Concrete Foundation Types Explained

The Texas Hill Country sits on some of the most geologically complex terrain in the state. Burnet, Llano, and San Saba counties feature a patchwork of Precambrian granite, Cretaceous limestone, and expansive Vertisol clay soils — sometimes all on the same 10-acre tract. Expansive clay (classified as CH or CL soils under the Unified Soil Classification System) can swell up to 10% in volume during wet seasons and shrink during summers that routinely push past 96°F. That movement generates soil pressure capable of cracking unreinforced slabs and shifting pier footings.

The Burnet County permitting office and the International Building Code (IBC 2021), Section 1803, both require a geotechnical investigation for most new residential and commercial construction. In practice, this means a soil boring or test pit report that identifies bearing capacity, plasticity index, and depth to rock — all of which directly determine which foundation system is structurally appropriate and code-compliant for your site.

In our experience working across Highway 281, RR 1431, and the back roads between Burnet and Marble Falls, no two lots behave the same. A site 200 feet from a creek bottom may have 6 feet of soft alluvial clay before hitting limestone, while a neighboring ridge lot hits solid rock at 18 inches. Choosing the right foundation from the start is the single most impactful structural decision you'll make.

A slab-on-grade foundation is a single, continuous concrete pad — typically 4 to 6 inches thick — poured directly on prepared subgrade. It is the most widely used foundation type for single-story custom homes, barndominiums, and metal buildings on relatively flat terrain in Central Texas. When properly engineered, a slab-on-grade is cost-effective, durable, and fast to construct.

In the Hill Country, virtually all slab-on-grade foundations should be post-tensioned rather than conventionally reinforced when expansive soils are present. Post-tensioned slabs use high-strength steel cables tensioned to approximately 33,000 pounds per cable after the concrete cures, creating a compressed slab that resists cracking under soil movement. The Post-Tensioning Institute (PTI) standard DC80.3 governs the design of residential post-tensioned slabs and is referenced by the Texas Residential Construction Commission guidelines.

Typical costs for a slab-on-grade in Burnet County range from $6 to $10 per square foot for a standard 4-inch residential slab, and $9 to $14 per square foot for a post-tensioned engineered slab with turndown edges and proper vapor barrier. A 2,000 sq ft barndominium slab runs roughly $12,000–$28,000 depending on site prep, rebar schedule, and engineer specifications. We always recommend a minimum 6-mil polyethylene vapor barrier per IRC Section R506.2.3 to address the Hill Country's occasional freeze events from December through February.

Pier and beam foundations elevate the structure 18 to 36 inches above grade on a grid of concrete piers topped by wood or steel beams. This system has been used in Texas for over a century and remains an excellent choice for sloped lots, sites with high moisture variability, or locations where future utility access under the floor is desirable.

On Hill Country sites with more than 3–4 feet of grade change — common on properties near Lake Buchanan, the Colorado River corridor, or the cedar-covered slopes between Burnet and Llano — pier and beam systems allow a level floor plane without the massive cut-and-fill earthwork that a slab would require. That earthwork savings alone can offset the higher foundation cost. Pier and beam construction typically runs $12,000–$22,000 for a 1,500 sq ft footprint in our service area, compared to $9,000–$18,000 for a comparable slab.

The crawl space beneath a pier and beam home requires proper ventilation per IRC Section R408 — a minimum of 1 square foot of ventilation per 150 square feet of crawl space area. In Burnet County's humid summers and occasional winter freezes, inadequate crawl space ventilation leads to wood rot, mold, and pest intrusion within 5–10 years. We install 6-mil ground cover vapor barriers and ensure cross-ventilation on every pier and beam project. Encapsulated crawl spaces with conditioned air are an upgrade option that dramatically improves moisture control and energy performance.

Drilled pier foundations — also called caissons or drilled shafts — are cylindrical concrete columns bored deep into the earth until they reach stable bearing strata. In the Hill Country, this typically means drilling through 10 to 25 feet of expansive clay or decomposed rock to reach the underlying Cretaceous limestone bedrock, which has an allowable bearing capacity of 8,000–12,000 psf or higher.

Drilled piers are the preferred solution when soil reports show a plasticity index (PI) above 35 — a threshold frequently exceeded in Burnet, Lampasas, and Bell County soils. They are also required by most structural engineers for two-story custom homes, commercial metal buildings, and any structure where differential settlement would be catastrophic. The Texas Department of Licensing and Regulation (TDLR) mandates that any drilled pier system for a commercial structure over a certain threshold be designed by a licensed Texas PE.

Pier diameters for residential use typically range from 10 to 18 inches, with depths of 15 to 30 feet. A grade beam — a reinforced concrete beam at or below grade — connects the pier tops and supports the floor system. Total cost for a drilled pier and grade beam system runs $18,000–$40,000+ for a 2,000 sq ft home, depending on pier count, depth to rock, and grade beam complexity. Based on our projects in Liberty Hill and Georgetown, where Blackland Prairie clays are especially severe, this investment consistently outperforms cheaper alternatives over a 20–30 year horizon.

A stem wall foundation combines a perimeter concrete wall (the stem wall) with an interior slab, creating a structural boundary that transfers loads from the building frame down to the footing. This system is extremely common for metal buildings, barndominiums, agricultural structures, and garages throughout our service area.

For metal building construction, the stem wall serves as the anchor point for anchor bolts that secure the structural steel frame. Most pre-engineered metal building manufacturers — including those compliant with Metal Building Manufacturers Association (MBMA) standards — require a stem wall or thickened-edge slab designed specifically to their anchor bolt templates and base plate reactions. A standard 6-inch stem wall with a 16x8-inch continuous footing and #4 rebar is typical for a single-story metal building up to 40 feet wide.

In Burnet County, stem wall foundations for metal buildings and barndominiums typically cost $8–$13 per square foot of enclosed floor area, which includes the perimeter stem wall, interior slab, and all concrete work. For a 40x60 foot barndominium (2,400 sq ft), that translates to roughly $19,200–$31,200 in foundation costs. TMB Construction handles all concrete and foundation work in-house with our own crew — no subcontractors — which keeps costs predictable and quality consistent. Call 830-289-3852 to get a line-item estimate for your specific building footprint.

Foundation costs in the Texas Hill Country are consistently 10–20% higher than statewide averages due to difficult terrain, rock drilling requirements, and the need for engineered solutions in expansive soil zones. Material costs for ready-mix concrete in the Burnet-Marble Falls corridor run approximately $155–$185 per cubic yard as of 2025, compared to $130–$160 in major metro areas.

Here is a realistic cost comparison for a 2,000 square foot residential footprint in Burnet County: A conventional slab-on-grade runs $12,000–$20,000. A post-tensioned engineered slab runs $16,000–$28,000. A pier and beam system runs $15,000–$26,000. A drilled pier and grade beam system runs $22,000–$42,000. A stem wall with interior slab runs $16,000–$26,000. These figures include labor, materials, rebar, forming, concrete, and basic site prep but exclude grading, excavation of rock, or engineered fill placement.

Permit fees through the Burnet County permitting office for residential foundation work typically range from $150 to $600 depending on project valuation. Most foundation inspections require 24-hour notice. Engineering fees for a site-specific foundation design from a licensed Texas PE run $800–$2,500 for residential projects. TMB Construction can coordinate the entire permitting and engineering process — our team has navigated Burnet County plan review dozens of times and knows exactly what documentation is required.

No foundation decision should be made without a soil investigation. IBC 2021 Section 1803.2 requires that the soil classification, bearing capacity, and any expansive, compressible, or shifting soil conditions be identified prior to foundation design. In Burnet County, this typically means hiring a geotechnical engineering firm to perform soil borings to a depth of 15–20 feet, collect samples at 5-foot intervals, and produce a report with bearing capacity recommendations, plasticity index values, and a foundation type recommendation.

Geotechnical reports for residential sites in the Hill Country typically cost $800–$1,800 and take 1–3 weeks to complete. The report will identify your soil as one of three general categories relevant to foundation selection: shallow rock (foundation solution is simpler and cheaper), deep expansive clay (requires drilled piers or post-tensioned slab), or mixed profile (requires site-specific engineering judgment).

In our 7+ years of Hill Country construction, we've seen property owners try to skip the geotech report to save $1,200 — then spend $35,000 fixing a slab that cracked within 3 years because the builder guessed wrong on soil conditions. The Burnet County permitting office increasingly requires engineered foundation plans stamped by a Texas-licensed PE (per TDLR requirements) for residential projects over 1,500 sq ft. That PE will require a geotech report regardless. Budget for it from the start.

Foundation construction timelines in the Hill Country are affected by rock drilling requirements, concrete cure schedules, permit lead times, and seasonal weather. Here is what to realistically expect for each foundation type as of 2025 in Burnet County.

A slab-on-grade takes 1–2 weeks from site prep through pour, then requires 7 days before framing begins (28 days for full 4,000 psi design strength). A post-tensioned slab adds 1–3 days for cable installation and tensioning after cure. A pier and beam foundation takes 2–3 weeks including pier drilling, beam installation, and subfloor framing. A drilled pier and grade beam system takes 3–5 weeks: 1 week for drilling, 1 week for rebar cage placement and concrete, 1 week for grade beam forming and pour, plus cure time. A stem wall foundation takes 10–18 days including footing excavation, forming, pour, cure, and interior slab.

Burnet County permit review typically takes 5–15 business days for residential foundation permits. Spring and summer (March through August) are peak construction season, and concrete contractor schedules and ready-mix delivery windows can add 1–2 weeks to any timeline. We recommend starting the permitting and engineering process at least 6–8 weeks before your desired start date. Call TMB Construction at 830-289-3852 to discuss scheduling and current lead times for your project.