ADJUSTABLE PITCH ROOF CURB – A Complete Guide!
By Nick Bombard & Abbie Stancato
Adjustable pitch roof curbs are a variation of Knockdown Curbs that can be customized during installation to suit a roof pitched up to 1” in 12” in ANY direction. There are many situations in which an adjustable pitch roof curb is preferable to a fixed pitch curb, especially if:
- The pitch is not known exactly.
- There are multiple pitches on the same roof.
- There is multiple rooftop (pitched) orientations on the same roof.
WHO CARES IF THE ROOF IS PITCHED… WHY DOES IT MATTER?
Uneven Weight Distribution… The first, most obvious reason is safety. A heavy rooftop unit placed on a roof with a steep pitch displaces the load on the curb and could cause the unit to crush or buckle the curb. Furthermore, intense winds increase the odds that a rooftop unit could tip and fall off a roof curb posing damage to property, injury, and/or death.
Vibration… A pitched rooftop could be affected in the long term by increased vibration from the angle of motors and compressors designed to be level. Excess vibration can also cause tubing connections to vibrate and loosen, resulting in the loss of expensive refrigerant. Uneven stress due to pitch could cause the compressor to vibrate loose causing un-necessary noise and damage. Lastly, fan motor bearings can untimely wear out causing fan noise & fan failure.
The Compressor… The motor inside the compressor requires oil, just like the motor in your car. If the unit isn’t level, then neither will the oil level inside the compressor. This can lead to internal parts on the high side of the compressor’s motor not being properly lubricated. The lack of lubrication can cause the compressor to overheat and run less efficiently. Additionally, there’s always some oil that escapes the compressor and travels with the refrigerant throughout the lines. It’s possible for the oil to separate from the coolant and pool in one area of an angled condenser section. As long as the unit is level, this oil returns to the compressor and gets redeposited where it’s needed. However, if the unit isn’t level, the oil may separate from the refrigerant and end up pooling in the lines and coils. Over time, this can lead to a shortage of oil in the compressor, causing damage and overheating. Additionally, oil coating the inside of the coils can make it harder for heat to transfer, decreasing the system’s efficiency. See “How Does An Air Conditioner & Heat Pump Work? – Basic HVAC Tutorial.“
HOW IS AN ADJUSTABLE PITCH CURB DIFFER FROM OTHER PITCHED CURBS?
Adjustable pitch roof curbs consist of two assemblies, whereas a fixed-pitch curb is one. The lower assembly gets attached to the pitched rooftop. The top assembly slides over the bottom assembly like a sleeve. The slope of the roof is easily offset by leveling the upper assembly, while the lower assembly remains fixed. Unlike a fixed pitched curb, which must be ordered with a specific pitch & orientation in mind, an adjustable pitch curb can simply be leveled on the spot with no prior pitch information.
WHAT IF I DO NOT KNOW THE PITCH OF MY ROOF?
If this applies to you, you’re in luck. All you have to do is confirm that your roof is pitched less than 1” in 12”, and you can install an adjustable pitch curb regardless of the specific pitch of the roof or the orientation of your rooftop unit. Please see our blog post about measuring the pitch of your roof if you have questions about measuring the pitch to discover if your pitch is more or less than 1” in 12”.
WHAT IF THERE ARE MULTIPLE PITCHES OR ROOFTOP UNIT ORIENTATIONS ON MY ROOF?
It is frequently the case that a roof will have a slightly different slope in certain areas. Sometimes these subtle differences can be difficult to measure, or it may be more convenient for you to find a single solution for all the different pitched curbs required. If you find yourself in a situation where you anticipate the need for multiple differently-pitched curbs for the same job, you can order adjustable pitch roof curbs to satisfy all your needs without worrying about the specifics.
Not all rooftop units face the same direction – Orientation… It’s common for units on the same roof to be rotated differently from each other. Without the availability of adjustable pitch curbs, this would result in the need to order fixed pitched curbs with different pitch orientations. You can, instead, order adjustable pitch roof curbs with confidence that your roof curb will work for the desired application regardless of the orientation of the rooftop unit.
HOW DO I ASSEMBLE AN ADJUSTABLE PITCH ROOF CURB?
Similar to knockdown roof curbs, adjustable pitch curbs ship disassembled. MicroMetl adjustable pitched curbs come in a single, compact package that includes all the required hardware. The sides and ends of the top and bottom assemblies are joined with sheet metal screws that fit tightly into pre-punched holes at the ends of each piece. The locations of the screw holes that hold each assembly together are specially embossed so that the heads of the sheet metal screws will have clearance as the top and bottom assemblies slide past one another during construction & setup. The embosses of the top assembly face outward, and the embosses of the bottom assembly face inward. The top assembly will be slightly larger than the bottom assembly.
When you’re ready to install the roof curb, fasten the bottom assembly to the roof. Make sure you measure the corner-to-corner distance in both directions, verifying that both measurements are the same, to guarantee that the curb is perfectly square. Then, the top assembly slides over the outside of the bottom assembly. The top assembly will be loose, allowing it to slide up or down. Lift up the lowest (downhill) end of the top assembly in order to make the curb level. Once the top assembly is level, self-drilling sheet metal screws (called “Tek Screws” — included) are screwed through the holes of the top assembly into the sidewall of the bottom assembly, locking the top assembly in place. Tek screws must be inserted around the entire perimeter of the curb to safely and adequately secure the sections. Once leveled and constructed, it’s time to install the duct supports and gasket in preparation for the rooftop unit.
ARE THERE HEIGHT RESTRICTIONS ON AN ADJUSTABLE PITCH CURB?
There is no upper limit for the height of an adjustable pitch roof curb, but there is a lower height limit. Because the top assembly slides over the bottom assembly to alter the pitch of the curb to match the pitch of the roof, the minimum height of the overall curb is dependent on the height of the lower assembly. The minimum height of the lower assembly is related to the length of the roof curb in its longest dimension. Since adjustable pitch roof curbs are intended for roofs with a slope up to 1” in 12”, the minimum height of the bottom assembly will be the length of the curb divided by 12, plus 1” of overlap for tek screw installation. We must also account for any clearance needed by the duct supports when the top assembly is placed onto the bottom assembly. For example:
70” x 40” roof curb
70” long dimension
70” / 12 = 5.83”
5.83” + 1” overlap for tek screw installation = 6.83”
6.83” + 1” clearance for duct supports (typical) = 7.83”
In this example, the minimum height of an adjustable pitch roof curb would be rounded to 8”. Of course, there is no reason that you couldn’t order a curb that was taller than 8” in order to meet your specific requirements.
Adjustable pitch roof curbs are an inexpensive and versatile roof curb option that can solve a variety of issues. They eliminate the need to stress over the pitch details of a certain roof. Adjustable pitch curbs remove the hassles associated with ordering the wrong pitched / orientation fixed roof curb. This catch-all roof curb design offers the peace of mind you deserve when facing a variety of situations & circumstances. Adjustable pitch roof curbs allow you to deal with uncertainty on the job site without breaking the bank, and without forcing you to custom order a more expensive job-specific fixed pitched roof curb. Most small-to-medium curb sizes are available as adjustable pitch curbs.
MicroMetl is the leader in HVAC Accessories and Energy Recovery. Please read the more elementary article “What Is An Adjustable Pitched Curb,” for a more basic understanding of Adjustable Pitched Curbs.