The uposatha (Lao sim; ordination hall) is always the most important structure in any Theravada Buddhist wat. The high-peaked roofs are layered to represent several levels (usually three, five, seven or occasionally nine), which correspond to various Buddhist doctrines. The edges of the roofs almost always feature a repeated flame motif, with long, fingerlike hooks at the corners called chaw faa (sky clusters). Umbrella-like spires along the central roof-ridge of a sim, called nyawt chaw faa or 'topmost chaw faa', sometimes bear small pavilions (nagos - mythical water serpents) in a double-stepped arrangement representation of Mt Meru, the mythical centre of the Hindu-Buddhist cosmos.

There are basically three architectural styles for such buildings - the Vientiane, Luang Prabang and Xieng Khuang styles.

The front of a sim in the Vientiane style usually features a large veranda with heavy columns which support an ornamented, overhanging roof. Some will also have a less-ornamented rear veranda, while those that have a surrounding terrace are Bangkok-influenced.

In Luang Prabang, the temple style is akin to that of the northern Siamese or Lanna style, hardly surprising as for several centuries Laos and northern Thailand were part of the same kingdoms. Luang Prabang temple roofs sweep very low, almost reaching the ground in some instances. The overall effect is quite dramatic, as if the sim were about to take flight. The Lao are fond of saying that the roof line resembles the wings of a mother hen guarding her chicks.

Little remains of the Xieng Khuang style of sim architecture because the province was so heav­ily bombed during the Second Indochina War. Pretty much the only surviving examples are in Luang Prabang and to look at them you see aspects of both Vientiane and Luang Prabang style. The sim raised on a multilevel platform is reminiscent of Vientiane temples, while wide sweeping roofs that reach especially low are similar to the Luang Prabang style, though they're not usually tiered. Cantilevered roof supports play a much more prominent role in the building's overall aesthetics, giving the sim's front profile a pentagonal shape. The pediment is curved, adding a grace beyond that of the typical Luang Prabang and Vientiane pediments.

A fourth, less common style of temple architecture in Laos has been supplied by the Thai Lii, whose temples are typified by thick, whitewashed stucco walls with small windows, two- or three-tiered roofs, curved pediments and naga lintels over the doors and steps. Although there are examples of Thai Lu influence in a few Luang Prabang and Muang Sing temples, their main location is in Sainyabuli Province.