Almost every “failed driveway” call we get traces back to the same root cause: the sub-base. The blocks, slabs or tarmac on top get the blame, but they’re rarely the problem. What’s underneath them is.
This is the bit no one photographs and no one talks about in sales pitches — which is exactly why corner-cutting contractors skip it. Here’s what should be there, why it matters, and how to spot a bad job before you’ve signed for it.
What “sub-base” actually means
A driveway is a layered system. From the bottom up, a properly built block paving driveway looks like this:
- Sub-grade — the natural ground, levelled and compacted
- Geotextile membrane — stops the layers above mixing into the soil below
- MOT Type 1 sub-base — typically 100–150mm of crushed limestone or granite, compacted in layers
- Laying course — usually 30–40mm of sharp sand
- Block paving — the bit you actually see
- Kiln-dried jointing sand — vibrated into the gaps between blocks
The blocks are 50–60mm thick. The sub-base is 100–150mm thick. Most of a driveway, by depth and cost, is what you can’t see.
Why sub-base failure shows up as “sinking blocks”
Here’s the chain of cause and effect when a sub-base is too shallow, poorly compacted, or laid without a membrane:
- Rain soaks through the joints into the laying course
- Without proper sub-base depth, water sits in the lower layers
- Repeated freeze-thaw cycles (every Glasgow winter) push particles around
- Over 1–3 winters, low spots form
- Blocks above the low spots drop slightly — they “sink”
- Once one block moves, its neighbours follow
- Water pools in the dips, accelerating the failure
By year 3–5 the driveway looks tired. By year 7–10 it’s dangerous (rocking blocks, ankle-twisting dips). The blocks themselves are still fine — but you’re paying for a full re-lay.
The three most common sub-base shortcuts
When a quote comes in suspiciously cheap, it’s almost always one (or all) of these:
1. Excavating too shallow
Proper depth for a domestic driveway is 200mm minimum (100mm sub-base + 30–40mm laying course + 50mm blocks + 10mm working tolerance). Skimping to 120–150mm saves a digger day and a couple of grab-load skips. It also guarantees movement within 5 years.
2. Skipping the membrane
A roll of commercial-grade geotextile costs about £80 for a typical drive. It stops the sub-base aggregate from migrating into the sub-grade and stops weeds from below. Skipping it saves the £80 and costs the driveway 3–5 years of life.
3. Not compacting in layers
MOT Type 1 needs to be laid and compacted in two passes of 50mm each, not dumped 100mm thick and rolled once. The vibrating plate compactor can only properly compact 50mm at a time. A single pass leaves voids that collapse later.
How to spot a bad sub-base before you sign
You can’t inspect a sub-base after the blocks go down. So ask the right questions before:
- “How deep will you excavate?” The honest answer is 200mm for a standard drive, deeper for very weak ground. Anything under 180mm is a warning sign.
- “What sub-base material and depth?” You want “100mm of MOT Type 1, compacted in two layers.” Anything vaguer than that means they’re guessing.
- “Will you use a geotextile membrane?” The answer should be yes, immediately. Hesitation = no.
- “How will you compact the sub-base?” A vibrating plate compactor is the minimum. For larger drives, a roller. “We just tamp it down” is a no.
- “Can I see during the build?” Any contractor who doesn’t want you on site between excavation and blocks is hiding something.
- “What’s your guarantee, and what does it cover?” Many guarantees specifically exclude “ground settlement” — which is exactly the failure mode poor sub-base causes. Ours covers it.
How to spot it on an existing driveway
If you’re buying a house and want to know whether the existing driveway is sound:
- Look for ponding. After heavy rain, walk the drive. Standing water = falls weren’t built in or the sub-base has dropped.
- Check the edges. Blocks creeping outward at the perimeter = no proper edge restraint.
- Tap blocks with your foot. They should feel solid. Any rocking = the laying course has migrated.
- Look for “flush patches.” Areas where blocks sit slightly lower than neighbours — early-stage sub-base failure.
- Check joints for weeds. A few isolated weeds is normal. Continuous weed growth in joints means the kiln-dried sand has washed out — usually because the surface holds water.
The honest take
Block paving is one of the longest-lasting hard surfaces you can put on a driveway — if the sub-base is right. We’ve worked on 25-year-old drives that still look excellent, and we’ve replaced 4-year-old drives that looked tired the day they were finished. The blocks were the same. The sub-base wasn’t.
If you’re getting quotes, ask the questions above and compare answers, not just prices. A driveway that needs replacing in 5 years isn’t cheaper than one that lasts 30 — it’s about three times more expensive.
If you’d like an honest opinion on a quote you’ve already had, WhatsApp it to us — we’ll tell you what looks right and what doesn’t, no pressure.