The LP-Gas Code is quite clear on the separation requirements for underground (UG) tanks. These distances are specified in Table 6.4.1.1. There is also more information in sections 6.5.2 and 6.5.3. 

Generally, all UG tanks of 2,000-gallon water capacity or smaller must be at least 10 feet from buildings and property lines that can be built on. This separation is intended to allow any gas that escapes from a leak in the tank or piping to find its way to the surface and disperse before it enters a house. That is also why the separation from the property line is important, to keep the tank at least 10 feet from whatever building might be on the other side of the property line.

There is a special situation that is very common along our coast and in the mountains. Many buildings in these environs are built entirely or partially above, and not touching, the ground. What makes them special for the purposes of this article is that their separation from the ground allows for dispersion. They may be entirely separated from the ground or it may be that the enclosed space is well back from the edge of the “footprint” of the building. By footprint I mean the area on the ground that is covered by some portion of the building above, or the shadow that would be cast if the sun was directly above the building.

Since the intent of providing separation between the tank and the building is to allow for dispersion of leaking gas from the tank before it gets to the building, then we have a different way of interpreting the separation for a building that does not touch the ground. To qualify, the building also must not have any skirting around the piles that support the building, because that would hinder dispersion of any gas that escapes under the building. In these cases the UG tank may be installed up to the footprint of the building. No part of the tank may be underneath the footprint of the building, a requirement of 6.4.2.3.

Some of these buildings have an enclosed portion in contact with the ground. The UG tank must be at least 10 feet from that portion of the building. This would allow the UG tank to be installed up to the footprint of the building if there is a wide deck or building overhang (more than 10 feet) on that side of the building. If the deck or overhang is less than 10 feet wide, then the tank must be of a distance away from the deck or building edge to make up the 10 feet.

See the simple sketches that illustrate the limit of how close the UG tank may be. Figure 1 is a building that has contact with the ground. Figure 2 is a building on supports and does not touch the ground.

The need for this first became apparent when a business near the coast had a UG tank within 10 feet of the footprint. There was clear daylight under the building. They requested a ruling that would allow the tank to stay. I checked with the National Fire Protection Association and got an opinion that leaving the tank in place did not violate the intent of the separation requirement since dispersion was not impeded.

There is another consideration for the UG tanks being outside the footprint. Section 6.7.2.3 requires that the relief valve on the tank not be obstructed. Cylinders are allowed to be underneath a structure as long as the perimeter is open at least 50 percent. (See Section 6.3.4.1) Also, ASME tanks of less than 125-gallon water capacity may be under a structure if the relief valve discharge is piped out from under the structure, addressed in Section 6.3.4.2.

This is differentiated because cylinders relieve at 375 pounds per square inch and ASME tanks relieve at 250 psi, which would be critical in the event of a fire near a tank.  

The key points to take away from this article are:

• No part of the living or enclosed part of the building can be on or in the ground within 10 feet of any part of the tank. Only solid building supports may be in the ground and within 10 feet of the tank.
• There must be no skirting around the perimeter of the building. Free flow of air under the building in any direction must be allowed.
• If the conditions of 1 and 2, above, are met, then a UG tank of up to 2,000-gallon water capacity may be buried just outside of the footprint of the building, with no part of the tank under the footprint of the building.

As always, questions on this interpretation are welcome. The question you ask may be the key that helps to clarify our interpretation and help others looking at the website to understand the requirements.