Conserving Drayton Hall’s Iconic Portico

By Special Contributor posted 05-06-2014 15:59

  
 Figure 1: Drayton Hall preservation staff investigate the iconic portico. | Credit: Drayton Hall
Figure 1: Drayton Hall preservation staff investigate the iconic portico. | Credit: Drayton Hall
by Trish Smith

Drayton Hall, a National Trust Historic site in Charleston, S.C., is widely considered the earliest and finest example of Palladian architecture in the United States. Chief among Drayton Hall’s character-defining features is its two-story, iconic portico which projects from, and recedes into, the house. A recent structural assessment of the portico has brought to light important information about the construction and subsequent alterations to this well-known feature.

Since 1974, several interventions have been made to stabilize and conserve the portico, which documentary evidence tells us has been plagued with structural issues almost since its initial construction more than 250 years ago. On May 2, 1815, Charles Drayton (1742-1820) wrote in his journal that one of the portico columns was “in jeopardy,” and was to be taken down. Four days later, he wrote, “Schnirle came [with] 26 fellows and took the pillar down safe and cleverly.” A stack of limestone columns in the cellar of Drayton Hall may be what remains of the original stone pillars discussed in Drayton’s diary. Architectural historians have long speculated about other alterations made to the portico.

 Figure 2: Detail of Gibbes Sketch Book, c. 1845 showing exposed brick above lintels; Gift of the Drayton family; Drayton Papers Collection. | Credit: Drayton Hall
Figure 2: Detail of Gibbes Sketch Book, c. 1845 showing exposed brick above lintels; Gift of the Drayton family; Drayton Papers Collection. | Credit: Drayton Hall
Paint analysis conducted in 2006 confirms that the ionic entablature on the interior of the second floor portico survives from Drayton Hall’s first period, while the interior entablature on the first floor as well as all of the exterior entablatures are clearly later alterations. An early watercolor of Drayton Hall, painted in 1765, showed that the first floor of the portico once had a Doric entablature adorning the exterior of the structure. A later sketch from the 1850s shows the portico with no exterior entablature and what appear to be several courses of exposed brick above the lintels. So, what happened to the exterior entablature and could its disappearance be related to the damaged limestone column reported in Charles Drayton’s diary in 1815? Furthermore, could the same conditions that led to these changes have continued to adversely affect the portico, contributing to the issues that compromise its structural integrity today?

In 2012, we began to explore these questions in depth as we launched a structural assessment of the portico. The assessment was prompted by several indications of structural stress. Oxide jacking, a phenomenon that occurs when iron rusts and expands, was severely compromising the concrete beams supporting the portico deck as the embedded reinforcing bars corroded and blew out large chunks of masonry. Additionally, we observed the obvious settlement of one of the large masonry piers below the back wall of the structure, and cracks that appeared in the stucco covering the lintels that support the second floor deck.

Craig Bennett and his team at Bennett Preservation Engineering, PE, were hired to conduct the structural assessment of the portico, and their findings indicated that there were indeed serious issues with settlement, water intrusion, and inappropriate modern materials. The portico was closed immediately to ensure visitor safety. Bennett then recommended stabilizing the portico until the most appropriate method of restoration could be determined. To that end, two jacks were installed below each of the five concrete beams in the cellar. These beams carry their loads to a solid masonry wall on one side and a system of arches that bear on masonry piers on the other side. Each of the ten jacks was raised approximately 1/10th of one millimeter to direct the load paths away from the compromised masonry piers.
 Figure 3: Cellar below portico with shoring posts and meter sticks used to measure height of deck relative to walls and piers. | Credit: Drayton Hall
Figure 3: Cellar below portico with shoring posts and meter sticks used to measure height of deck relative to walls and piers. | Credit: Drayton Hall
Before raising the jacks, it seemed prudent to be sure that the concrete slab below the stone pavers on the first floor was not tied into the masonry walls of the house. Thus, a stone paver immediately adjacent to the front wall of the house was removed.

We discovered that not only was the slab tied into the wall, but an entire course of brick, one wythe deep, had been removed to let the concrete in, such that the front wall of the house appears to be suspended over a large void. Ultimately, it was decided to press on with lifting the slab just a fraction of a millimeter to direct the load path to the jacks instead of the masonry piers.

Concurrent with the stabilization of the portico, an investigation phase was launched to gather information that might inform the ultimate preservation plan. First, stucco was selectively removed from the sandstone lintels in order to determine whether cracks evident in the stucco were broadcasting through from the underlying stone. Fortunately, this was not the case, and the exposed areas of sandstone appear to be in good condition.

The next step involved removing the dropped ceiling from the first floor portico to expose the framing system supporting the second floor portico. The ceiling, which appears to have been installed toward the end of the 19th century, had been so thoroughly sealed with caulk and several layers of white latex paint that it was almost completely watertight, trapping moisture from the second floor and rotting the wood framing. Thus, the benefits of removing the ceiling were twofold. First, it allowed moisture to escape, halting the wood rot that was compromising the structure. Second, it enabled us to examine the structure and begin to understand how it has been modified over the years.
 Figure 4: Drayton Hall preservation staff removing dropped ceiling from 1st floor of portico. | Credit: Drayton Hall
Figure 4: Drayton Hall preservation staff removing dropped ceiling from 1st floor of portico. | Credit: Drayton Hall
Hidden within the dropped ceiling were wooden joists that appear to be hand-hewn, making them much earlier than the current ceiling. But curiously, measurements of the joists as well as the orientation of the lap—the “L” shaped notch that extends over the member on which it rests—on one end indicated that they were all installed upside down and backward, that is, they all taper toward the interior rather than the exterior, and the lap is on the top rather than the bottom. Infilled joist pockets immediately adjacent to each joist further indicate that the existing hand-hewn joists are reused from another location if one assumes that they were installed next to the original joists before they were removed.

In the cellar, a strip of concrete extends downward from the ceiling approximately one foot around the entire perimeter of the portico. In an effort to better understand the sequence of changes to the structure, the concrete was removed, exposing the original joist pockets. Careful study of these joist pockets indicates that two phases of construction predate the current system of five large concrete beams. Initially, wooden joists spaced 17 inches on center lapped over a wood plate or masonry arch, depending on their location. It is evident that this configuration was abandoned in the 19th century as mortar impressions indicate the presence of circular sawn lumber placed in the same joist pockets but elevated approximately 3 inches above their previous location. Fragments of extant joist plates show significant deterioration which points to water intrusion as the likely culprit in the failure of the first two wooden joist systems, and the impetus for the 20th-century concrete repair.
 Figure 5: Exposed joist pockets in cellar below portico. | Credit: Drayton Hall
Figure 5: Exposed joist pockets in cellar below portico. | Credit: Drayton Hall
A final determination regarding the method of restoration for the portico has yet to be made. Most of the concrete beams and first floor deck will likely be removed to prevent further deterioration to the existing structure by way of settlement and oxide jacking. A new support structure in the cellar will mimic the original configuration by inserting joists into the abandoned joist pockets. These joists could be salvaged lumber protected by a waterproof membrane above or pre-cast concrete inserted into lead-lined joist pockets. The final determination will be made upon the conclusion of the intensive research phase which is ongoing.

By taking the time to study the architectural evidence, we hope to better understand the issues that have long plagued Drayton Hall’s iconic portico. In this way, we can identify a solution which respects the historic integrity of this remarkable resource and ensures its preservation for generations to come.


Trish Smith is curator of historic architectural resources at Drayton Hall, a site of the National Trust for Historic Preservation located in Charleston, South Carolina. She holds a master of science in historic preservation from the Clemson University & College of Charleston joint graduate program in historic preservation.

#Architecture #HistoricSites #DraytonHall #conservation

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