One of the more common shed foundations is the concrete slab. They provide a solid foundation not only for the shed structure itself, but also provide a solid, even base for a floor. Flooring can be easily put over the concrete or the concrete slab itself can be used as the shed floor.
The concrete foundation, however, is not an end in and of itself. In order to provide maximum stability, the shed must be attached firmly to the concrete. While some may believe their shed may be heavy enough by itself or that their lawn tractor is sufficient to weigh it down, one strong gust from a thunderstorm may just change their minds.
In this article I will go over the 5 most common types of concrete anchors for anchoring a shed to a concrete slab.
Since a shed is something, you want to keep around and use for a long time, it makes sense to secure it to its base. There are several ways to anchor a shed to the concrete, both before and after the slab is poured.
The 5 Most Common Types of Concrete Anchors
1] Wedge Concrete Anchor – Wedge anchors can also be known as expansion anchors or expansion bolts. They use a mechanical wedging effect to secure the structural element to the concrete. A hole is drilled using a hammer drill equipped with a carbide-tipped masonry drill bit. A hammer drill can be rented for a day at a reasonable cost. The bit should be the same diameter as the anchor.
The bottom wall stud can already be in place, allowing you to drill through both at the same time, or they can be drilled separately. Make sure the hole is clear of all dust and debris, using a vacuum or compressed air if necessary. Many sources urge drilling a deeper hole than the desired depth of the anchor to allow for the possible inability of getting all the concrete dust out of the hole. For more information on permanent foundations read my article Should You Use a Permanent Foundation for a Shed?
Hammer in the anchor into the hole to the proper depth. Turn the top nut by hand until firm, then use a torque wrench until it is tightened to the required value. This will cause the bottom sleeve to wedge against the concrete, providing a firm attachment.
2] Split Drive Anchors – Split drive anchors are manufactured of heat-treated, high-strength carbon steel, preventing possible rust problems. They are usually ¼ nch in diameter with a choice of head shape. They are usually designed for light-duty fastening, and so may not be suitable for your shed size or construction materials. They are a bit easier to install than some other types, requiring only a drilled hole, cleaning out of the hole, then hammering in to set. If in doubt, consult your supplier to find out if this type of anchor is suitable for your shed.
3] Lag Shield Anchor – This is a two-part anchor, designed for light to medium-duty fastening. Again, consult your supplier to see if this type of anchor is what you need for your particular shed size and construction. They not only work with concrete but also brick and block bases. They are available in several diameters and lengths.
They are installed by drilling the proper size hole, clearing out the hole, then inserting the anchor until it is flush with the material. Set the shed piece you want to attach over the hole with a properly sized hole drilled in if necessary. Then insert the lag screw through both and into the anchor and tighten it. The bottom of the anchor will spread out against the concrete. The screw will be removable, but the anchor base remains in place.
4] Concrete Nails – Concrete nails may look like wood nails at first glance, but there are some differences. They have thicker shanks than wood nails, and also usually have vertical ribs, making penetration easier. They are made from hardened steel, meaning that when you are driving then with a hammer that has a head also made of hardened steel, tiny metal shards may fly up and get into your eyes or into your skin.
Using eye protection and gloves is a must. Instead of using a framing hammer, a small sledgehammer is recommended. These have heads made of slightly softer steel, which cuts way down on the danger of metal shards shearing off.
A lot of care must be taken to drive concrete nails absolutely straight. If it wobbles at all when being struck, it will chip the concrete instead of making a hole. If you’re using it to attach part of the shed, for instance, a wooden wall base plate, the wood will help guide the nail, but you still must make sure the hammer hits the nail straight every time. Hitting the nail at an angle will create vibrations that will cause chips in the concrete.
If you’re driving the nails into green concrete, it shouldn’t be too hard, but if the concrete is fully cured, you probably will want to drill a pilot hole for each nail. This will take up some time, but it’s a lot easier than trying to drive the nail otherwise. Use a drill bit at least an eighth-inch smaller than the nail shank or the nail won’t hold. Use a vacuum to remove the dust before driving the nail. If you don’t, the dust will actually act as a lubricant, enabling the nail to work its way out of the hole eventually.
5] Machine Screw Anchor – These are constructed and used much as the lag shield anchors. Like the lag shield anchor, they are meant for light to medium-duty purposes and are available in various diameters and lengths. They are usually constructed of the same material, and installation is similar. Drill a hole into the concrete with the proper diameter, then clean out the hole. Insert the anchor with the threaded cone-shaped end first.
A setting tool is used to hammer the anchor into the hole until the setting tool’s lip aligns with the top of the anchor. Place your attaching piece over the anchor and insert your screw through the top piece and into the anchor. The screw will be removable. Check out my article The Ultimate Guide to Choosing a Shed Anchor Kit for more options and ideas.
What Type of Concrete Anchor Should I Use?
There are several types of anchors that can be used, as we have seen. Some are termed as light or medium-duty anchors while others are heavy-duty. The size and structure of your shed will help determine what kind you should use. For instance, you may be able to use a lighter duty type to anchor a shed if it is a smaller plastic or metal one.
For a large, heavy, wood-framed shed, you’ll probably want to use anchors rated for heavier use, much as stud walls are attached to a concrete slab when building a house. Your supplier will be able to help you decide which kind you need according to the type of shed you plan to have.
If your shed is to be constructed with joists or skids instead of having wall plates setting directly onto the concrete slab, you can investigate using T or L brackets with expanding concrete bolts. These bolts are installed through the bracket’s pre-drilled holes, then attached to the joists or skids. Brackets can also be used if you are building a shed featuring wooden posts instead of a wooden frame assembly with a sill plate. These are attached to the concrete using bolts, then attached to the post.
They are often used with metal sheds that are meant to have insulation added inside to provide a basis for the insulation, while providing a vertical element to attach the siding. Some are made to be sunk partially into wet concrete. There are also T-shaped brackets with pre-drilled holes. The bracket is used upside down and actually attaches to the side of an existing slab with concrete bolts. The upright part then is attached to the sill plate of the shed or the outer skid or joists. Check out my article Is a Gravel Shed Foundation a Good Idea? for more information on anchoring a shed to a gravel foundation.
What is The Strongest Type of Concrete Anchor?
Wedge anchors are usually considered to be the strongest concrete anchors. Those that are set into wet concrete are generally considered to be stronger than those used with an existing slab, although there is some disagreement on this. If you have a shed with heavier construction, such as wood-framed walls, you probably will want to use these. For lighter sheds, such as smaller plastic ones, a lighter type of anchor may be used. If your still considering a concrete slab get more information from my article Should I Pour a Concrete Pad for a Shed?
What Is the Difference Between a Concrete Fastener and a Concrete Anchor?
Many people don’t distinguish between the two terms. However, there is a difference. If you’re attaching items that are non-structural to the concrete, that is generally termed to be fastening. For instance, you can attach a picture frame to a concrete wall using a fastener.
When you are attaching an element of a structure to the concrete, that is considered to be anchoring. Anchors tend to be larger and heavier than fasteners, although some of each can be used for either application.
How Deep Should Concrete Anchors Go?
Figuring the length of fasteners to anchor a shed can be a rather involved process. There are charts providing minimum depth the anchor should be embedded into the concrete for different diameters of the anchor. The total anchor length is then figured by adding the minimum depth to the depth of the material being fastened, plus any extra that is needed.
For instance, for a wedge anchor 1/2-inch in diameter, the minimum depth the anchor should go into the concrete is 2-1/4 inches. Fortunately, the minimum depth should be listed on the packaging. Add that to the 1-1/2 inch actual width of a 2 x 4 or whatever you are using for your wall base plate plus whatever length is needed for the top nut and washer.
It all starts, of course, with the construction and size of your shed. Your supplier can advise you on the diameter of the anchors you should use to hold the type of wall you will have. He can also tell you how far apart the anchors should be. The type of force your particular wall will exert on the anchors will determine both of these. Other factors may come into play, such as the case of living in an area routinely subjected to strong winds.
Is it Better to Use Anchors That Go into Wet Concrete?
There is much discussion about this topic. Some people swear by using wedge anchors set into the solid concrete. Many cite instances in which an anchor set in wet concrete was knocked out of kilter before the concrete set. Others point out that anchors drilled into hard concrete only work on loads in a certain direction and won’t help with rotation loads due to wind or other factors.
These anchors are very durable and can bear heavier loads than many other types. They come in several types, including a threaded bar with a plate at the bottom held by hex nuts. Others are long rods with an L, T or J shape at the end. Even heavy hex-head bolts can be used with the head facing down into the concrete.
The threaded part holding the nut would protrude above the concrete to attach the shed. These set-in anchors are the type used in home building. If you are building or buying a wood-framed shed or a metal shed, you probably should consider these. They are less work to install than the type that requires drilling into the cured concrete.
Your shed plans may come with recommendations as to spacing, or you can consult a supplier for recommendations. The trick with these anchors is that they must be placed correctly, so using guidelines is a must.
Many sources urge setting the bolts into the wet concrete soon after it is poured, but if the concrete is too soupy, the bolts may lean over a bit before it sets enough to hold them. If that happens, they should be straightened before the concrete is hard. You may want to wait until the concrete starts to set, insert the bolts to the proper depth, then slowly move them up and down a few times to encourage the concrete to flow snugly against them.
Securely anchoring your shed to its base is just common sense. If you went to the trouble to have a slab poured for it, it doesn’t take much extra effort to add the attaching materials. You want your shed to last as long as your home, so give it the same foundation care and security it needs.