Barndominium Builders in Charleston, SC

Barndominium builders who take on a Charleston project start with three numbers before they talk about floor plans: a 150 mph design wind speed, seismic design category D2 and the flood zone on the parcel. Those come straight from the City of Charleston's own design criteria table, and they shape everything from the steel frame to the slab height. Charleston is a big, low city, covering about 135.5 square miles (roughly 20.5 of them water) at an official elevation of just 3 feet, and it grew from 120,083 residents in 2010 to 150,227 in 2020.

Very little of that growth is on the old peninsula. Most of the land where a barndominium with a real shop makes sense is out on the edges: Johns Island between the Stono and Kiawah rivers, the far reaches of West Ashley between the Ashley and the Stono, and the Cainhoy Peninsula between the Wando River and Nowell Creek. Each of those areas is ringed by tidal rivers, which the city describes as evidence of a drowned coastline. That geography is the reason a Charleston barndo is an engineering project first and a design project second.

The design table, line by line

The City publishes the criteria its reviewers expect to see on plans under the 2021 South Carolina building codes. Here is what each line means for a metal or post-frame home:

• Ultimate design wind speed, 150 mph. Roof purlins, wall girts, fastener spacing, the big shop doors and the anchor bolts all get sized for this. Overhead doors and windows need design-pressure ratings that match.
• Seismic design category D2. This is unusual for the East Coast, and it means lateral bracing and connections deserve the same attention as wind uplift.
• Ground snow load of 5 pounds per square foot and a frost depth of 0 inches. Snow will not drive the roof design here; wind will.

Why D2 is on the table: August 31, 1886

The seismic category is not a paperwork quirk. On the night of August 31, 1886, an earthquake with an estimated moment magnitude around 7.0 struck just northwest of the city. Maximum intensity reached X on the Mercalli scale, and roughly 2,000 buildings were damaged across the Southeast. In Charleston nearly all of the 8,000 city structures had interior damage or broken windows, and brick buildings made up 81 percent of the building damage. Sand boils from liquefied ground damaged roads, bridges and fields.

For a barndominium, the lesson is about mass and ground. A steel or wood frame with metal skin is light compared with masonry, which works in its favor, but the foundation still sits on soft coastal soil. We plan the slab and footings around what the site actually is, and we want the soils assumption on the plans to be a real one.

Hugo and the 150 mph line

Hurricane Hugo came ashore at Charleston Harbor on September 21, 1989, as a Category 4 storm. About three-quarters of the homes in the historic district were damaged to some degree, and total damage ran over 2.8 billion dollars. Today's wind design is the response to storms like that. On a barndominium, the weak points are usually the large openings: a 12- or 14-foot rollup door that is not rated for the design pressure can fail and pressurize the whole building. We spec doors, windows and the connections that hold the roof down as one system.

Flood maps on two sides of the county line

Most of Charleston is in Charleston County, but part of the city lies in Berkeley County, and the City's criteria list a separate current effective flood map for each: January 29, 2021 for Charleston County and December 17, 2018 for Berkeley County. The city first entered the National Flood Insurance Program in 1971. Before a footprint is drawn we look up the parcel on the effective map, find the flood zone and base flood elevation, and set the finished floor and the shop slab from that. A barndo shop can often be designed with flood-resistant materials and openings on the ground level while living space sits higher, but the zone decides what is allowed.

Soft ground at the marsh edge

Land near the marsh is a different material from upland pine ground. The USDA Bohicket soil series, a typical tidal-marsh soil on the South Carolina coast, is described as very poorly drained, very slowly permeable and flooded twice daily by sea water; it is too soft even for cattle grazing. Nobody builds on the marsh itself, but transition ground near it can be wet and compressible. That is why a geotechnical look, and sometimes deeper foundations, belongs in the budget conversation for waterfront lots.

Sewer, septic and city versus county

Inside city limits many parcels can reach public sewer. Out on rural Johns Island or in unincorporated Charleston County, a lot may need a septic system. South Carolina requires an SCDES septic permit on property not served by public or community sewer, and the county cannot issue a building permit without it. On wet coastal soils, the septic evaluation can decide where, or whether, the house goes.

If you own land in or around Charleston, call (803) 784-7083 or

Get a Free Estimate

. Send us the parcel number and we will start with the flood map and the design criteria, before anyone falls in love with a floor plan.