A septic tank cap, also called a lid or access hatch, is normally located on the top of the tank near the center. Most caps are square in shape and are approximately 16 inches across. This cap acts as an access point to the interior of the tank for cleaning and other maintenance. Persons should never enter a septic tank as the fumes and lack of oxygen can result in rapid suffocation. Locating this cap can also prove to be a challenge in certain situations.
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Enter the crawl space or basement of your home and locate the main drain line coming from the toilets. Determine the location where the drain leaves the house and its direction of travel. Take measurements if needed to ensure you can identify the exact location of the drain from the exterior of the house. On the exterior of the house, mark the location of the drain line. This drain line will empty into the septic tank and in most cases it will be a straight shot from the house to the septic tank. If the drain takes unexpected turns, the task becomes much more difficult.
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Check the area building codes. Most require the septic tank to be located a minimum of 10 feet from the foundation of the home. For ease of installation, most contractors will install the tank as close as possible to the home. The closer the tank is located to the house, the shallower it will be buried. Measure out 12 to 14 feet from the foundation directly along the suspected path of the drain line. Mark this location. Double check your measurements and angles to be certain of your accuracy.
Dig down to a depth of no more than two feet at the location of your mark. You should strike the top of the septic tank before reaching this level. If not, re-fill the hole and dig another hole a little further out on the suspected path of the drain line. If you continue to have trouble finding the septic tank, dig closer to the house to account for contractors who do not follow codes when locating septic tanks.
Expend the hole once you have located the septic tank to expose more of the tank. Continue digging until you locate a seam in the tank lid, then remove enough dirt to expose the entire seam. This will be the tank cap. On most units the cap sits flush on top of the tank and may have two wire handles. Be certain to remove all dirt in the area prior to opening the hatch to prevent any dirt or other debris from entering the tank.
Depending on the model and type of tank you have, the cap can be very heavy. In some instances the cap will be heavy enough to require multiple individuals to remove it.