What the 345 Rule Means for Your Build

Ever heard someone mention the 345 rule on a building site and wondered what it actually does? In plain terms, the rule is a quick way to check if your foundation, walls or slab meet basic safety standards. It looks at three key numbers – a 3% slope, a 4‑inch drainage layer and a 5‑inch buffer of compacted material. If those three lines line up, you’re probably on solid ground.

Why Builders Use the 345 Rule

The rule saves time. Instead of pulling out a calculator and digging through specs, you just glance at the numbers. A 3% slope keeps water from pooling under the slab, which means fewer cracks later. A 4‑inch drainage layer of crushed limestone or grit lets water flow away, and the 5‑inch compacted fill creates a firm base that spreads loads evenly.

At Lime Hillock Construction Material Resources we supply the limestone and fill that fit these specs perfectly. Using the right material means the 345 rule works every time, not just on paper.

How to Apply the 345 Rule on Site

Step 1: Check the ground slope. Use a simple level or a laser grade line. If the rise over run is about 3% (roughly 3 cm rise per 1 m run), you’re good.

Step 2: Lay a drainage layer at least 4 inches thick. Spread crushed stone evenly, then tamp it down. This layer acts like a sponge, moving water away from the foundation.

Step 3: Add a 5‑inch layer of well‑compacted fill. Choose a mix that matches your soil type – sand‑loam works for most UK sites. Compact in 2‑inch lifts with a plate compactor, checking each lift for firmness.

Step 4: Pour the concrete slab or start wall footings. Keep an eye on the slab thickness and reinforcement – those details stay separate from the 345 rule but are just as important.

Step 5: Inspect after the concrete cures. Look for any spots where water still gathers. If you see puddles, add more drainage or adjust the slope.

Following these steps doesn’t guarantee a perfect build, but it cuts down the biggest causes of settlement and moisture problems. That means fewer foundation cracks, lower repair costs, and a house that stays stable for decades.

Remember, the 345 rule is a guideline, not a law. Complex sites with high clay content or steep gradients may need extra measures. When in doubt, get a structural engineer to weigh in – especially for larger projects.

Bottom line: keep the slope at 3%, the drainage layer at 4 inches, and the compacted fill at 5 inches. Those three numbers are easy to remember, cheap to achieve, and they protect your investment.

Looking for the right limestone or fill to meet the rule? Lime Hillock has local quarries that supply the exact grades you need, delivered straight to your site. Call us to chat about how the 345 rule can fit into your next project plan.