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Light fixtures sometimes need to be fastened to concrete in both outdoor and indoor applications. Installation may be needed in a home or commercial building. The weight of the light fixture will determine the diameter of concrete anchor that should be used. The configuration of the mounting plate and space provided by the cover will also help determine the type of anchor that will be required. The following article describes the various types of concrete fasteners commonly used for fastening a light fixture to concrete as well as the considerations that need to be addressed.
A concrete fastener's holding value is a factor of the diameter of the anchor being used. As the weight of the lighting fixture increases, so will the size/diameter of the concrete anchor that should be used. The heavier the light fixture, the deeper the embedment will need to be as well. Each diameter anchor has a minimum embedment to meet minimum holding values in the concrete. As the diameter increases, so does the minimum embedment. For example, a 3/8" diameter wedge anchor has a minimum embedment of 1-1/2" while a 1/2" wedge anchor has a minimum embedment of 2-1/4". The deeper the embedment, the better the holding values. The deeper the penetration into the concrete, the more likely it will be to hit rebar. Rebar can be drilled through for anchor placement, but this makes the job much more costly and involved.
The mounting fixture that comes with the light fixture will have holes in it for mounting it to the base material. Depending on the size of the fixture, the holes can range from 1/4" to 5/8" or larger. The concrete fasteners must fit through the fixture holes. The diameters of the anchors will not match up with the diameters of the holes in the fixture. A 3/8" wedge anchor will not fit through a 3/8" hole in a mounting fixture. The threaded part of the anchor matches up but the part that goes into the concrete will be slightly larger than the nominal diameter of the anchor. It is important to check that the anchor will fit through the mounting hole. A good rule of thumb is to use an anchor one diameter smaller than the holes in the fixture. For example, a 3/8" hole would require the use of a 5/16" diameter anchor.
If the light fixture is to be placed outdoors or in an area where there is constant moisture or other chemicals, it is best to use stainless steel concrete anchors. Fasteners that are zinc plated will rust over time. Zinc plated fasteners will work fine for indoor use but not outdoors.
Concrete fasteners are set into concrete with different and sometimes inconsistent results in terms of how far the anchor protrudes from the concrete. The type of fastener chosen may be determined by the amount of space allowable for the anchor to protrude into the cover of the light fixture. Some smaller diameter fasteners have been designed so that they don't protrude while others are designed in such a way that it allows the anchors to be countersunk in the mounting plate. In heavier and larger light fixtures, the fixture is designed to accept larger concrete anchors with space for the anchor to protrude into.
The wedge anchor is a stud type concrete anchor that is designed to be used in concrete only. Wedge anchors have the best and most consistent holding values of any type of mechanical concrete anchor and come in a wide variety of diameters and lengths. Wedge anchors are available in zinc plated, hot dipped galvanized and stainless steel. The hole that needs to be drilled for a wedge anchor is the same as the diameter of the anchor. Wedge anchors may protrude from the base material at different lengths and may not be suitable for use with certain light fixture covers.
Tapcon® is a registered brand name for a concrete screw. Concrete screws tap threads into a predrilled hole in concrete, brick or block. The standard Tapcon® comes in two diameters- 3/16" and 1/4". They are available in two head styles- slotted hex washer and flat phillips countersunk. Tapcons have excellent holding values in most base materials and are very versatile. Concrete screws are available with a Climaseal® coating that provides excellent rust resistance as well as a 410 stainless steel screws. Hole size for Tapcons needs to be smaller than the designated diameter of the screw. Large diameter Tapcons are also available in a variety of diameters and finishes (zinc plated and stainless steel). Tapcons have a finished head and will not protrude into the light fixture cover.
Hammer Drive Anchors
Hammer drive anchors are excellent light duty fasteners that can be used for light fixtures. For this application, they are available in 1/4" diameter with a zinc plated or stainless steel nail. The low profile head is perfect for most light fixture plates and covers. Anchor diameter equals hole size and the wide mushroom head provides an excellent bearing surface for pullout strength.
All of these concrete anchors and fasteners would work when fastening light fixtures to concrete. The type of anchor chosen will depend on the unique application and the type of light fixture being installed.
As with any anchoring project, it is important to keep safety in mind and follow instructions carefully. Always remember to wear safety goggles, handle all tools with extra care and follow all technical specifications. This article is meant to serve only as a basic explanation of concrete fasteners. Always refer to manufacturer's instructions or consult a contracting expert during any anchoring project.