Shores are the backbone of site-cast concrete construction, yet many contractors underestimate their importance. At Ladder 48, we’ve seen firsthand how improper shoring leads to structural failures, safety hazards, and costly delays.

This guide walks you through everything you need to know about selecting, installing, and managing shores on your projects. We’ll cover the calculations, placement strategies, and common pitfalls that can derail your concrete work.

What Shores Actually Do in Concrete Construction

Shores are temporary vertical or angled supports that hold up formwork and wet concrete until the material gains enough strength to support itself. In site-cast concrete work, shores transfer the weight of fresh concrete downward through a system of posts, beams, and bases to the ground or existing structure below. The load path matters tremendously.

When you pour concrete, it weighs roughly 150 pounds per cubic foot in its wet state. A typical floor slab 8 inches thick exerts about 100 pounds per square foot of pressure on the formwork below. That pressure doesn’t just disappear-it flows directly through your shore system. If shores aren’t properly sized and positioned, that massive downward force finds the weak point and failure happens fast.

Contractors often underestimate how much load a single shore must carry, which leads them to space shores too far apart or use undersized posts. The concrete industry standard, found in ACI 347 guidelines, recommends specific shore spacing based on concrete strength, slab thickness, and formwork type. Ignoring these calculations is how accidents happen.

Steel Versus Timber Shores

Steel shores dominate modern construction because they’re reusable, predictable, and their load capacity is clearly marked. Adjustable steel shores with screw jacks let you fine-tune height and make them ideal for uneven substrates. Timber shores still appear on some projects, but they’re less reliable because wood strength varies by species, moisture content, and grain direction.

A timber shore rated for 50,000 pounds might only handle 25,000 pounds if it absorbed moisture on site. Steel shores eliminate this guesswork. Most contractors prefer steel for anything larger than small residential pours because the consistency reduces liability and inspection headaches.

The Real Cost of Early Shore Removal

Concrete gains strength gradually. At three days, it typically reaches only 25 to 40 percent of its design strength, depending on mix design and temperature. Contractors often remove shores at seven days-a common but dangerous practice-which frequently occurs before concrete reaches 75 percent strength.

Temperature swings matter significantly. Cold weather slows hydration dramatically. Concrete poured in 40-degree conditions takes roughly twice as long to reach target strength compared to 70-degree conditions. Removing shores on a fixed schedule without testing actual concrete strength has caused slab failures weeks after removal.

You must test concrete strength on-site before you approve shore removal. Either rebound hammers or breaking test cylinders provide accurate readings. This step adds days to the schedule but prevents catastrophic failures that cost far more than the testing expense. With proper strength verification in place, you’re ready to explore the specific calculations that determine how many shores your project actually needs.

How to Calculate and Position Shores Correctly

Calculate Total Load on Your Shore System

Calculating load capacity is where most contractors go wrong, and it’s the single biggest reason shores fail. The weight your shores must support depends on four factors: concrete density, slab thickness, formwork weight, and construction live load. Fresh concrete weighs 150 pounds per cubic foot, but your formwork and temporary bracing add another 10 to 20 pounds per square foot depending on the system you’re using.

ACI 347 requires you to account for a 20-pound-per-square-foot construction live load on top of everything else. For an 8-inch slab with standard formwork, that’s roughly 130 pounds per square foot total pressure on your shore system. If you pour a 20-by-20-foot section, that’s 52,000 pounds of downward force that must travel through your shores to the ground.

Spacing shores too far apart concentrates this load on fewer posts, which is why contractors often see sudden failures when they try to stretch shore spacing beyond recommended limits. The industry standard from ACI 347 specifies maximum shore spacing of 4 feet on center for most concrete work, though this can decrease to 2 feet on center for thicker slabs or softer substrates. Calculate the total load on each individual shore by multiplying the tributary area by the total pressure per square foot, then verify that your shore’s rated capacity exceeds this number by at least a 25 percent safety factor.

Account for Temperature Effects on Strength Gain

Temperature during curing affects strength gain dramatically, which means your shore removal timeline must account for actual conditions on your job site. Concrete poured in 50-degree weather gains strength at roughly half the rate of concrete poured in 70-degree conditions according to standard concrete strength development curves. This means the seven-day rule doesn’t apply universally; cold weather projects need extended shoring periods that many contractors underestimate during initial scheduling.

Inspect Shores Weekly for Settlement and Movement

Inspection and maintenance during construction separate contractors who finish projects safely from those who experience costly delays or failures. Walk your shore system weekly and look for settlement, tilting, or visible deflection in the formwork above. Even 1/8 inch of deflection signals a problem that needs immediate attention. Check all connection bolts and wedges at least twice weekly because vibration from concrete placement loosens fasteners over time.

Test Concrete Strength Before Removal

Test concrete strength before you remove any shores, using either on-site test cylinders broken at the lab or rebound hammers for quick field measurements. Don’t rely on calendar days alone; a 14-day-old slab in cold weather may have less strength than a 7-day-old slab poured in warm conditions. Many contractors remove shores in stages rather than all at once, which reduces the sudden load increase on remaining shores. Remove shores from the center of the span outward, leaving perimeter shores in place longer to support edge conditions. This staged approach also lets you monitor deflection as you progress.

Document Your Shoring Process

Keep detailed records of when shores were installed, when concrete was placed, ambient temperature during curing, and when each shore was removed. These records protect you if structural issues emerge later and provide data for improving your process on future projects. With your shores properly calculated, positioned, and monitored, you’re now ready to identify the mistakes that derail even experienced contractors and learn how to avoid them on your next project.

Common Mistakes That Destroy Shore Systems on Job Sites

Test Soil Bearing Capacity Before Installation

Most shore failures happen because contractors skip the verification step between calculation and installation. You calculate the load correctly, but then you install shores without confirming that the actual ground conditions match your assumptions. Soft soil, uneven substrates, or saturated ground can reduce bearing capacity by 50 percent or more compared to what you assumed during planning.

Before you place a single shore base, test the soil bearing capacity with a simple bearing plate test. Place a 12-by-12-inch steel plate on the ground where your shore base will sit, load it gradually, and measure settlement. If the soil settles more than 1/4 inch under 50 percent of your calculated load, you need thicker base plates or additional shoring.

This testing takes two hours and costs almost nothing compared to a slab failure that forces you to demolish and repour thousands of square feet of concrete. Contractors often skip this step because they’re behind schedule, then spend three weeks dealing with structural problems that could have been prevented with one afternoon of soil testing.

Account for Temperature When Planning Shore Removal

Removing shores too early happens constantly because contractors mistake calendar days for concrete strength. A seven-day-old slab poured in 50-degree weather has roughly 35 to 40 percent strength according to ACI strength development curves, while a seven-day slab poured in 75-degree weather reaches 60 to 70 percent.

Cold weather projects need 14 to 21 days of shoring, not the standard seven. Temperature fluctuations matter equally. A slab that gains strength for five days in warm conditions, then experiences a cold snap, loses strength development momentum completely. You must test actual concrete strength on your job site before removing any shores.

Rebound hammers provide quick field results in 30 seconds, or you can break test cylinders at a local lab within 24 hours. The cost runs 150 to 400 dollars depending on your method, and it eliminates the guesswork that causes failures. Document the ambient temperature during the entire curing period and correlate it with your strength test results. This data protects you legally and gives you accurate timelines for future projects in similar conditions.

Remove Shores in Stages to Catch Problems Early

Staged shore removal reduces risk significantly. Remove shores from the center of the span outward over a three-to-five-day period rather than stripping all shores at once. This approach reveals deflection problems immediately while you still have shores in place to add support.

If deflection exceeds 1/4 inch during staged removal, stop immediately and leave remaining shores in place longer. The extra labor cost for staged removal is negligible compared to the liability of a catastrophic failure (and the schedule delays that follow). Staged removal also distributes load increases gradually, which prevents sudden stress spikes that crack concrete or damage formwork systems.

Final Thoughts

Proper shoring separates contractors who deliver safe, on-schedule projects from those who face structural failures and costly delays. In site-cast concrete construction, shores are used to transfer massive loads safely from wet concrete to the ground, and this responsibility demands precision at every stage. The calculations matter. The soil testing matters. The strength verification before removal matters.

We at Ladder 48 work with contractors across the industry, and we’ve seen how the best performers treat shoring as a non-negotiable foundation for their concrete work. They calculate loads accurately using ACI 347 standards, test soil bearing capacity before installation, and verify concrete strength with actual on-site testing rather than guessing based on calendar days. They remove shores in stages to catch problems early, which costs slightly more upfront but eliminates the catastrophic failures that destroy schedules and budgets.

Your next site-cast concrete project starts with a shoring plan that accounts for your actual site conditions, not generic assumptions. Temperature during curing, soil conditions, formwork weight, and construction live loads all factor into how long shores stay in place and how many you actually need. Ladder 48 helps contractors attract more local customers and generate qualified leads through tailored SEO strategies that boost your online visibility.