Search for exterior self levelling compound and you mostly get product pages. They tell you what one bag does, not whether your patio, balcony or path should get a compound at all. This guide answers that question, using UK manufacturers' data sheets and Approved Document H of the Building Regulations.
Short answer: yes, self levelling compound can go outside, but only a product sold for external use, laid thin over sound, mature concrete, and protected from frost while it cures. Indoor smoothing compounds are not designed for weather. If the area needs a fall to shed water, more than about 20mm of build up, or a new base, it needs an external screed instead.
Can you use self levelling compound outside?
Only if the bag says so. Most compounds sold for floors are made to go under vinyl, LVT, carpet or tiles inside a heated building. F. Ball's data sheet for Stopgap 700 states "This product is for internal use only", and the sheet for Stopgap 300 says it "is not recommended for external use".
Exterior products exist, and they are a different thing. ARDEX K 301 is sold as an "Exterior Self Levelling Concrete Resurfacing Compound". Its April 2025 data sheet lists balconies, patios, domestic driveways, garages and walkways among its uses, at 2mm to 20mm thick. It also sets clear limits, which are covered below.
So the real question is rarely "compound or not". It is whether the outdoor surface needs resurfacing (a thin, smooth, sound top layer on concrete that already drains properly) or reshaping (new levels, falls, or a new base). A compound can resurface. Reshaping is a screed's job.
Compound or screed: side by side
| Indoor smoothing compound | Exterior levelling compound | External screed | |
|---|---|---|---|
| Example | F. Ball Stopgap 700 or 300 | ARDEX K 301 | Bonded or unbonded sand and cement |
| Outdoor use | Not recommended, or internal only | Yes, per the data sheet | Yes, with the right finish and joints |
| Thickness | Typically 2mm to 10mm or 20mm | 2mm to 20mm, combined layers no more than 20mm | 25mm to 40mm bonded; from 50mm unbonded |
| Can form falls? | No | Only very short, shallow ones | Yes, falls are set in the screed |
| Base needed | Sound, dry, indoor base | Concrete at least 6 weeks old, mechanically prepared | Sound slab, prepared for bonding, or a membrane |
| Ready for foot traffic | Around 60 to 90 minutes at 20°C | 2 to 3 hours at 20°C | Usually 24 to 48 hours |
| Our service page | Floor levelling | Floor levelling | Sand and cement screed |
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Product figures come from the F. Ball and ARDEX data sheets listed in the sources. Screed depths are the site's own figures and match the Mineral Products Association's summary of BS 8204.
Why indoor compounds fail outdoors
Indoor compounds are made for a warm, dry, stable room. Outside, three things work against them.
Frost and cold. Cement based compounds need warmth to set and gain strength. F. Ball's Stopgap 700 sheet says the floor must be kept above 10°C throughout application and drying. Even the exterior ARDEX product must be applied above 5°C and protected from frost, strong sunlight and fast drying air until it hardens. A British winter night does not meet either condition, and freeze thaw cycles keep attacking a surface for its whole life.
Water. Indoor compounds are designed to sit dry under a floor covering. Outside they sit in rain and puddles. Gypsum and calcium sulphate products are worse still: our own rule is that a cement based material is the choice wherever a floor may get wet.
Movement. Outdoor slabs heat up in the sun and cool at night. ARDEX says any structural, movement or live joints must be carried up through K 301 to the surface, and that the product may be unsuitable for aged or frost damaged concrete that deflects with thermal cycling or loading. A thin layer poured straight over a joint will crack along it.
The result is the classic failure: a smooth skim that looks fine for a few weeks, then crazes, flakes at the edges, and lifts in sheets after the first hard frost.
What exterior levelling compounds are for, and their depth limits
An exterior compound is a resurfacing layer. It makes rough, pitted or slightly uneven concrete smooth and sound again. Going by the ARDEX K 301 data sheet, it suits:
- Tired concrete paths, patios and drives with surface scaling or a rough finish, where the slab itself is sound.
- Balconies and garage floors that drain properly but need a new top surface.
- Pre-smoothing before a coating, paint or sealer.
The limits matter as much as the uses:
- 2mm to 20mm, and the combined thickness of all layers should not exceed 20mm.
- Concrete at least 6 weeks old, hard, sound and mechanically prepared by grinding, scabbling or shot blasting, then vacuumed clean.
- Primed with the manufacturer's primer at the external dilution.
- Not over asphalt or tarmac, and not for heavy industrial floors, public highways or solid or metal wheels.
- Light vehicles after 48 hours at 20°C; walkable after 2 to 3 hours.
- Slip resistance: a smooth trowelled surface can be slippery when wet. ARDEX suggests a stiffer mix with a broom finish, or a slip resistant coating, for wet or external areas.
For small repairs and local ramps, ARDEX also sells A 46, a repair mortar for internal and external use from 2mm to 50mm, described as freeze thaw and salt resistant, with building ramps among its listed uses. That is a patch repair product, not a way to reshape a whole terrace.
If your outdoor surface fits inside those limits, a compound can be the cheaper, quicker answer. Our guide to latex, self levelling compound or a new screed covers how compounds compare indoors.
When you need an external screed instead
Short answer: choose a screed when the surface needs a fall, more depth than the compound allows, or a new base.
The depth limit is what usually decides it. Approved Document H recommends a gradient of at least 1 in 60 on impervious paved surfaces so water drains quickly, with water directed away from the building. A 1 in 60 fall drops 50mm over 3 metres. A compound capped at 20mm in total can only create a 1 in 60 fall over about 1.2 metres, and that is before you allow for any existing hollows. Most patios and terraces are larger than that.
You need a screed when:
- Water ponds or runs towards the house. The fall has to be rebuilt, not skimmed.
- The surface needs raising by more than about 20mm, for example to meet a door threshold or new tiles.
- The slab is cracked through, hollow or breaking up. A compound only bonds to what is under it. Damaged concrete needs repair or a new base first.
- It is a balcony or roof terrace where the deck has to drain to an outlet or gutter and a waterproofing system is involved.
- Tiles or paving are going on top and need a true, level bed at the right height.
Outdoors, that is almost always a cement based screed. A bonded screed at 25 to 40mm suits a sound slab where height is limited. Where the screed sits on a membrane or separating layer, it is unbonded and needs at least 50mm, per the MPA's summary of BS 8204.
Two limits are worth stating plainly. We do not lay structural concrete slabs, so a failed base needs a groundworker or structural advice first. On a roof terrace or balcony, the falls, outlets and waterproofing membrane are set by the designer or the waterproofing manufacturer, and the screed is laid to their details.
Falls, drainage and finishes on outdoor screeds
Short answer: set the fall first, keep the screed above its minimum depth at the thinnest point, carry the joints through, and finish to suit what goes on top.
- Survey the levels. Find where water must go: a drain, gully, gutter or outlet, and away from the building. Approved Document H also suggests a reverse gradient for at least 500mm from a wall where levels would otherwise send water towards it.
- Work out the depth at both ends. The thinnest point, usually at the outlet, still has to meet the minimum for the construction. A 1 in 60 fall over 3 metres adds 50mm, so a screed that starts at 25mm at the outlet is about 75mm at the high end. That is well beyond the 25 to 40mm a bonded screed is normally laid at, so a long fall may need a thicker unbonded build up or a base that already falls the right way. Our screed calculator helps with volumes once you know the average depth.
- Prepare the base. A bonded screed needs a clean, mechanically prepared slab with the laitance removed. See floor preparation.
- Lay and compact to the falls. Screed rails or levels are set to the fall line, and a semi-dry sand and cement mix is compacted and ruled to them.
- Form joints. Movement joints in the slab are carried up through the screed.
- Protect while it cures. Cover against rain, frost and direct sun for the first days. Cold weather slows everything.
- Finish. A screed is normally covered with tiles, paving or a waterproofing or coating system. Leaving a levelling screed exposed as the final walking surface outdoors is a different specification and should be designed as one.
| Area | Typical approach | What to check |
|---|---|---|
| Sound patio, good falls, rough surface | Exterior compound, 2mm to 20mm | Slab age, joints, slip resistance |
| Patio that ponds or slopes to the house | Screed laid to falls, bonded where the depth allows | Gradient of at least 1 in 60, drainage point, depth at the high end |
| Balcony or roof terrace | Screed to falls to the designer's details | Outlets, membrane position, upstands |
| Path or step with local damage | External repair mortar | Depth of damage, edges |
| Slab cracked through or sinking | Not a screed problem | Needs a new base or structural advice |
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What an outdoor screed costs
Published UK cost guides cover internal floors, not outdoor work, so treat them as a starting point only. MyJobQuote (updated August 2026) puts sand and cement screed at about £18 to £25 per m² supplied and laid. trade2base (June 2026) gives about £12 to £18 per m² for a bonded sand and cement screed at 25 to 40mm, and says jobs under 20m² should carry a minimum charge of £300 to £500.
Outdoor jobs vary more than that: falls add depth and material, access through a house or up to a balcony takes time, and preparation of weathered concrete can be a large share of the cost. That is why we price outdoor work from photos and measurements rather than a rate card.
Get a fixed price for your floor
Send us photos of the outdoor area, its size in m², the finish it will get (tiles, paving, a coating or left as it is) and the postcode. If you know where the water drains to, or how far it needs to rise, include that too. We will tell you whether it needs an exterior compound or a screed, and price it.
You get a same day call back and a written quote within 24 hours, with a fixed price agreed before any work starts. Contact us here or call 07466 650936.
Sources
- ARDEX K 301 Exterior Self Levelling Concrete Resurfacing Compound, product data sheet, ARDEX UK, April 2025, accessed October 2026
- ARDEX K 301 product page, ARDEX UK, accessed October 2026
- ARDEX A 46 Multi-Purpose Concrete Repair Mortar, ARDEX UK, accessed October 2026
- Stopgap 700 technical data sheet, F. Ball and Co., April 2024, accessed October 2026
- Stopgap 300 technical data sheet, F. Ball and Co., March 2026, accessed October 2026
- Approved Document H: Drainage and waste disposal, 2015 edition, HM Government, accessed October 2026
- Mortar data sheet 22: Screeds, Mineral Products Association, Issue 3 October 2023, accessed October 2026
- BS 8204-1:2003+A1:2009 Screeds, bases and in situ floorings, BSI, accessed October 2026
- Floor screeding cost guide, MyJobQuote, updated August 2026
- Floor screed costs UK, trade2base, June 2026
