What's the difference between render and plaster?

People often use the words interchangeably, but render and plaster are different materials for different sides of the wall.

Why the materials differ

Render goes on the outside of a building and has to cope with everything the weather throws at it — driving rain, frost, heat and UV. So render mixes are built for durability and water management: traditionally cement and sand (sometimes with lime added for workability and flexibility), or pure lime on older properties, and increasingly modern through-coloured or polymer-modified systems. The finish is often textured, floated or scraped, and may be painted with a breathable masonry paint.

Plaster goes on the inside, where there is no weather to resist, so it is optimised for a fine, smooth, decorative finish. Indoors that usually means gypsum plaster — an undercoat plus a multi-finish skim — giving the flat, paint-ready surface you see in most UK rooms. Gypsum is not weatherproof and would quickly fail if used outside, which is exactly why it stays indoors and render stays out.

Cement render versus lime render

Within rendering, the big distinction mirrors the lime-versus-gypsum debate indoors. Cement render is hard, strong and widely used on modern masonry, but it is relatively rigid and not very breathable, so on old solid walls it can crack and trap moisture. Lime render is softer, more flexible and breathable, which is why it is the right external coating for period and solid-wall buildings that need to manage moisture — the same breathability argument that applies to internal lime plaster.

Indicative guidance for UK buildings. On listed or pre-1919 properties, lime is often the appropriate choice both inside and out.

Finish, application and durability

Indoors, plaster is trowelled to a smooth, polished finish ready for paint or paper, prioritising appearance. Outdoors, render is usually left textured or floated rather than glass-smooth, because a slightly open texture sheds water, hides minor movement and takes masonry paint well. Render is generally applied thicker and tougher than internal skim, because it is the building's protective skin, and it is built up in coats with a weather-resistant finish coat.

Durability expectations differ too. Internal plaster, kept dry, lasts for decades with no weathering. External render is exposed to constant weather cycles, so it is designed to be robust but will eventually need maintenance — repainting, repair of cracks, or patching where water has got in. Choosing the right render for the wall (breathable lime on old fabric, appropriate cement or modern systems on suitable modern masonry) is what gives it a long, trouble-free life, just as choosing gypsum or lime correctly does indoors.

Where they overlap

For most buildings the rule is simple: render outside, plaster inside, with render tougher and weatherproof and plaster smooth and decorative. The clearest overlap is lime. On old solid-wall and period properties, breathable lime is often used both as the external render and the internal plaster, because the same breathability and flexibility that protect the wall outside also suit it inside — it is essentially the same family of material doing both jobs.

So if someone says they are 'plastering the outside', they almost certainly mean rendering, and the material will be a cement or lime mix rather than indoor gypsum. Understanding the distinction matters when specifying work: using indoor gypsum outside would fail rapidly, while using hard cement render on a breathable old wall can cause damp. Match the coating to the side of the wall and the building's construction, and both the inside finish and the outside protection perform as intended.

The external rendering world has also broadened well beyond traditional sand-and-cement. Modern through-coloured renders — silicone, acrylic and mineral systems — carry the colour all the way through the finish coat, so they do not need painting and resist fading and cracking better than a painted cement render; many are also designed to be more breathable than plain cement, which suits a wider range of walls. There are also thin-coat renders applied over external wall insulation, which both weatherproof and insulate a wall in one system, much as insulated drylining does inside. None of this changes the basic divide — these are still external, weather-resistant coatings, not indoor plaster — but it means 'render' now covers a spectrum from traditional lime and cement through to engineered coloured systems. As with the lime-versus-cement question, the right external choice still depends on the wall: breathable systems for older or solid walls, and the appropriate cement or modern render for suitable modern masonry.

Maintenance expectations are the other big practical difference between an inside and an outside coating. Internal plaster, kept dry, is effectively maintenance-free for decades — you redecorate over it, but the plaster itself rarely needs attention. External render lives in the weather, so it is a wear surface: hairline crazing, cracks, blown or hollow patches and staining all appear over time, and water finding its way behind render is a common cause of damp inside. Catching and repairing render cracks early, keeping gutters and downpipes from soaking one area, and repainting breathable masonry paint when it tires are all part of looking after a rendered wall. So when comparing render and plaster, remember you are comparing not just two materials but two very different lives: a protected internal finish that lasts quietly, and an exposed external skin that protects the building but needs periodic care to keep doing its job.

Frequently asked questions

Sources & further reading

• Historic England — renders and external finishes
• The Property Care Association — damp and external walls

Figures on this page are typical UK ranges drawn from published sources and depend on your specific room. They are guidance, not a quotation.