One of the common misconceptions you'll hear about fiber cement siding is that it's hard to work with, specifically very difficult to cut. Granted when using a circular saw there is a fair amount of dust that is given off but that can be remedied with a debris collector. A shear attachment however eliminates almost all of the debris kickback altogether. One of the biggest things that siding installers have to overcome however is cutting holes for light fixtures and then safely securing those items to the siding.
So how does installing light fixtures work on a material that is supposedly difficult to work with in fiber cement siding? The methods of installing a light fixture on fiber cement are very similar to other materials, it's just the tools and the process that vary slightly:
The Use of Mounting Blocks
It should be noted that you won't be attaching the light fixture to fiber cement siding at all – rather a fiber cement mounting block. The mounting block is used for both aesthetic and functional reasons. For example, the mounting block provides a contrast to the siding and makes the light look 'like it belongs.' The different shapes and sizes available (rectangular, octagon, square, etc.) create an attractive exterior feature behind the fixture.
The mounting blocks are applied directly to the sheathing which also helps to eliminate water exposure. In order for water to get behind the fixture it would have to get through starter strip, flashing, weather barrier, caulk, and the siding itself.
Mounting blocks are also installed before the siding whenever possible. Vinyl light fixtures are much easier to retrofit because the material is easier to cut and special considerations can be made for waterproofing. With fiber cement, instead of attaching a light fixture over siding, you're siding up to a light fixture.
Align the mounting block hole over where the light wiring comes out of the house. Level the block parallel with how the siding is going to be and nail it into place with 2-1/2” roofing nails. Make sure the weather resistant barrier is behind the mounting block to avoid water exposure to the sheathing.
Proper Sealing of the Blocks and Preparation for Siding
The nice thing about light fixture mounting blocks is that they are self contained units (electrical wiring code) that come pre-flashed and with a drip edge for a complete pre-assembled waterproofing solution – on the exterior of the siding. You still need to perform the proper prep work underneath the siding layers however.
Note: Most contractors wait to install the light fixture mounting blocks until they have sided up to that point. This allows for slight adjustments to the position of the light, either centering between rows or avoiding a fixture that falls on a 'sliver' of a row.
Once the mounting block is nailed into place, it's time to apply flashing tape over the flange edges to get rid of any cracks that may attract water. It's best to get a mountain block that is pre-assembled with flashing but if you do not have this you'll need to install a 'z' flashing above the block. A starter strip may need to go on top of that too to push the siding away from the home to simulate the overlap of one row on top of another.
Applying the Siding
If it seems like 75% of installing a light fixture to fiber cement siding has to deal with a simple mounting block – it's because it does. Once the block is in place you can side around it just like you would at a door or near the posts at the end of a wall. Unlike a vinyl siding mounting block that clips apart and when attached will cover up gaps near the fixture, the mounting block on fiber cement siding is what it is. What this means is that you'll want to make sure cuts are straight and even because they will be somewhat visible.
To clarify, the siding should be installed with an 1/8” to 1/4” gap around the sides and ends to account for any expansion and contraction with humidity changes. These gaps on the sides and bottom will be caulked to prevent any water damage. This is also why it's so important to apply flashing tape where the mountain block meets the sheathing just in case the caulk fails. The caulk is then painted to provide a seamless blend between the block and the siding.
Note: the top of the mounting block is not caulked. Rain should be whisked away by the top z-flashing or 'drip edge.' Caulk on the top of fixtures, windows, etc. can actually trap water which will eventually permeate its way behind the fixture.
Retrofitting a Fiber Cement Light Fixture
If you've inherited a light fixture or need to replace a damage one, retrofitting the fiber cement is possible although not ideal. The best plan of attack is to remove the individual planks from multiple rows until the light fixture nailing lip area are fully exposed. Of course, that's a lot of work and you risk damage when removing the boards. Level up the size of the mounting block with pencil (remembering to account for a 1/8” to 1/4” gap) to prepare for the dusty task of cutting the boards in place. Depending on the condition of your siding the best cutting methods might include a rotozip with a carbide blade or an angle grinder with a metal cutting disc or cement wheel.
Once the hole opening is cut out, you'll have to navigate the mounting block into place situating the nailing lip behind the existing rows of fiber cement. After that, reverse engineering of the flashing tape, z-flashing, starter strip, etc. makes the project just like a new install.
Always make sure to check with a home inspector or licensed electrician to make sure your installation is following local codes and regulations. Since so many fixtures, mounting plates, and siding applications can vary this guideline should only be used as an example of how a light fixture is installed into fiber cement.