Finished in 1824, the rounded South Portico was a near-perfect addition to the river face of the White House. With a full width of 61 feet, the portico gave visual focus to the 168 foot expanse of the south elevation. It created a protected extension, a terrace or viewing platform, for the state parlors, as well as a serviceable entrance by means of an elegant double flight of stairs, rising almost 13 feet from the ground.

The South Portico is defined by six Ionic columns that step out and around the center bow or bay, as a fully three-dimensional continuation of the monumental “pilastrade,” or row of pilasters, that rings the house on three sides. Like the pilasters, the columns of the portico are raised on square plinths. The shafts are Seneca sand- stone, made in sections and pinned at their centers with iron. They are smooth, in contrast to the heavily rusticated treatment of the Ground Floor wall where the portico base, extending outward, becomes a high podium. Seven arched openings pierce the thick walls of this podium, providing the only outward expression of Hoban’s groin vaults, which support the portico.

A groin vault is formed when two barrel or “tunnel” vaults cross. The name is derived from the arcing edge produced at the intersection of the vaults, which is called a “groin.” In a perfect crossing, the groins define four equal-sized wedges that are the vault. Each of these elements is known as a “groined arch.” When used in small spaces, groin vaults sometimes have a dome-like quality, a loftiness that can belie dense masonry construction.

Why did Hoban trouble himself to build vaults for a more or less self-supporting addition like the portico? As with the original Ground Floor corridor, the vaults of the portico were intended to most efficiently direct the weight of the First Floor’s stone pavers. They also added assurance that the large addition would not compromise the physical integrity of the load-bearing walls of the residence. For both practical and aesthetic reasons, the vaults under the portico floor were a non-negotiable part of Hoban’s design. A solid base would have been materially wasteful and structurally unnecessary in addition to eliminating because of sheer bulk any ground-level access to the residence. The openings behind the seven archways lightened the heavy mass of the portico and also established visual interest, giving an element of shadowy depth to contrast with the out- ward curve and dramatic rustication of the podium. Hoban crafted these voids with a series of groin vaults and groined arches.

Vaulting is visible in six of the seven bays at the South Portico, while a modern dropped ceiling covers visible evidence in the seventh. Now used for an interior vestibule, the center bay most legibly conveys the manner in which Hoban conceived the vaulting system and some of its quirks.

A transverse, north-south, section through the residence and Ground Floor plan indicates that the groin vault that arches over the space is in fact irregular. It might even be better described as a barrel vault intersected by two groined arches, as the north-south vault
is smaller than the east-west one. consequently, the north and south groined arches can never fully intersect.

The complex vaulting beneath the South Portico is testimony to Hoban’s skill. Here he tailored conventional forms of groin vaulting to the extraordinary needs of the curving South Portico. Though irregular in form, his vaults still perform their primary function in shouldering and transferring weight safely through the arch to the wall. The barrel vault extending east to west, following the curve of the portico, effectively carries the compressive weight of the upper portions downward. Groined arches intersect this semicircular vault over each of the openings, redirecting the outward and down- ward thrust around them. Hoban used the resulting groin vaults for bays with an exterior opening for each. The remaining interior bays were firmly fixed on the solid south walls of the house and required only a single groined arch over the doorway. Portions of the vaulting were obscured when the outermost bays were enclosed as rooms, but the doorway part remains exposed.

Five years after the 1824 completion of the South Portico, Hoban was called back to build the North Portico on the Pennsylvania Avenue side of the house. Here a modest vault already existed, a narrow, single span built by Latrobe for Jefferson to support a bridge over the north areaway or light well to the front door. Hoban’s challenge was therefore less complex than with the South Portico. The sharp decline in the White House grounds from the north or Pennsylvania Avenue side to the south necessitated the areaway, so that the basement’s kitchens and service rooms would have natural light and ventilation, sunk as they were some 12 to 13 feet below grade on that side.

In 1807–8, Latrobe’s new bridge was an elegant, well-made addition to the President’s House and is still part of the support of the great North Portico. Latrobe’s broad segmental vault, made of brick faced on the east and west sides with stone to match the house, is still visible. A groined arch over the lower door—the kitchen door—joined engaged buttressing that flanked it to dis- tribute the load of the heavy masonry bridge above. The north end of the vault required no groining, for it met a “pier” formed by a small storage room of brick and rubble built into and extending into the earth beyond the stone retaining wall of the areaway.

In enlarging the bridge to serve the new portico in 1829, Hoban merely duplicated the segmental vault one bay to the east and one to the west, using an essentially identical arrangement of groin arches to clear the windows flanking the kitchen door.

Although the full arc of the vault under the North Portico remains visible, twentieth-century partitions obscure much of the underlying structure. Late in 2006, the Historic American Buildings Survey (HABS) of the National Park Service had the opportunity to document a segment of this vault. Fortunately, the area recorded by HABS contained part of Latrobe’s central vault with its stone facing with Hoban’s duplicate work beyond. This was a rare and welcome opportunity to see the construction methods and materials that have provided the grand North Portico a stable foundation for the better part of two centuries.