D.C. Code § 43-122: Mode of burial.

No body shall be buried in said District in any vault unless the coffin be separately entombed in properly cemented stone or brick work, so as to render such vault airtight; such vault, after having been sealed, shall not be opened within 10 years; no body shall be temporarily deposited in any vault for a longer period than 1 month, unless such body is in an hermetically sealed metallic case, nor in any instance for a longer period than 1 year.