Newgrange is a passage tomb in the Boyne Valley, and it holds one of the oldest sound solutions to a specific architectural problem — roofing an interior in stone: the corbel vault.

At the heart of the monument, at the end of a narrow passage some 19 metres long, lies a cross-shaped chamber. Its ceiling is not an arch; the true arch is not yet known. Instead, the upper courses of the wall each oversail the one below (corbelling); successive slabs slide inward, slowly narrowing the opening until a single capstone closes it at the top. The load is balanced by the weight of each slab and carried down and outward. The whole is mortarless; even so, the vault has stood for 5,000 years beneath the thousands of tonnes of earth and stone heaped over it, and remains watertight — the builders cut channels into the upper stones to throw off rainwater.

What also makes Newgrange a light instrument is the "roof-box" above the passage: on the mornings around the winter solstice alone, the rising sun enters through this narrow slot, runs the full 19 metres of the passage, and reaches the inner chamber. This is a structure aligned to the millimetre against an astronomical event. Newgrange thus unites corbelled roofing, waterproofing, and oriented design in a single dry-stone work — thousands of years before the true arch.