Fastening to Concrete Block Walls

Whether you're a DIY homeowner hanging a new TV mount or a contractor installing a handrail, fastening to concrete block walls requires a different approach than fastening into wood or drywall. Also known as concrete masonry units (CMU and not cinder blocks), understanding the right techniques for proper fastening is crucial for safety and a durable result.

Fastening to Concrete Block Walls? Here’s What You Need to Know

First Things First: Know Your Wall

Before you even touch a drill, you need to understand the type of concrete block wall you're working with. A typical CMU assembly is constructed with hollow units, however, portions of the hollow cells within the assembly may be filled with grout (similar to concrete) and may contain reinforcing bars.

• Ungrouted CMU: When anchoring to a section of a CMU assembly that does not contain grout within the hollow cells of the unit, specialized anchors or fasteners are embedded into or through the outer face shell to engage the masonry and lock the fasteners in place.

• Solid CMU or Grouted CMU: At locations where the cells of the hollow CMU are filled with grout, anchors and fasteners can be embedded deeper into the assembly providing a more robust connection.

  
  
  
  

Choosing the Right Fastener: A Breakdown

The correct fastener depends on what you're attaching, its weight, and whether the wall section where the connector will be installed is hollow or grouted. Always follow the fastener manufacturer’s specific instructions on weight capacity and installation.

For Light-Duty Applications (under 25 lbs.)

These are perfect for hanging pictures, small shelves or lightweight fixtures.

• Plastic Anchors: Simple, inexpensive and easy to use, these work best in the face shell of a hollow block or in the mortar joint. They expand when a screw is inserted, gripping the sides of the hole. For instance, to hang a small mirror, you would drill a hole in the face shell, insert the anchor, and then drive a screw into it.

• Toggle Bolts: Ideal for fastening to hollow blocks where you need more holding power, the "toggle" (a wing-like mechanism) expands behind the face shell, distributing the weight across a larger surface area. This is a great choice for a flat-screen TV mount in a hollow CMU wall.

Toggle bolts work well in hollow walls. The FlipToggle (pictured) requires a 1/2 in. hole and can be used with any bolt length, making it ideal for various applications. The unique flipping action combined with the tilting of its metal toggle simplifies installation.

For Medium-Duty Applications (25-100 lbs.)

This category includes items like wall-mounted cabinets or outdoor light fixtures.

• Self-Tapping Screws (Tapcons): A favorite among contractors and DIY'ers alike, these screws are designed with unique threads that cut into the concrete masonry as you drive them, creating their own "tapped" threads. For a strong, reliable hold, use them in a solid section of the block, such as where the cross webs of the unit connect with the face shells or at solid grouted sections. A common use would be attaching a heavy bookshelf to a grouted CMU wall. Remember to use the correct masonry bit and clean the hole of all dust before inserting the screw.

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Concrete screws perform in concrete, block, and brick applications. Brands such as Tapcon feature a blue corrosion-resistant coating that enables them to withstand the harshest conditions.

• Sleeve Anchors: These fasteners use an expander sleeve that is compressed against the masonry as you tighten the nut. They work well in both hollow and solid blocks. A key tip is to always use a washer under the nut to prevent the anchor from pulling into the masonry.

• Lead Anchors: These require a pre-drilled hole and are set into the masonry before a lag screw is inserted. They provide a very strong hold, making them suitable for attaching heavy signs or machinery.

For Heavy-Duty and Structural Applications (over 100 lbs.)

When you're dealing with structural connections, such as anchoring a deck ledger or a sun roof to the house, or supporting heavy loads that may be subject to vibration, you need the strongest solutions. For these applications, the fasteners should be used in solid grouted CMU, not hollow, unless an engineer has designed the fasteners for your application.

• Epoxy Anchors: They involve injecting a chemical bonding compound (epoxy or resin) into a pre-drilled hole, then inserting a threaded rod or rebar. The compound cures to create an adhesive bond that is often stronger than the concrete itself. This is the go-to method for critical, heavy-duty applications in grouted CMU.

  
  

  

  
  

• Powder-Actuated Fastening Systems: These tools use an explosive powder or compressed gas to drive hardened fasteners directly into solid grouted concrete block. They are not recommended for DIY'ers.

Essential Techniques for a Perfect Fastening

No matter the fastener, proper technique is key.

Choose the Right Drill: A hammer drill is essential for drilling into grouted CMU. It not only rotates the bit but also provides a rapid hammering action that pulverizes the masonry, making drilling faster and less strenuous. A regular drill will simply not work effectively. However, a hammer drill will be too damaging if drilling into only a hollow CMU.

Screw and anchors for concrete blocks require a pilot hole. Light-duty drilling can be accomplished using a drill/driver with a hammer-drill setting.

Larger fasteners in grouted block require drilling with a rotary hammer, which is a larger tool that feature a hammer-only setting that can be used for chipping and demolition. Rotary hammers are often available to to rent.

Use a Masonry Bit: Always use a masonry drill bit that matches the fastener diameter. The packaging of most concrete screws and anchors will specify the bit size.

Drill With Care: Drill the hole perpendicular to the surface. It is crucial to drill to the specified depth. For screws and sleeve anchors, the hole should be slightly deeper than the fastener's embedment length to allow for dust accumulation.

Clean the Hole: This is one of the most important and often overlooked steps. After drilling, use a vacuum, brush, or compressed air to remove all dust from the hole. This ensures the fastener can achieve maximum grip or, in the case of epoxy, a proper bond.

Don't Overdrive: When driving a screw or tightening an anchor, stop as soon as it is flush with the material. Overdriving can strip the threads or cause the masonry to spall or crack, which can compromise the entire connection. If a fastener spins, it means the hole is either too large or the fastener has stripped. A longer or larger diameter fastener might work, or you may need to move the hole.

By following these guidelines and understanding the different fastener options, both homeowners and contractors can confidently and safely fasten into concrete masonry, ensuring a strong and reliable result for any project.

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This article appears courtesy of the Concrete Masonry Checkoff , an industry funded initiative to support education, research, and promotion of manufactured concrete masonry units in the United States. The information is taken from TEK 12-05, Fasteners for Concrete Masonry by the Concrete Masonry & Hardscapes Association (CMHA).  You can also consult your local masonry supply company for products and information.