Cement Boards for Rendering UK: Fixing and Specification Guide

Why Framed Walls Need a Cement Board Before Thin-Coat Render

Cement boards for rendering give thin-coat systems a dimensionally stable base on framed walls, holding linear variation to 0.16% under change in moisture content to BS EN 12467:2016 + A1:2016 — the flatness a mesh-reinforced basecoat depends on. That single figure is why the substrate exists: timber and light-gauge steel move with heat and moisture, and the board plus mesh layer absorbs that movement before it reaches the finish coat.

The boards in the rendering cement boards range at Renders World are the STS NoMorePly 12 mm fibre cement carrier, specified for timber frame, lightweight steel frame and modular or offsite construction. The manufacturer's own composition statement is short and worth knowing, because it explains the board's behaviour on an exposed elevation.

• Composition: fibre cement — sand, cement, water and organic fibres. The guide confirms the board contains no MgO and no fibreglass mesh, which is also why it is described as fully recyclable.
• Durability declaration: a minimum life expectancy of 30 years, with freeze-thaw, warm water and soak-dry durability recorded as complied and heat-rain as passed.
• Bending strength barely moves when wet: 12.46 MPa dry against 12.91 MPa after warm-water saturation, so the sheet keeps its working properties through the spell of site exposure before rendering.
• Racking contribution: Category 1 racking strength, classified to BS EN 594:2011 — useful context when the board is over-boarding an existing substrate.

Sand and cement render is not the intended finish here. The manufacturer notes the bonding surface suits cement-based products well, but that this render type tends to crack at the joints because it sets completely solid, and recommends a silicone thin-coat system instead for its flexibility over movement.

Specification Figures That Change How You Fix the Board

Three numbers govern the day's work: coverage, weight and the declared fixing count. The two formats share thickness, mechanical class and test scope, so the choice between them is a handling and joint-count decision rather than a performance one. Both are stocked by Renders World and both take the same screw.

The density figure divides the two documents: the February 2024 installation guide prints 1.23 g/cm³ and the November 2025 technical data sheet prints 1.28 g/cm³ for the same 12 mm sheet. Where a calculation depends on it — dead load on a batten framework, or scaffold loading schedules — work to the lower 1.23 g/cm³ and confirm the current figure against the data sheet revision supplied with your batch.

Read the unprimed impermeability line alongside the primed one, because the pair sets the programme. The unprimed face resists water formation for 24 hours but does develop a damp patch across the tested area, which is exactly why the manufacturer's joint treatment for delayed rendering matters. Thermal conductivity of 0.24149 W/mK also means the board is a substrate rather than an insulant: at 0.04928 m²K/W it adds very little thermal resistance, and any U-value belongs to the whole build-up calculation. Specification detail for each format sits on the STS full-size 2.4 × 1.2 m carrier board and the half-size 1.2 × 0.8 m carrier board pages.

How to Fix Cement Boards for Rendering, Step by Step

The method below follows the STS render carrier board installation guide. Working through it in this order keeps the cavity, the movement allowance and the fixing schedule consistent with the manufacturer's detailing, which is the basis any render system warranty is assessed against.

Set Out the Battens and the Ventilated Cavity

1. Fit the breather membrane first. For most timber frame buildings a breather membrane is required on the external walls before battens go on. Confirm the need and the type with building control, the project architect or the structural engineer, since it varies with the wall build-up.
2. Batten vertically. The board must have a cavity behind it, vented top and bottom, so vertical battens are the recommended arrangement. Cross-battening — horizontal to the substrate, then vertical — is available where a specific build-up or an existing cladding line has to be matched.
3. Set the centres to exposure. 600 mm centres suit most cases; move to 400 mm centres where the building carries higher wind load or exposure. Every batten must be fixed directly through to the studwork or frame, not into sheathing alone.
4. Size the batten and the cavity by nation. The most common batten is 60 mm wide by 25 mm deep, and that applies in England and Wales. For Scotland, Northern Ireland and the Republic of Ireland the minimum cavity depth is 50 mm. Where the boards would sit flush with the wall below, batten out to keep a minimum 15 mm airflow gap.
5. Terminate the cavity properly. The base detail runs a stop bead above DPC with an insect render mesh and ventilation section; the eaves detail terminates on a stop bead into the soffit void. Sizing those profiles is covered in the render stop bead installation guide.

Lay the Boards With Movement Gaps and Staggered Joints

1. Leave 3–5 mm on all sides of every board. Timber is among the most movement-prone substrates, and a butt joint with no gap lets adjacent boards push or grind against one another. With the gap in place, the mesh layer in the render system transfers little or no movement through to the topcoat.
2. Stagger the joints, including at corners, so the joint line breaks rather than running through.
3. Never align a joint with the edge of an opening. Cut boards around the window or door and fix onto a batten of at least 25 × 60 mm in the centre of the opening, with the board supported by a batten of that size all the way around. Most render manufacturers then call for a single or double layer of mesh over the corner stress points.
4. Fill the gaps to suit the programme. Rendering soon after boarding, fill with the basecoat mix. The wider sequence of reveals, sills and bead junctions above the board is covered in our guide to render detailing around windows and doors.

Choose the Right Screw and Work Out the Box Count

The guide specifies two different fasteners, which is the detail most often simplified on site. For timber framing it recommends a 3.95 × 38 mm self-drilling, self-countersinking screw; for light steel framing a 4.8 × 38 mm self-drilling, self-countersinking wingtip fixing. Either way the fixing must be stainless steel or suitably treated against corrosion and approved for external use — the specification the Ruspert-coated STS 38 mm render board screws are built to, with their coating classified at R1000 or R1500 grade in neutral salt spray and conforming to 12–15 cycles of the DIN 50018 corrosive-gas test.

• Edge distance: fix 20 mm in from board edges as the recommendation; the manufacturer also reports successful testing at 15 mm from the edge.
• Declared counts per 2400 × 1200 sheet: 28 fixings at 400 mm batten centres in horizontal or vertical board orientation, and 27 fixings at 600 mm centres in horizontal orientation.
• Box arithmetic: 250 screws ÷ 27 per sheet = 9.2, so one box covers nine full sheets at 600 mm centres; 250 ÷ 28 = 8.9, or eight sheets at 400 mm centres.
• Worked order for a 120 m² elevation: 120 m² ÷ 2.88 m² per board = 41.7 → 42 boards; adding a 10% cutting allowance you set from your own opening count gives 46 boards; 46 × 27 fixings = 1,242; 1,242 ÷ 250 = 4.97 → five boxes.

One gap is worth naming rather than filling with an estimate: the guide gives no fixing count for 600 mm centres in vertical board orientation, so plan that arrangement from the 400 mm figure or confirm the schedule with STS.

Trade Tips for a Crack-Free Finish on Render Carrier Board

Exposure timing is the detail that most rewards planning. The board can be left exposed to the elements for up to six months, and the manufacturer's instruction turns on the two-month mark: if rendering will not start within the first two months, run PU adhesive between the boards at the point of install, which gives a watertight joint and removes the need to tape. Where boards are already up and the render is delayed beyond two months, an external-grade joint filler with weatherproof tape achieves the same protection.

Key Takeaway: leave a 3–5 mm gap on all sides of every board, keep the cavity vented top and bottom, and treat the joints with PU adhesive at install if rendering will start later than two months — three documented steps that hold movement and water out of the render system.

Before any render goes on, the guide is unambiguous that the board must be dry, dust free and undamaged in every instance, so a walk of the elevation with a brush and a moisture check is time well spent. For cutting, the manufacturer recommends its polycrystalline diamond blade for a clean, low-dust cut that lasts considerably longer than a multi-purpose circular saw blade; pairing it with on-tool extraction keeps the working area clear.

Movement joints are the last thing to design and the hardest to add later. For most structures the guide calls for horizontal and vertical movement joints at a maximum of 15 m spans, or matched to movement joints in the substrate behind, with an expansion joint gap of 25 mm maximum. Use, spacing and detail should be clarified with the render manufacturer, architect or building control before the boards are fixed, which keeps the decision cheap and the elevation clean.

How the Board Fits the Complete Render Build-Up

The typical silicone render build-up over the carrier board runs five layers in a fixed order, and the board is only the first of them.

1. The carrier board, fixed and jointed as above.
2. Render mesh embedded in the base coat.
3. Render base coat.
4. Primer.
5. Render finish.

For layers two and three, the fibre-enhanced EPS adhesives basecoats stocked by Renders World give the crack-bridging behaviour that suits board joints and fixing points, and the mesh is embedded into wet basecoat and trowelled flat rather than laid dry against the board. Overlap discipline at the joint lines is set out in the fibreglass mesh overlap guide. Because the board sits inside a wider system of beads, primers and finishes, it is worth specifying the whole build-up together from the rendering materials range so the layers are compatible from the outset.

Two external references sit above the product documents. Reaction to fire on a rendered facade is a build-up and project fire-strategy question rather than a board-level one, and it is assessed under current Approved Document B guidance, which is now withdrawing the national BS 476 test classes in favour of BS EN 13501. Classification data for the specific board should be taken from the manufacturer's current documentation for the sheet being supplied and confirmed against the project fire strategy. On warranty-registered new homes, detailing also has to satisfy the NHBC Standards 2026, which apply to homes whose foundations begin on or after 1 January 2026.

Order the Board System and Plan the First Fix

Cement boards for rendering are the right substrate wherever thin-coat render meets timber frame, light steel frame, SIP or modular walls. On SIP panels there is no internal studwork, but the outer OSB layer is structural enough to take the carrier system directly — membrane, vertical battens at 400 or 600 mm centres, then the standard sequence. Solid masonry does not normally need a carrier at all.

• Board count: divide the elevation area by 2.88 m² for full sheets, and hold half-size sheets for reveals, dormers and soffit returns.
• Screw count: nine full sheets per 250-piece box at 600 mm centres, eight at 400 mm centres.
• Delivery planning: Renders World ships boards by pallet from its Southampton base, and because pallets are shrink-wrapped rather than weatherproof and all deliveries are curbside only, arrange covered storage and unloading labour before the drop.

Specify the boards, the matched Ruspert screws, basecoat, mesh, primer, beads and finish as one order from the STS render carrier board collection, and the fixing schedule and the render system arrive matched to each other.

Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.

FAQ — Fixing, Exposure and Ordering Cement Render Boards

How many boards and screws should I order?

Divide the elevation area by 2.88 m² for the full-size sheet count, then add a cutting allowance you set from the number of openings on the elevation. Screws follow the sheet count directly at 27 per sheet at 600 mm batten centres or 28 at 400 mm centres, so a 250-piece box covers nine or eight full sheets respectively.

Which screws suit timber and which suit steel framing?

Timber framing takes a 3.95 × 38 mm self-drilling, self-countersinking screw, and light steel framing systems take a 4.8 × 38 mm wingtip fixing of the same type. Both must be stainless steel or suitably treated for external use, which is why a corrosion-coated render board screw is the matched choice rather than a general-purpose fastener.

How long can the boards stay exposed before rendering?

Up to six months. If rendering will not begin within the first two months, run PU adhesive between the boards at install for a watertight joint; if the boards are already fixed and the programme slips past two months, an external-grade joint filler with weatherproof tape does the same job. In every case the board is dry, dust free and undamaged before render goes on.

Does the board need a ventilated cavity behind it?

Yes — the cavity must be vented top and bottom, which is why vertical battens are the recommended arrangement. Where the boards would finish flush with the wall course below, batten out to hold a minimum 15 mm airflow gap. In Scotland, Northern Ireland and the Republic of Ireland the minimum cavity depth is 50 mm.

Can sand and cement render go onto the board?

The board bonds well to cement-based products, but the manufacturer notes this render type tends to crack along the joints because it cures completely rigid. A silicone thin-coat system is the recommended finish, since its flexibility accommodates the small seasonal movements a framed structure makes.