The following is an excerpt from Green Restorations by Aaron Lubeck (New Society Publishers, 2010). Room by room and system by system, Lubeck discusses the steps for restoring historic buildings using sustainable practices and green building techniques, including the massive financial incentives of residential historic tax credits. This excerpt is from Chapter 9, “Structural.”
Old homes were designed with structural logic much different from today’s. New homes are designed with strict adherence to the lumber sizing and spans listed in the building code. Old homes were built with common sense, logic and feel. Most homes built between 1880 and 1930 were built on inferior footing. These old footings offer foundation walls little support, which then may support often overspanned joists and girders. Historic home foundations were subject to improper load calculation, inferior footings and substandard mortar. Old framing systems can be vastly over- or under-built. Each is a potential point of failure. It is crucial to understand such risks during a rehabilitation.
Structural failure is a phrase that scares the average Joe. Unfortunately, most structures subject to a century of seasonal expansions, water, humans, animals, deferred maintenance, improper storage and poor footings are destined to have some structural issues that need to be addressed. Combine construction flaws, time and the dreadful soil of central North Carolina, and it’s rare that I see an old home that doesn’t have some sort of structural problem. Structural issues can all be addressed, however, and most are simple (but laborious) fixes.
The structure of a building is formed by foundation and framing. The foundation is a structure that transfers loads to earth. It keeps earth and wood apart. The concept is simple: A house is heavy, so a foundation spreads that load over an area suitable for the earth to handle. The average two-story Queen Anne Victorian weighs between 30 and 60 tons, enough for three 20-ton jacks to support the whole thing (theoretically, but don’t try it at home).
The framing forms the structure, defines separate rooms and carries the floor, wall and roof loads to the foundation.
I’ll discuss the most common foundation and framing techniques, common problems and how each is typically fixed. I’ll also discuss basic preservation and sustainability issues related to the structure.
Footings and Foundations
Foundations are constructed of footings and foundation walls. A footing is the belowground mass, generally made of concrete or brick, that supports the foundation wall. It is sized to transfer the weight of the entire structure to ground. A footing must sit on stable soil and not backfill, which compresses easily. If the soil is not stable, the footing is more likely to fail. Today footings are eight inches wider than the wall or pier they support (e.g., a 12-inch-wide wall requires a 20-inch-wide footing) and deep enough to sit below the frost line, the depth at which groundwater is expected to freeze in a respective climate.
Footing construction varies greatly on old homes. Larger stately homes may well be on large and well built footings, though it would be rare to find the metal reinforcing bar (rebar) required today. Many houses sit on a soldier course, which is nothing more than an extra course of bricks at the bottom of the brick pier or foundation wall. Soldier courses are commonly found above the frost line and are prone to mortar breakdown, especially under pressure of water.
The foundation wall carries loads from the exterior sill beam framing to the footing. Piers support interior girders and are made of a variety of materials — stone, masonry and poured-in-place concrete are all common. Mortar joints offer little resistance to unbalanced lateral forces (such as a backfilled basement wall), so tall, thin, unreinforced masonry curtain walls are prone to failure.
The foundation wall also defines the area underneath the main living space. In the northern United States, basements are common, while in the South, crawl spaces are more typical. A below-grade basement or crawl space is intrinsically unstable and problematic; the pressures of earth and groundwater are predisposed to assault its footing and wall. Based on their porosity, soils hold and shed varying amounts of water. The basement floor can actually be below the water table, most likely in spring when snow is melting both on the roof and ground.
Water causes nearly all problems in foundations. Water against a foundation wall exerts hydrostatic pressure — water trying to get from areas of high pressure (poor draining soil) to areas with less (your basement). Frost heave happens when water freezes in poorly draining soil, then expands and pushes the footing, foundation and house upward. Any footing above the frost line will rise and fall with the freeze thaw cycle. Typical frost lines vary from four feet in Maine to less than a foot in the Southeast, and footings must be at least as deep as the frost line to avoid frost heave. The best solutions for water problems are to grade, divert roof runoff, dampproof or waterproof the foundation.
Grading refers to the slope of earth around the foundation. Code requires a 5 percent slope to 6 feet around the foundation, and many old homes fail this bench mark. Any place where a slope does not meet such grade is subject to water problems. Solutions include swales, which create a low point six to 12 feet from the foundation to capture water, and French drains, a subsurface swale covered with perforated pipe and drainage
gravel, allowing surface grading to remain unaltered.
Roof runoff (rainwater) is diverted away from the foundation by either gutters or proper grading. In cold climates gutters cause ice dams which can result in roof leaks and eave damage, which is why some forgo gutters in favor of ground-based drainage often involving plastic water barrier protection covered with decorative gravel or a continuous pitched concrete grade.
Dampproofing keeps most water out of the foundation, but allows water through in a torrential rain. A perforated pipe is set just below the exterior of the footing, sloped to direct water away via gravity or sump pump.A dampproofing approach may or may not include a latex waterproofing paint on the foundation wall, now required by many local ordinances on new construction.
Waterproofing keeps all water from entering the foundation and is necessary if using the basement as finished space. It is much more involved than dampproofing. Waterproofing can be done inside or outside the foundation wall; it’s better to stop water before it enters the structure though that does requires a more expensive exterior waterproofing. First, a thick, impermeable dimple sheet is installed to keep water out of the foundation. Next, just below the footing, a perforated pipe is set which captures groundwater and drains it to either daylight or a sump pump. A sump pump is used to remove water accumulated in a sump pit, commonly placed at the low point in a basement, crawl space or exterior.
Bentonite clay is a natural waterproofing material that functions by suspending water in a gelatinous form. Less natural but more common is extruded polystyrene foam board insulation (XPS), which is a cheap and common detail on new foundations, particularly in northern climates. It is nonpermeable (except at its seams), helping resist water infiltration.
It’s important to differentiate between problems caused by surface water runoff and a high water table. The first can be fixed rather simply, the latter may be impossible. High water tables enter the structure through the wall and the basement floor. Concrete is porous, and no match for such pressure. Today, vapor barriers of 4- or 6-millimeter-thick polyethylene are installed under slabs and are an excellent tool against such an assault. Old homes won’t have such a barrier. A possible fix is to install a vapor barrier and pour a new slab. Still, while it’s feasible to waterproof a new footing and all its transition points it’s nearly impossible to waterproof an old footing. Footing drains outside should relieve some of the pressure, but not all. If there is evidence of a high water table, it’s recommended that you leave the basement unfinished. Finished basements require a 100 percent success rate against water, and the costs to insure such a rate would be too excessive.
Lastly, a wet basement can occasionally be caused by a blocked drain tile, failed sewer or stormwater line. Unfortunately, private lines are not easily explored by anything short of excavation. Plumbers do have pipe camera tools that can avert a major dig, though many are cautious about sending an expensive piece of equipment up a pipe with an unknown blockage.
Fixing the foundation, or rebuilding it entirely, requires the temporary transfer of the house’s loads above in order to perform the work. Holding the house consists of temporarily lifting the load-bearing girders or sills on a portion of the house just enough to remove failed members. One-half inch is usually enough. Raising the house consists of lifting the entire house at once. One foot elevation to an entire story is typical. Raising strategies might be considered when a usable basement is desired in tandem with major foundation reconstruction. The house may be raised as much as eight to ten feet to allow for the addition of a new floor below.
The merits of permanently raising a house are an ongoing debate in the preservation community. An argument that the house must be raised to ensure its longevity as a healthy structure makes a stronger case than arguing it must be raised because the owner wants a game room. Generally speaking, lifting the house significantly disrupts the streetscape by creating an unusually tall structure, particularly so if raising more than a few feet. In New Orleans, of course, it’s argued that raising houses is necessary now to survive potential future flooding, so the debate continues.
It also is impossible to raise a house without reconstruction of chimneys, since the practice will throw all your hearths off elevation. Floors are raised while chimneys are not. Balloon-framed homes can be more difficult to jack than platform-framed structures. Since a balloon-framed floor may be tacked onto the studs with nothing more than a few nails, jacking up the floor may lift only the floor system, while not lifting the walls. To correct this, jacking may be required from the inside and out, and sometimes a temporary wall between floors is needed to ensure the whole structure rises in tandem.
New Foundation Construction
After the house has been lifted, a new foundation wall can be built. Often a pier and curtain wall will be replaced by a continuous masonry wall made of either brick, CMU block or both. Wood wicks water from masonry, which is why building codes now specify that any wood in contact with masonry now must be pressure-treated. If a new foundation wall is supporting an existing non-treated sill, termite protection should be installed — either pressure-treated wood or a termite flashing made of 20 gauge aluminum. Be sure to install sleeves for utilities. Short PVC stubs suffice for electric, water and HVAC lines.
As with all exterior features, try to match any foundation detailing. If the piers protruded beyond the curtain wall on the exterior, or had some masonry corbelling for example, it would be good to restore that detail. I’ve found preservation boards to be reasonably flexible so long as the old foundation wasn’t extremely distinctive.
Reprinted with permission from Green Restorations, published by New Society Publishers, 2010.