Preparing the ground under a terrace: gravel, sand and slope
The Abimeister team · Updated · 5 min read
Most terrace problems in Tallinn gardens do not start with the decking. They start under it, with ground that was never dug out properly, gravel that was never compacted, or a surface that slopes towards the house instead of away from it. This guide goes through the groundwork step by step: how deep to excavate, what layers to lay and in what order, why compaction is not optional, and where the slope usually goes wrong. It is the one part of building a terrace that is worth doing slowly.
How deep to dig, and why it matters
The excavation depth depends on what is under your current lawn or paving and what the terrace will carry. As a rough guide, a domestic terrace with a gravel and sand base needs digging out to around 25–30 centimetres below the finished surface level, more if the existing soil is soft, peaty, or has a lot of organic topsoil in it that will keep settling.
Estonian topsoil is often shallow, but under it you can hit clay that holds water, or old fill from when the plot was levelled. Both are a problem: clay stops water draining through, and loose fill will keep compressing under the frame for years no matter how well you build on top of it.
If you dig down and find soft, dark, springy soil rather than firmer subsoil, keep going until you reach something solid. Skipping this because the hole is already deep enough is the first place people cut corners, and it shows up later as an uneven terrace.
The gravel and sand layers, and the order they go in
Once you are down to firm subsoil, the base is normally built in layers, each one compacted before the next goes on. A typical build-up is a crushed gravel layer of around 15–20 centimetres, using a graded stone (roughly 0–32 mm) rather than uniform round gravel, because the mix of sizes locks together when compacted.
On top of that goes a levelling layer of coarse sand, usually 3–5 centimetres, which is what you actually screed level and set your slope in. The sand layer is not there to bear the load; the gravel underneath does that. Sand is there so you can get a true, even surface for the joists or the pedestal system to sit on.
Lay a permeable geotextile membrane under the gravel if the subsoil is fine or silty, to stop the gravel working its way down into the ground and the ground working its way up into the gravel over time. This is a small step that is easy to skip and mildly annoying to fix later.
Why compaction is not an optional step
Gravel that has simply been tipped in and raked flat looks solid but is not. It has air pockets throughout, and every one of them will eventually collapse under weight, foot traffic, rain, and frost. That is what causes a terrace to develop dips and soft spots eighteen months after it was built, usually right where people actually walk.
Compact each layer separately, not just the top one. For a domestic job, a plate compactor from a tool hire shop is enough; do a pass, water the layer lightly if it is very dry, and compact again. Two or three passes per layer is normal. You should not be able to leave a footprint in the surface once you are done.
This step takes an afternoon and costs almost nothing beyond the hire fee, which makes it the step people are most tempted to shorten. It is also the step that most decides whether the terrace still looks flat in three years.
Getting the slope right: where most mistakes happen
A terrace needs a fall away from the building, not towards it. A slope of around 1–2 centimetres per metre is normal for a decked terrace, enough to move rainwater off without being noticeable underfoot. Set this slope in the sand layer with a straightedge and a spirit level or a laser level, checking it at several points, not just at the two ends.
The mistake we see most often is a terrace built dead level, or worse, with a very slight fall back towards the house wall, because it was easier to match the door threshold that way. Water then sits against the foundation, and over a few winters it finds its way into the wall or under the base.
Check the slope again after compacting, because compaction can flatten out a fall you thought you had built in. It is far easier to correct at this stage than after joists and decking boards are down.
- Excavate to firm subsoil, typically 25–30 cm below finished level
- Lay and compact gravel in one layer, roughly 15–20 cm
- Add a levelling sand layer of 3–5 cm and set the slope in it
- Compact each layer separately, not just the final surface
- Aim for a fall of 1–2 cm per metre, away from the house
- Re-check the slope after compacting, before building on top
When to do this yourself, and when to call someone in
A small, flat terrace on ground that drains reasonably well is a realistic DIY job if you have a weekend, a plate compactor, and the patience to check levels properly rather than by eye. Marking out, digging, and laying gravel and sand is straightforward manual work; the skill is in checking the slope repeatedly, not in any special technique.
It is worth calling for help once the ground is uneven, once you are dealing with a large area, or once the terrace sits close to a boundary fence that also needs attention, since fence repair and installation is charged from 45 € per hour and is often worth pairing with groundwork if both need doing at the same visit.
If the excavation reveals persistent water, very soft clay that will not compact, or the terrace needs to tie into a slope that affects drainage around the whole house, that is the point to get a second opinion rather than build on and hope. Our decking and terrace work starts from 90 € per job, and a look at the ground beforehand is often what decides whether the price stays there.
Dig to firm ground, compact every gravel and sand layer separately, and keep checking the slope until the surface is set — that is what keeps a terrace flat and dry for years.