Slope, aspect and buildingHow the lay of a tract decides where a house, a drive and a septic site can go

Every slope rule in this guide starts from the same arithmetic: slope as a percent, the rise in height over the distance measured, which Floyd County's Land Division Ordinance also uses for a road's grade.

So ground that climbs 20 feet over 100 feet measured is a 20 percent slope, and ground that climbs 50 feet over the same 100 feet is 50 percent. A percent is not an angle, and mixing the two up makes steep ground sound gentler or harsher than it is. The short definitions of slope and grade are in the slope and grade glossary entry.

The numbers matter here because so much of the ground in these counties is steep. For the short answer on whether a steep lot can carry a house at all, see whether you can build on a steep lot. What follows is the order in which a steep site gets read.

Aspect is the compass direction a slope faces: a slope that falls away to the south has a southern aspect. The Virginia Department of Forestry's one-line definition is quoted in the aspect glossary entry.

No Virginia rule ties where a house may be built to the direction its slope faces. None of the building, septic, well, erosion, subdivision or ridge rules in this guide turns on aspect, and no source behind it connects aspect to a house's energy use or its price.

Aspect still earns its place in a read of the ground for one practical reason. A slope faces the direction it falls, so aspect tells you which way is downhill from any point. The well rule in the next step turns on exactly that question, whether a well sits downslope of a sewage system, and it measures the answer in percent of slope, not in compass points.

A level house pad on sloping ground is made by cutting the high side and filling the low side, and a drive that climbs to it is made the same way. Grayson County's plan describes it directly: developing excessive slopes into a level grade "requires grading, cut and fill and unique challenges for addressing hydrological features, storm water runoff and prevention of erosion."

The Virginia Erosion and Stormwater Management Act defines land disturbance to include the clearing, grading, excavating or filling of land. Under 9VAC25-875-70 and 9VAC25-875-250, disturbing 10,000 square feet or more outside a Chesapeake Bay Preservation Area is regulated, and a locality may reduce that threshold. Carroll County applies its plan requirement at 10,000 square feet or more of exposed soil, and the plan, its single-family substitute and the county's land-disturbing permit are explained in what an E and S plan is.

The state's 19 minimum standards for erosion and sediment control include two that bear directly on hillside grading:

• Cut slopes and fill slopes: designed and constructed to minimize erosion.
• Runoff on those slopes: concentrated flow goes down a cut or fill slope only inside an adequate channel, flume or slope drain.

Another standard protects properties and waterways downstream from sediment deposition, erosion and damage. Where a new drive meets a hillside public road is a separate VDOT question, taken up in what VDOT considers a safe entrance.

A house site near the top of a mountain can meet one more rule, but only where a locality has chosen to write it.

Va. Code 15.2-2295.1 lets a locality with a protected mountain ridge adopt an ordinance governing the height and placement of tall buildings on that ridge. The statute permits a ridge ordinance; it does not require one. What the statute counts as a ridge, measured down from the crest, is set out under ridge line, and what that means for a building site near the top is taken up in whether you can build on a ridge top.

The foundation comes last because it depends on everything above it: where the pad was cut, how much fill went in, and what the ground underneath turned out to be. The building code puts the call with the building official: "When determined necessary by the building official, construction documents shall include adequate detail of the structural, mechanical, plumbing or electrical components." Engineered documents, including relevant computations, are sealed by the registered design professional responsible for the design. The distance a footing keeps from a slope, and the design of a foundation on one, are for the building official and a design professional to settle.