How do you anchor concrete to wood?

Anchoring concrete to wood is a common requirement in construction projects when building concrete structures on top of wooden foundations or framing. Proper anchoring ensures a strong, long-lasting connection between the concrete and wood. There are several methods for anchoring concrete to wood, with the best approach depending on factors like load requirements, type of wood, and more. This article will provide an overview of the most common techniques for joining concrete and wood securely.

Why Anchor Concrete to Wood?

Concrete is an extremely strong and durable material, but it can be prone to cracking and shifting if the foundation underneath is not stable. Wood provides a cost-effective sub-structure for concrete but lacks the stiffness and compression strength on its own. Anchoring the concrete to the wood composite provides:

• Increased resistance to lifting and shifting from wind, seismic activity, and foundation movement
• Reinforcement for the wood to prevent buckling, warping, or cracking under the weight of the concrete
• A composite structure taking advantage of both the strength of concrete and the natural flexibility of wood

Proper anchors create a positive connection, allowing force transfer between the concrete and wood while preventing separation. This composite behavior results in a structure stronger than either material on its own.

Considerations for Anchoring Concrete to Wood

There are several factors to take into account when choosing an anchoring method:

Type of Wood

The species, grade, and moisture content of the wood all impact its structural properties. Denser hardwoods provide more holding power for anchors compared to softwoods. Treated lumber with preservatives can corrode certain metal anchors over time. The moisture content also affects strength, with green wood being weaker than seasoned wood. Checking the wood’s specifications is important in selecting suitable anchors.

Load Requirements

The expected live and dead loads on the structure influence the anchoring requirements. Heavy static or dynamic loads demand higher anchor strength and stiffness. More anchors may also be needed to distribute the load sufficiently and resist shear and uplift forces.

Concrete Type and Strength

Higher strength concrete can allow thinner slabs but requires stronger anchoring. The aggregate size and pouring method for concrete also determines how closely anchors can be spaced. These factors are considered when positioning anchors for concrete-to-wood connections.

Environmental Conditions

Outdoor concrete structures experience freeze-thaw, moisture, and temperature fluctuations. Anchors must withstand corrosion, sustained vibration, and fatigue stresses. In coastal areas, stainless steel anchors and connectors may be preferred.

Construction Method

The concrete installation process impacts the anchor selection. Pre-embedded anchors cast into concrete require coordination with forms. Post-installed anchors are driven after concrete is cured but can fracture the slab if not done properly.

Methods for Anchoring Concrete to Wood

There are several suitable methods for joining concrete and wood. The optimal anchoring approach depends on the specific project’s requirements and construction process.

1. Concrete Screws

Concrete screws provide a quick and easy post-installed anchoring option. They are inserted through pre-drilled holes in the concrete into the wood below using an impact driver or drill. Pros:

• Simple installation with basic tools
• No need to align with internal wood members
• Can be used for thin concrete slabs and overhead applications

Cons:

• Not recommended for heavy loads beyond 200 lbs
• Risk of concrete cracking during installation
• Corrosion prone in harsh environments

Use for: Light duty residential projects, fences, posts

2. Concrete Anchoring Adhesive

Adhesives designed for concrete bonding provide a simple anchoring method. After surface prep, the adhesive is applied to the concrete/wood interface and allowed to cure per the manufacturer instructions. Pros:

• Requires no drilling into concrete
• Works with uneven concrete surfaces
• Bonds wood and concrete chemically

Cons:

• Lower load capacity than mechanical anchors
• Environmental factors can degrade bond over time
• Specialized installation requirements

Use for: Supplement to mechanical anchors, tilt-up panels, decorative concrete

3. Lag Bolts

Lag bolts are large wood screws that can anchor concrete forms or sill plates to wooden members. The coarse threads provide high pullout strength. Pros:

• Simple installation with pre-drilled holes
• Immediate loading possible after tightening
• Allows wood shrinkage/movement

Cons:

• Not recommended for tension loads
• Large pre-drilled holes required in concrete
• Corrosion potential in wet environments

Use for: Anchoring foundation forms, connecting floors to walls

4. Expansion Anchors

Wedge or sleeve expansion anchors embed themselves in pre-drilled concrete holes for tension loads. Torquing the bolt expands the anchor’s sleeve outward. Pros:

• High strength in tension applications
• Immediate loading after installation
• Available in several materials (carbon steel, stainless steel, etc.)

Cons:

• Hole drilling can crack thin slabs
• Loose concrete reduces effectiveness
• Load capacity is lower for redundant anchors

Use for: Column bases, machine mounts, heavy equipment

5. Adhesive Anchors

Adhesive anchor systems use threaded rods or rebar embedded in an epoxy adhesive inside drilled concrete holes. The adhesive cures under normal conditions to bond the anchor. Pros:

• High strength in tension and shear
• Performance not affected by hole size
• Minimal concrete cracking potential

Cons:

• Long cure time before loading
• Installation variables affect strength
• Two-component adhesives have short pot life

Use for: Structural retrofits, heavy machinery, tie-downs

6. Concrete Screws with Steel Plate Washers

For heavy duty anchoring, large washer plates are used with concrete screws. The plate provides a greater bearing surface to distribute load. Pros:

• Ease of installation like standard screws
• High pullout strength
• Allows wood shrinkage movement

Cons:

• Concrete damage potential during installation
• Rebar location affects performance
• Not recommended for dynamic/shock loads

Use for: Machine equipment, large structural sections

7. Through Bolts

Through bolts pass completely through the concrete and wood members to provide a positive connection. The bolts have washers and nuts on both ends. Pros:

• Supports very heavy loads
• Direct transfer of forces from concrete to wood
• Easy to install on form boards

Cons:

• Requires access below concrete
• Form work must accommodate bolts
• Not suitable for solid concrete foundations

Use for: Large structural beams, heavy equipment

Best Practices

Following best practices helps ensure proper concrete anchoring to wood for load transfer:

• Check wood moisture content – Should be below 19% for most anchors
• Use manufacturer recommended embedment depths
• Observe edge distance and anchor spacing requirements
• Ensure drilled holes are clean before setting anchors
• Allow full adhesive cure time prior to loading
• Retorque expansion anchors after concrete is loaded
• Use corrosion resistant anchors in wet environments
• Avoid anchors at rebar locations to prevent interference

Conclusion

Anchoring concrete to wood provides an economical design option to take advantage of each material’s strengths. The multitude of available anchor types allows structural engineers to cater connections to specific project needs and loading conditions. With proper installation that follows manufacturer specifications, anchors provide robust performance and safely secure concrete structures to wood framing or foundations.

Understanding the capabilities, pros and cons, and best uses for different concrete to wood anchors enables selection of the optimal method. Safety factors should still be included in designs, and periodic inspection is recommended for any concrete anchorage. Following construction and installation best practices also ensures anchors achieve their full load capacity and service life.

With the right approach to connecting concrete and wood, contractors benefit from fast construction and designers get the performance needed for lasting structures. Concrete anchoring continues to evolve with new engineered anchor products from companies like Hilti, Simpson Strong-Tie, and Powers bringing more options to jobsites. Innovations in anchoring equipment have also made installing threaded rods and rebar easier. Specifying the optimal concrete anchoring system and using proper techniques allows taking full advantage of concrete and wood’s complementary properties.

Anchor Type	Installation Method	Performance Rating*
Concrete Screws	Post-Installed with Impact Driver	**
Adhesive Anchors	Post-Installed with Dispensing Gun	****
Expansion Anchors	Post-Installed with Wrench	***
Through Bolts	Cast-in-Place or Post-Installed	*****

* Performance rating is based on load capacity, with more stars indicating higher strength

Related Questions

What types of wood can concrete be anchored to?

Concrete can be anchored to most common construction wood types like treated southern pine, Douglas fir, laminated veneer lumber (LVL), and oriented strand board (OSB). Some important considerations are wood density, moisture content, and preservative treatments that could corrode metal fasteners.

What is the best anchor for attaching a concrete porch to an existing wood house?

For attaching a concrete porch slab to a wood structure, use of epoxy adhesive anchors or through bolts is recommended. Adhesive anchors allow drilling and anchoring the concrete without accessing the interior space. Through bolts provide maximum strength but require access behind the wood framing to install nuts and washers.

Do you need rebar when anchoring concrete to wood?

Rebar is not mandatory but can provide additional reinforcement. Rebar resists tension loads better than plain concrete. Vertical rebar dowels drilled and epoxied into an existing wood foundation help transfer loads when pouring a new slab. Rebar can interfere with closely spaced wood anchors, so coordination of layout is important.

What is the difference between anchoring new poured concrete versus attaching to existing cured concrete?

For new pours, cast-in place anchors like threaded rods allow securing forms and embeds anchors directly in the concrete. For existing concrete, drilled-in anchors are used but loading is often delayed to allow the epoxy or expansion anchors to fully cure. Drilling cured concrete also risks cracking, blowouts, and other damage.

How deep should concrete anchors be embedded for maximum holding strength?

Anchor depth depends on the anchor diameter but the general rule is 10-15x the bolt diameter. So a 1/2″ anchor would need 5-7.5″ embedment. Edge distance from any concrete edges should be a minimum of 7-10 bolt diameters. Following manufacturer specifications for minimum spacing, edge distance, and embedment is critical.

Can concrete be anchored to wood vertically like for a concrete wall on a wood stud base?

Yes, concrete can be anchored vertically to wood framing. Lag screws or epoxy anchors provide good performance for attaching concrete walls to the wood top and bottom plates. The wood framing must be designed to resist the out-of-plane wall forces, and additional post-tensioning is often needed for tall walls.

What inspection or testing should be done on concrete anchors into wood?

For critical anchors, proof load testing to 125-150% of the working load should be done. Torque checks of nuts/bolts ensures proper installation. Periodic visual inspection checks for corrosion, wood separation, and concrete cracking around anchors. Checking for loose nuts/bolts and adding sealants helps improve durability.

How much stronger is anchored concrete to wood versus just pouring concrete on top of a wood platform?

Anchoring can make the concrete-wood connection 5-10 times stronger compared to just pouring concrete over a wood base. The anchors prevent separation and resist uplift forces. Proper anchoring allows the concrete and wood to work together while plain concrete on wood risks cracking, shifting, and complete failure under loads.

Can you anchor new concrete to existing wood, like a concrete topping slab on an old wood subfloor?

It is possible to anchor a new thin concrete topping to an existing wood subfloor, but the old wood must be inspected for rot or weakness first. New sleepers can be installed on top of the wood subfloor and epoxy or expansion anchors used to tie the concrete to the new wood framework.

What equipment is needed to install various concrete to wood anchors?

Basic equipment like rotary hammers, impact wrenches, torque wrenches, and drill bits are needed. Concrete screws require an impact driver. Adhesive anchors need a dispensing gun for the epoxy cartridges. Through bolts need regular wrenches or sockets. Always use the drill or bit size specified by the anchor manufacturer.

Should you grease the threads on anchors into wood like lag bolts to make installation easier?

Grease or oil should not be used on the threads for anchors into wood, only anchors into concrete. The lubricant can get absorbed into the wood and reduce pullout strength. Use sharpened anchor points, correct drill hole sizes, and reasonable torque to install wood anchors without lubrication.

What is the best way to attach nameplates, low voltage conduits, or decorative trim to existing concrete on a wood frame building?

For lightweight attachments to cured concrete on wood framing, consider using concrete screws placed into plastic anchors. Plastic anchors insert into pre-drilled holes and provide good holding strength with minimal concrete damage. Concrete screws into the anchors provide an easy way to fasten small items.