Clay Court
Manual
Spring, summer and fall — achieving consistent bounce, controlled slide and minimal maintenance. A clay court is an integrated system of geometry, materials, moisture and routines.
Spring Start
From frozen ground to playable court — week-by-week guide covering structural repairs, clay application and line installation.
Summer Maintenance
Daily dragging, weekly rolling, monthly checks — the routines that keep the court in top condition from May to September.
Autumn Closure
Winter preparation is where next spring is won or lost. Leveling, compaction, checklist and the case for a closing work party.
Konglungen: 40–45 mm active clay layer
The thickness of the clay layer is a deliberate choice that affects the court's character and maintenance demands.
- Faster court with cleaner bounce
- Less plowing and clay migration
- Inspired by Swedish style (Båstad)
- Adapted to Norwegian climate
- Requires precise watering and routines
- Less forgiving of sloppy work
- Provides a "sweet spot" for Norwegian conditions
Function over name — and brand matters too
Suppliers use overlapping terminology. "Tennisgrus", "tennismassa", "clay", "top dressing", "Båstad clay" — the same word can mean different materials with different functions. The court is built from four mineral products stacked in a specific order. We focus on what each one actually does, what it is made from, and which brand we trust for which job.
Why does 0–2 mm bind worse than 0–3 mm? It's all clay anyway.
It feels intuitive that finer material should bind better. More clay, more "cement". But on a tennis court the opposite happens once you go too far down in fraction. The reason is not how much clay is in the mix — it is how the particles pack and how water moves between them.
Both fine and slightly coarser particles. The coarse grains form a structural skeleton; the fines and the clay fill the voids between them. The result is a structure that compresses hard, still drains, and locks together mechanically.
Missing the slightly larger particles. Everything becomes uniform and flour-like. When it dries it loses internal structure: loose surface, dust, weak shear strength, and grit that rolls under the shoe instead of locking.
Same principle as why pure sand binds badly, pure clay cracks and turns to mud — but a mix of several fractions becomes stable.
Water matters as much as the material
- 0–3 mm holds enough moisture to bind, while still leaving pores so water distributes evenly.
- 0–2 mm can go either too tight when wet, or too loose when dry — sometimes both in the same day.
Slightly paradoxical: too much fine material can make the court "dead". It looks compact, but lacks the microscopic reinforcement that the larger grains provide. Courts with a slightly coarser red top often feel firmer and more stable; very fine red clay tends to go powdery as the day wears on.
That is also why good clay systems use multiple layers, different fractions, and usually a top dressing that is finer than the carrier layer below it — not the other way around.
A clay court is a stack — not a slab
From the bottom up, a properly built outdoor clay court at our latitude is five layers. The deeper layers handle drainage and frost; the upper two are what the player actually touches.
| Layer | Material | Fraction | Thickness | Function |
|---|---|---|---|---|
| 5 | Coarse gravel (singel) | 40–70 mm | ~300 mm | Frost-free foundation |
| 4 | Fine gravel (singel) | 3–15 mm | ~50 mm | Capillary break |
| 3 | Coarse brick (Tennisgrus 2–10) | 2–10 mm | ~40 mm | Drainage / load-bearing |
| 2 | Playing layer (Lawnit / 0–2 mm) | 0–2 / 0–2.5 mm | ~40 mm | The active surface body |
| 1 | Top dressing (0–1 mm or finer) | 0–1 mm | ~3 mm | Seal, slide, ball mark |
A · The playing layer — Lawnit, not just any "tennisgrus"
This is the warm red mass that gives the court its character. It must bind when rolled and watered, slide cleanly under a sneaker, and leave a clear ball mark. The finest distinction in this whole manual sits here:
Crushed, partially-burnt red shale from Sweden. The shale carries natural clay minerals through the firing process, so the material binds on its own when wetted and rolled. Manufactured since 1942; standard on Båstad and Davis Cup tie courts in the Nordics. Grain 0–2.5 mm.
Crushed and screened red brick (tegl). Looks identical at a glance. But brick particles are inert — there is no clay to bond them together. On its own, this material packs but does not bind. We have proven this on court: lay it in a hole, roll it, wet it — and it crumbles back out within a week.
B · The top dressing — the 3 mm that the racket sees
A few millimetres of fine 0–1 mm material spread across the playing layer. This is what gives the slide, the ball mark, and the visual finish. Thin and frequent — never thick, never one-time-per-year.
- Lawnit fine fraction (preferred) — same shale family as the playing layer below it, so the layers fuse rather than separate. Spread 1–3 mm at a time, brushed in.
- Tennisgrus 0–1 (Utomhus) — pure red brick at 0–1 mm. Works as visual top dressing and gives the same warm rust colour, but does not repair structural problems beneath it. Treat it as paint, not putty.
C · The binder — Bara Clay 0–1 (Leire 0–1)
The product that fixes the brick-doesn't-bind problem from section A. Bara Clay 0–1 (sold in Norway as Leire 0–1 by utomhus.no) is not a tennis surface — it is a pure mineral clay used as an additive. 35 % illite, 25 % smectite/vermiculite, the rest mostly quartz and feldspar. The high clay-mineral content gives it a cation-exchange capacity (CEC) of 20–25 meq/100 g, which is what makes it bond particles into a hard surface.
Manufacturer's stated use on tennis courts: "used in combination with outdoor tennis brick and calcium chloride to form a hard surface". In practice that means: when you must repair a worn spot using brick-based 0–2 mm, mix in roughly 10–20 % Bara Clay 0–1 (and optionally a calcium chloride solution) to give the patch the binding it would otherwise lack.
D · The drainage layer — Tennisgrus 2–10
Coarser red brick (2–10 mm) installed once during construction at ~40 mm thickness, directly above the fine gravel. This layer never sees a player. Its only job is to drain water away from the playing layer and prevent capillary rise of moisture from below. It is also the layer responsible for the long-term stability of the court geometry — if it shifts or settles unevenly, every layer above will follow.
- You will not touch this in normal seasonal maintenance.
- If the playing surface develops a permanent low spot that does not respond to topping up — that is a sign of trouble in the 2–10 mm layer below.
- Replace only during a full reconstruction, not as a routine item.
Quantities per court per season
Practical annual maintenance for an established Norwegian clay court:
- Spring main top-up: approx. 0.5–1 ton of playing-layer material (Lawnit 0–2.5 mm), spread thin and rolled in.
- Summer / autumn top-up: 0.5–1 ton if there is significant wear, wind loss or depression formation.
- Top dressing (0–1 mm): several small applications throughout the season — frequency over volume.
- Bara Clay 0–1 binder: typically 1–2 sacks (20 kg each) per court per year is plenty. It is an additive, not a primary material.
Total: 1–3 tons per court per season, usually closer to 1–2. The historical figures of 8–12 tons of structure + 6–8 tons of playing layer that older guides cite apply to major repair or new construction, not regular maintenance. Always start low and observe.
Quick reference — what to order
| Need | Right product | Wrong-product warning |
|---|---|---|
| Spring main top-up of playing layer | Lawnit tennismassa 0–2.5 mm (storsekk via Unisport NO) | Brick-only "tennisgrus 0–2" will pack but not bind — court goes loose within weeks |
| Light top dressing during season | Lawnit fine fraction or Tennisgrus 0–1 (Utomhus) | 0–2 mm or coarser as top dressing — feels rough, kills slide |
| Filling a hole / repairing a worn spot | Lawnit 0–2.5 mm or brick 0–2 + 10–20 % Bara Clay 0–1 + roll + water | Plain 0–2 mm brick on its own — comes back out in a week, often worse |
| Sub-construction repair (rare) | Tennisgrus 2–10 mm | This is a build item, not a maintenance item |
Suppliers we use: Lawnit (Sweden) via Unisport Norge for the playing layer and top dressing — and utomhus.no for Bara Clay 0–1 binder and the brick fractions when needed for sub-layers or budget top-ups.
Seasonal line practice
Remove all lines before winter
- Prevents frost heave damage
- Allows unrestricted spring work
- Protects line material from winter
Lines stored away
- Court surface settles naturally
- Spring prep work unhindered
- Lines remain in good condition
Reinstall after new clay
- Complete structural repairs first
- Apply and roll new surface
- Install on prepared stable surface
- Secure with stakes and water
Big bags vs small bags
- Lower total cost per ton
- Efficient — crane / lifting
- Reduced manual labour
- More consistent material quality
- Less plastic waste
- Requires lifting equipment
- Requires dedicated storage space
- Requires planning of deliveries
- High flexibility
- Easy storage of small quantities
- Gradual refilling during season
- No machinery needed
- Higher cost per ton
- Significantly more manual labour
- More plastic waste
| Method | Material cost | Labour | Total operational cost |
|---|---|---|---|
| Big Bags | Lower | Lower | Usually lowest |
| Small Bags | Higher | Higher | Usually higher |
Geometry: controlled slope
A good clay court is not flat. It must slope in one single plane (true plane) for even runoff and the best playing experience.
- Side to side: Preferred direction — least impact on play, best line visibility
- End to end: Good alternative if terrain dictates — accepted by ITF/ASBA
- Slope: Typically 0.25–0.35% for clay (approx. 1 cm per 3–4 metres)
- Bowl shape — water collects in middle
- Tent shape / crown — high centre, unpredictable ball path
- Slope toward or away from the net
Place a straight board (3–4 m) across the court. You should see a faint light strip at the outer edges — even slope without a clear bulge in the middle.
Green growth on court: the right tool
Green growth on a red clay court is typically a mix of algae, moss and weeds along the edges — and they behave very differently from ordinary garden weeds. The right treatment depends on what you are actually trying to remove.
UgressNIX Effekt — contact herbicide (pelargonic acid)
UgressNIX Effekt is based on pelargonic acid (a fatty acid) and works as a contact product: it "burns" the green growth on the surface but does not kill roots from within. It breaks down quickly and leaves no lasting residue in the clay — far gentler than glyphosate.
- Kills surface green fast — weeds, algae, moss
- Works almost immediately
- Breaks down quickly in the clay
- Leaves no lasting residue
- Does not kill roots — regrowth is possible
- Effectiveness depends on dry weather and sun
- May require several treatments per season
- Dry court + sun — do not spray on a wet surface
- Light brushing first to remove loose material
- Apply evenly — do not over-apply, it is not paint
- Let it work — do not water afterwards
- Brush again after a few days
Roundup (glyphosate) — when and where
Roundup kills weeds including roots and is effective in border zones, but it is rarely the right tool for the playing surface itself. It does not effectively kill algae (it is not designed for that), can bind to the clay and leave dead organic matter that makes the surface more slippery.
Playing surface → UgressNIX Effekt + brushing
Edges, fences, behind benches → Roundup (here only — effective against deep roots)
The real winning formula
Run a consistent routine: light brushing 1–2 times per week + UgressNIX 1–2 times per season. Think of it as giving the court a regular shave rather than major surgery every year.
Prevention is more effective than treatment: better drainage, removing shade from shrubs and trees, and regular brushing are what actually reduce green growth over time.
Bulk purchasing: structure beats enthusiasm
If KTK coordinates purchasing together with other clubs (Holmen, OTK, Asker TK etc.) you can significantly reduce both cost per ton and transport costs. The key is to present as one buyer with volume and predictability.
- Establish a clear mandate — confirm in writing with the clubs: number of courts, expected tonnage (top dressing + surface dressing), delivery period and delivery location. The result: "We represent X clubs, Y courts, Z tons total" — that is your negotiating muscle.
- Standardise the product early — specify fraction (0–2 mm), colour/properties (light, low silt) and reference court ("Båstad type") before negotiations begin. Avoid comparisons that are not like-for-like.
- Calculate two scenarios — Scenario A: individual deliveries (price per ton + transport per club). Scenario B: bulk purchase (total tonnage, fewer trucks, one logistics plan). Suppliers love Scenario B.
- Set the frame — do not open with price — open with: "We are organising a joint purchase for X clubs in Oslo/Akershus. Combined volume approx. YY tons. We want a long-term partnership." The subtext is volume + stability + future.
- Negotiate in the right order — first price per ton, then transport cost, then a bundle price (material + delivery), finally an option on the same price next season. Many clubs lose money by starting with transport.
- Play suppliers against each other — cleanly — "We are in dialogue with several suppliers in Sweden and Norway. The total price is decisive for us." That is when discounts appear.
- Agree on one coordinator — one contact, one order, one invoicing model. Suppliers value low administration highly.
Homemade sand sifter
Sifting clay by hand is exhausting. With an oscillating sand sifter — driven by an old drill and an eccentric disc — the basket does the work itself. Fine sand falls through, stones and twigs migrate out at the end.
Drill rotates eccentric disc → eccentric bolt travels in a circle → connecting rod converts rotation into linear back-and-forth motion → carriage oscillates on rails.
- Adjustable stroke via three holes (40 / 60 / 80 mm)
- Estimated cost: 400–900 NOK
- No specialist skills required
Complete build guide, interactive mechanism animation (three views), detailed parts list and key tips.
Read full guide →Spring, Summer & Fall
Spend one extra day on structure now — it saves five weeks of problems later.
Apply main 0–2 mm quantity late April or May, when night temp > +8°C and wind is calm.
Read full guide →Little and often beats big emergency fixes. Light daily brushing is far more effective than heavy work once a week.
Circular dragging — start outside, work toward centre.
Read full guide →Avoid giving up toward the end — fall quality directly affects the following spring.
Full winter covering with tarp is not standard. Let the court breathe, but finish with good geometry.
Read full guide →