What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
Hollow concrete block looks solid from the outside, but its thin face shell and empty cells make anchor selection more important than it is in poured concrete. A standard expansion anchor can spin, crush the block, or pull out when its expansion cone reaches the hollow space. The right method depends on the load, the block condition, and whether you can access the back of the wall.
What we cover
Choose the right anchor
First estimate the real load, including leverage. A small picture frame may weigh 10 pounds, but a shelf projecting 12 inches from the wall puts much more stress on the fasteners than a flat object. Use two or more anchors where practical, and keep holes away from block edges and mortar joints. As a general rule, stay at least 2 inches from an edge and avoid placing a heavy fastener directly over a joint.
| Anchor type | Best use | Typical trade-off |
|---|---|---|
| Nylon toggle or strap toggle | Light to medium loads on a hollow wall | Good holding area, but requires a larger hole and space behind the face shell |
| Metal hollow-wall toggle | Brackets, cabinets, and moderate static loads | Stronger than basic plastic, but can be difficult to install and remove |
| Masonry screw | Light fixtures, clips, and items where the screw can reach solid material | Fast and inexpensive, but unreliable if the threads only grip a thin shell |
| Chemical anchor with mesh sleeve | Heavy, permanent loads in hollow block | High holding potential, but slower, more expensive, and dependent on careful installation |
| Sleeve or wedge expansion anchor | Solid concrete or filled block only | Usually a poor choice for an unfilled hollow cell |
For a light hook or small cable bracket, a hollow-wall toggle anchor is often the sensible choice. If the load is safety-critical, supports a handrail, or will be repeatedly pulled, do not treat a small plastic anchor as adequate. Attach to a filled cell, add blocking, or use a through-bolt with access to the opposite side.
Tools and materials
You will need a tape measure, pencil, level, safety glasses, a vacuum or blow bulb, the anchor and screw, and a drill bit that matches the anchor manufacturer’s stated diameter. For most holes, use a masonry drill bit set. Carbide-tipped bits are suitable for ordinary concrete block.
A cordless drill is enough for small holes. Use hammer mode only when the bit is cutting slowly; hammering can chip the face shell and enlarge the entry hole. A cordless hammer drill makes repeated holes faster, but start at low speed and light pressure. An impact driver is generally the wrong tool for drilling masonry. It can drive the anchor screw, but use a regular drill for making the hole so you can control depth and prevent damage.
Mark and drill the hole
Mark the anchor locations with a level. Check for electrical cables, plumbing, and concealed conduit before drilling. Avoid the center of a block’s thin face if the load is high; locating the fastener near the middle of the web or in a filled cell can provide better support, but the internal layout is not always obvious. If you can inspect the opposite side or an unfinished block, do so.
Wrap masking tape around the bit at the required depth. For a typical hollow-block anchor, the hole only needs to pass through the face shell, often about 1/2 to 1 inch, plus enough depth for the anchor. Do not blindly drill 2 or 3 inches as you might in solid concrete. You can hit reinforcement, utilities, or the far face of the block, and excessive clearance can let the anchor rotate.
Begin at low speed with the drill held square to the wall. Let the carbide tip cut instead of forcing it. If the face shell starts to chip, stop hammer mode and finish with rotary drilling. A clean, round hole matters: an oversized hole reduces the anchor’s grip, especially with plastic expansion sleeves.
Install toggle anchors
For a toggle, drill the exact hole size specified on the package. Fold or collapse the toggle, push it through the face shell, and allow it to open inside the hollow cell. Pull the screw or strap gently toward you so the toggle bears against the back of the shell. Hold that tension while tightening the fixture.
Do not overtighten. Stop when the bracket is firmly seated. Excess torque can crush the block face, bend a metal toggle, or pull a plastic toggle through the hole. If the anchor spins while tightening, the toggle may not have opened, the hole may be too large, or the back of the face shell may be damaged. Remove it and move the hole at least an inch rather than trying to rescue a spinning anchor.
Install masonry screws
Masonry screws are convenient when the threads can bite into sound material. Drill the pilot hole to the manufacturer’s diameter and depth, clear the dust, and drive the screw at low speed. Keep the drill aligned with the hole. A screw that wobbles during installation can strip the block.
These screws are often fine for cable clamps, small signs, and lightweight brackets. They are a poor choice for a shelf loaded with books if the threads engage only the thin face shell. Never substitute a longer screw on the assumption that it is automatically stronger; in a hollow block, extra length may simply pass through empty space.
Use chemical anchors for heavier permanent loads
For heavier loads, use an injectable anchoring adhesive with a perforated mesh sleeve made for hollow masonry. Drill the specified diameter, brush and vacuum the hole, insert the sleeve, and inject adhesive from the back of the sleeve forward so voids are filled. Insert the threaded rod with a slow twisting motion.
Observe the product’s cure time and temperature limits before applying load. Dust, water, a missing sleeve, or premature loading can cause failure. Chemical anchors are not a quick fix for crumbling block. If the block itself is cracked or deteriorated, repair or relocate the attachment first.
Test the installation
After tightening, pull the fixture firmly by hand and check for movement. A small amount of screw flex is different from the anchor moving in the wall; movement at the anchor means the installation has failed. For a shelf or cabinet, load it gradually and inspect the fasteners again after 24 hours. When failure could injure someone, use a structural attachment method rather than relying on a hollow-wall anchor.