Clay Tile Roofing: Types, Pitch & BS 5534 Fixing Guide

Summary

Clay tiles are one of the oldest and most durable pitched-roof coverings in the UK, lasting well over a century when laid correctly. They come in two families that behave completely differently: plain tiles, which are small flat tiles laid double-lap so that every part of the roof has at least two thicknesses of tile over it, and interlocking (single-lap) tiles, which use side and head interlocks to shed water with one thickness, covering far more area per tile. Choosing the wrong type for the roof pitch, or the wrong fixing for the exposure, is the most common cause of clay roof failure — tiles slipping, water ingress at low pitch, or wholesale lifting in a storm.

This matters to every roofer, but also to builders, extension specialists and anyone re-roofing or matching an existing roof. The defining change in modern practice is BS 5534. The standard was substantially revised (2014, amended 2018) after a series of wind-damage failures, and the headline outcome is that the old habit of hanging plain and small tiles purely on their nibs is gone — the standard now drives mechanical fixing of essentially every tile, with the fixing spec coming from a wind-load calculation specific to the building's location, height, topography and roof zone. Mortar bedding alone is no longer accepted as the sole fixing for ridges, hips and verges; these now need mechanical or dry-fix systems.

The big misconceptions are: that clay and concrete tiles are interchangeable (they differ in weight, pitch, sizing and detailing); that you can re-bed a ridge in mortar and call it compliant (you generally need mechanical/dry fixing too); and that minimum pitch is a single number (it depends on tile type, headlap and exposure, and the manufacturer's figure is authoritative). Get the pitch, lap, batten gauge and fixing right and price the dry-fix kit in — it's no longer optional.

Key Facts

• Governing standard — BS 5534: Slating and tiling for pitched roofs and vertical cladding — code of practice. Covers fixing, laps, battens, underlay and wind loading.
• Two tile families — plain tiles (double-lap, ~265 × 165 mm traditional size) and interlocking single-lap tiles (larger coverage, side/head interlocks).
• Plain tile minimum pitch — generally about 35°, rising to 40°+ in exposed/severe areas (manufacturer figure governs).
• Interlocking clay tile minimum pitch — commonly 30° and above; some profiles down to around 22.5° subject to manufacturer approval and headlap.
• Every tile mechanically fixed — since BS 5534:2014, designs are based on wind-uplift calculation; in practice nearly all tiles are nailed and/or clipped. Nibs/gravity alone are not relied upon.
• Plain tile headlap — typically around 65 mm (varies with pitch and exposure; increase lap as pitch falls or exposure rises).
• Plain tile gauge — batten gauge = (tile length − headlap) ÷ 2 for double lap (each course is half-bonded). For a 265 mm tile at 65 mm lap, gauge ≈ 100 mm.
• Interlocking tile gauge — set by the manufacturer's min/max gauge range for that profile and the required headlap; never exceed the maximum gauge.
• Side lap / bond — plain tiles are laid broken-bond (half-bond), with tile-and-a-half tiles at verges and abutments to maintain bond.
• Battens — graded to BS 5534 (factory-graded, marked battens); minimum sizes typically 25 × 38 mm or 25 × 50 mm depending on rafter spacing; fixed with the correct length nails.
• Underlay — breathable membrane or traditional 1F felt to BS 5534/relevant standards; ventilation must satisfy condensation control.
• Fixings — aluminium or silicon-bronze/stainless nails for clay (clay is more abrasive/acidic than concrete); tile clips for perimeter and high-uplift zones.
• Dry fixing — ridges, hips and verges increasingly use dry (mechanical) systems instead of, or as well as, mortar; mortar alone is not an acceptable sole fixing under current BS 5534.
• Perimeter/zone uplift — eaves, verges, ridges and corners experience the highest wind uplift and need enhanced fixing (more clips/nails).
• Building Regs — Part A (structure — roof loads), Part C (resistance to moisture), Part L (where roof affects thermal element), Part B (fire — roof coverings near boundaries).

Quick Reference Table

Detailed Guidance

Plain tiles vs interlocking — choosing the right one

Plain tiles give the traditional, fine-textured look expected in conservation areas and on period properties; they're small, double-lapped, and need a steeper pitch and many more tiles per square metre. Interlocking single-lap tiles cover far more area per tile, can be laid at lower pitches, and are quicker and lighter to lay, but have a more modern, larger appearance. For matching an existing roof, identify the exact tile (type, size, profile, maker) before quoting — mixing plain and interlocking, or clay and concrete, looks wrong and detail differently.

Pitch, lap and the relationship between them

The flatter the roof and the more exposed the site, the more water tries to track back under the tiles, so you increase headlap as pitch falls or exposure rises. Never lay below the manufacturer's stated minimum pitch for that tile and lap — it's the single most common low-pitch failure. Exposure is assessed by the building's location, the driving-rain index and local topography. Treat the manufacturer's pitch/lap table as authoritative.

Batten gauge

For plain (double-lap) tiles, the working gauge is:

Gauge = (tile length − headlap) ÷ 2
e.g. (265 − 65) ÷ 2 = 100 mm

For interlocking tiles, the gauge is set within the manufacturer's min/max range to achieve the required headlap — set it out so the courses land neatly at eaves and ridge, and never exceed the maximum gauge (which would under-lap the tiles). Always dry-set a few courses and mark the battens accurately; consistent gauge is what makes the roof look right and shed water.

BS 5534 and mechanical fixing

Before the 2014 revision, plain and many small tiles were often hung on their nibs and only perimeter tiles nailed. After a run of wind failures, BS 5534 was rewritten so the fixing specification comes from a wind-uplift calculation for that specific roof — its location (wind map), height, topography, roof pitch and zone. The practical result is that every tile is mechanically fixed (nailed and/or clipped), with perimeter zones (eaves, verges, ridge, corners) fixed more heavily than the field. You cannot assume a generic "nail every third course" rule any more — do (or obtain) the calculation. Manufacturers provide fixing specifications and software to satisfy this.

Fixings and battens

Clay is more abrasive and chemically aggressive to ferrous metal than concrete, so use aluminium, silicon-bronze or stainless nails — not bright steel. Battens must be BS 5534 graded and marked, the correct cross-section for the rafter spacing, fixed with appropriate-length nails, and supported/jointed only over rafters. Counter-battens may be needed with some membranes for ventilation/drainage.

Ridges, hips and verges — dry fixing

Mortar bedding alone is no longer an acceptable sole fixing for ridges, hips and verges under current BS 5534 — they must be mechanically fixed, typically with dry-ridge and dry-verge systems (ridge battens, screws, unions and ventilated ridge rolls; dry verge units). Mortar may still be used decoratively or in conjunction, but the mechanical fixing carries the wind load. This is one of the most common compliance gaps on re-roofs.

Underlay, ventilation and condensation

The roof underlay (breathable membrane or traditional felt) plus ventilation must control condensation in the roof void to satisfy Building Regulations Part C/F principles. Get the ventilation strategy right (eaves, ridge and/or breathable membrane) — sealing the roof tight without provision causes interstitial condensation and timber decay.

Frequently Asked Questions

Can I lay clay plain tiles at 30°?

Generally no — plain clay tiles usually need about 35° minimum, and 40°+ in exposed areas. Going below the manufacturer's stated minimum pitch invites water tracking back under the tiles. If you need a lower pitch, switch to an interlocking tile rated for it.

Do I really have to fix every single tile now?

In practice, yes. Since BS 5534:2014 the fixing is driven by a wind-uplift calculation, and for most UK roofs that calculation results in every tile being nailed and/or clipped, with perimeter zones fixed more heavily. The old reliance on tile nibs and gravity is gone.

Is mortar bedding still allowed on ridges and verges?

Mortar alone is no longer an acceptable sole fixing. Ridges, hips and verges must be mechanically fixed (dry-fix systems). Mortar can still be used, but it can't be the only thing holding the ridge or verge against wind uplift.

What nails should I use for clay tiles?

Use aluminium, silicon-bronze or stainless steel nails. Clay is abrasive and more chemically aggressive than concrete, so bright/galvanised steel corrodes too quickly. Match nail length to batten and tile so the nail bites the batten without splitting it.

Clay or concrete tiles — which should I quote?

Clay lasts longer, keeps its colour (concrete can fade), and is expected on period/conservation roofs, but costs more. Concrete is cheaper and heavier. They aren't interchangeable on the same roof — sizes, pitch and detailing differ. For matching, identify exactly what's on the roof first.

Regulations & Standards

• BS 5534 — Slating and tiling for pitched roofs and vertical cladding: code of practice (fixing, laps, battens, wind loading). Key revisions 2014/2018.

• BS 8000-6 — Workmanship on building sites: slating and tiling of roofs and claddings.

• BS EN 1304 — Clay roofing tiles and fittings: product definitions and specifications.

• BS EN 539 (water impermeability / frost resistance test methods for clay tiles).

• Building Regulations Part A — structure (roof and tile loadings).

• Building Regulations Part C — resistance to moisture / weather.

• Building Regulations Part B — fire, including roof covering performance near boundaries.

• Building Regulations Part L / F — thermal elements and ventilation/condensation control in roofs.

• NHBC Standards — chapter on pitched roofing (warranty requirements often exceed minimum).

• BSI — BS 5534 slating and tiling code of practice — primary UK fixing standard

• NFRC — National Federation of Roofing Contractors technical guidance — UK roofing trade body

• Marley / Redland / Sandtoft technical manuals — manufacturer pitch, lap, gauge and fixing data

• BS EN 1304 clay roofing tiles — BSI — clay tile product standard

• HSE — working at height in roofing — safe roof access

• slate vs concrete tiles — comparing tile coverings and where each suits

• roof tile types — broader overview of UK roof tile profiles

• dry verge and dry ridge systems — mechanical ridge/verge fixing required by BS 5534

• roof ventilation — condensation control and eaves/ridge ventilation