) mechanical, 2) powder-actuated and 3) chemical. It's unlikely that you'll need a chemical anchor unless you're building a bridge, and in many states, certification is required to use powder-actuated fasteners. This document describes how to use mechanical anchors.
  
  
• Products do vary from one manufacturer to the next; if the manufacturer's installation instructions are different than the information in this document, always follow the manufacturer's instructions. One caution when installing masonry anchors–always wear eye protection and follow the manufacturer's safety instructions.
  
  
• Most masonry anchors work in one of two ways–either by expanding against the sides of the hole and gripping the concrete (Fig. 1) or by friction against the sides of the hole (Fig. 2). The holding power of any anchor depends on the quality of the concrete and on where the anchor is placed. If the concrete is old and crumbly, the holding power of the fastener will be reduced.
  
  
• Likewise, if the anchor is placed near the edge of the concrete–or two anchors are placed too close together–the force generated by the anchor may break the concrete. The general recommendation is that any anchor should be placed no closer than five diameters from the edge of the concrete. In other words, a 1/2" diameter anchor should be no closer than 2-1/2" (1/2" x 5) from the edge of the concrete.
  
  
• Two anchors should never be placed closer than 10 diameters from each other. In other words, two 1/2" anchors should be at least 5" (1/2" x 10) apart.
  
  
• The most important factor in choosing the proper anchor is the type of load it will carry. An anchor is rated for two types of loads. Shear loads are caused when the weight of the fixture exerts force parallel to the surface of the concrete (Fig. 3). Tensile loads are caused when the fixture exerts force perpendicular to the surface (Fig. 4).
  
  
• Naturally, a load may exert a combination of the two. An anchor that carries a pipe hung from the ceiling is under a tensile load, while a mirror hanging from a basement wall exerts almost a purely shear load. A shelf hung from the same wall exerts a combination of the two forces, as does a floor-mounted anchor that holds a table saw in place in the garage.
  
  
• Other load factors are also important. They include:
  
  
• Static loads are loads that are steady and constant–for example, the weight of the circuit breaker box fastened to your basement wall. They place the least stress on the anchors.
  
  
• Dynamic or vibrating loads are loads that are constantly changing. For example, if you hang a shelf from a concrete wall in order to mount your electric grinder, the load on the anchors will be a dynamic load. The strength rating of the anchors will be reduced by the constant vibration of the load.
  
  
• Impact loads are loads that change suddenly–such as a box tossed onto a shelf anchored to the basement wall.
  
  
• Because of the inexact quality of concrete, the standard recommendation is that the anchor you choose should be rated for about four times the weight it will carry if it will bear a static load and eight times the weight if it will carry a dynamic or impact load.

USING MASONRY ANCHORS

• When choosing anchors, remember that the total load should be divided by the number of anchors that will carry it.
  
  
• Whatever type of anchor you decide to use, you'll probably want to rent a hammer drill to drill the holes for it. Masonry drill bits work by chipping concrete away (as opposed to wood bits, which cut wood away). If you use a standard electric drill, you'll find that it not only drills much more slowly, but you'll be much more likely to ream the sides of the hole and wind up with a hole much larger than you intended.
  
  
• The holes have to be exact in diameter and sometimes even an exact depth in order for the anchor to work properly. Some manufacturers' anchors must be installed with special drill bits. For best results with masonry anchors, it is important to "blow out" any excess dust from the drilled holes. One of the easiest ways to do this is with a kitchen blaster.
  
  
• This document covers four basic types of masonry anchors: 1) concrete screws, 2) hammer anchors, 3) one-piece expansion anchors and 4) two-step expansion anchors that are used with standard screws.
  
  
• They are called "two-step" because the holes must be spotted before installation. In other words, the anchor is larger than the fastener that will go into it (Fig. 5). As a result, you'll have to position the material to be anchored and spot the locations of the holes, then set the material aside so you can drill holes and insert the anchors. Finally, you can place the material in position again and fasten it down.
  
  
• One advantage to these anchors is that the screws can be removed and reinserted. Also, they are relatively inexpensive.
  
  
• If you're using lag shields (Fig. 6), you'll have a choice between short or long shields. Use the short shields in hard masonry (usually older concrete) or the long shields in softer masonry. Long shields are generally about 30 percent stronger than short shields, but drilling through old, hard concrete is not an easy task.
  
  
• To install a two-step anchor, drill a hole the specified diameter and depth (usually slightly deeper than the length of the anchor). You can mark the depth on your drill bit by measuring the length of the anchor, then wrapping a piece of tape around the bit at that point. Insert the anchor, tapping it lightly with a hammer to seat it. Position the material, then drive the screw into the anchor until it is snug. Do not over-tighten the screw.
  
  
• One-Piece Expansion Anchors– Two-step anchors have become a thing of the past with professional builders, however. One-piece expansion anchors not only require no hole-spotting, but, since they are steel, they provide a much stronger grip than plastic or lead anchors.
  
  
• Two types of one-piece anchors are popular (Fig. 7). Sleeve anchors have a steel sleeve on the shank, split at the bottom so it can expand. The bolt has a cone-shaped plug at the base, and a nut on the top. When you place the anchor in the hole and tighten the nut, it draws the bolt upward, pulling the plug into the sleeve and expanding it against the hole.
  
  
• Once installed, sleeve anchors cannot be removed. They do come with a variety of heads, however–a removable hex head, an acorn nut, or either round- or flat-head screws.
  
  
• The shank of a wedge anchor is similar to a sleeve anchor–a solid shank, threaded at the top and with a cone-shaped plug at the bottom. But the shank of a wedge anchor is grooved on opposite sides. In each groove is a rectangular shank with a spade-shaped wedge on the end. As the nut on top is tightened, the washer pushes the rectangular shanks down, which spreads the wedges over the plug.
  
  
• Like a sleeve anchor, a wedge anchor cannot be removed once it is installed. Wedge anchors always have a hex head with a washer so the material can be removed and reinstalled.
  
  
• To install wedge or sleeve anchors, first position the material you want to anchor. Drill a hole in the masonry behind the bolt holes. Make sure the hole is the specified diameter, at least 1/4" deeper than the length of the anchor. Insert the anchor in the hole. Tighten a sleeve anchor two to three turns to expand it. Tighten a wedge anchor three to five turns. Manufacturers' instructions may specify that the anchor is tightened with a torque wrench to a certain number of foot-pounds.
  
  
• As a rule, use sleeve anchors when you're working with soft concrete or installing them in the mortar joints between block or brick. Also use sleeve anchors when you suspect that the concrete may have voids in it. Sleeve anchors have a larger bearing surface than wedge anchors.
  
  
• Use wedge anchors for maximum holding power in hard concrete.
  
  
• Hammer Anchors– For lightweight applications such as hanging furring strips or conduit, a hammer anchor is an excellent alternative to a plastic or fiber anchor. There are a number of styles (Fig. 8).
  
  
• The most common consists of a hardened steel ring-shank nail with either a nylon or zinc sleeve over the shank. Like one-step expansion anchors, hammer anchors require no hole-spotting.
  
  
• To install a sleeve-type hammer anchor, simply drill a hole through the material you're anchoring, large enough to accept the sleeve but small enough that the sleeve flange won't slip through. A newer type of hammer anchor looks like a heavy nail with a short bend near the end of the shank.
  
  
• To install either type of hammer anchor, set the material in position and then drill the masonry behind it. Be sure to use the size drill bit specified by the manufacturer. The hole can be any depth as long as it is deeper than the length of the anchor.
  
  
• Insert the anchor through the material and into the hole, and drive it down tight with a hammer. Hammer anchors obviously don't have the strength of larger expansion anchors, but while they aren't meant to be removed, you can pry them out if necessary.
  
  
• Concrete Screws– Concrete screws came onto the market in the mid-1970s and have become a staple of lightweight applications. They look like any other screw, except that they are made of hardened steel that will cut its own thread in the masonry (Fig. 9).
  
  
• To install them, set the material in position and then drill the masonry behind it. Be sure to use the size drill bit specified by the manufacturer–concrete screws require a precise pilot hole with a slightly smaller diameter than the screw. The hole can be any depth as long as it is deeper than the length of the anchor.
  
  
• The big advantage of concrete screws over hammer anchors is that the screw can be removed and then reinstalled. You will lose some holding power if you do so, however.
  
  
• There are no hard and fast rules about which anchor to use in what situation, but the following guidelines will help:
  
  
• Machinery to a concrete floor–As a rule, you'll want to use a heavy expansion anchor such as a sleeve or wedge anchor.
  
  
• 2" x 4" sleeper over a concrete floor– Powder-actuated fasteners (PAFs) are the most common because they are fast. If you're not certified for PAFs and only laying a small area, use hammer anchors.
  
  
• 2" x 4" framing around a door or window opening–Use hammer anchors.
  
  
• Furring strips on a foundation wall– Again, most builders use powder-actuated fasteners for the speed. If you're not certified for PAFs, use hammer anchors or concrete screws (Fig. 10).
• Shelf brackets on a foundation wall–Concrete screws are generally the best choice because they can be removed if necessary.
  
  
• A deck ledger on a masonry wall–Use edge anchors on concrete or sleeve anchors into the horizontal mortar joints of a brick or block wall.
  
  
• Conduit to a foundation wall–Use either hammer anchors or concrete screws.
  
  
• Mudsill to foundation–Use a sleeve anchor as a substitute for an occasional missing anchor bolt. If you're starting from scratch and there are no bolts, use either sleeve anchors or wedge anchors (Fig. 11).