Question 1

What's the real difference between galvanised, Zincalume, and colour-coated roofing sheet?

Accepted Answer

Galvanised steel sheet is coated with zinc through a hot-dip process, which protects the underlying steel by sacrificial corrosion (the zinc corrodes preferentially, protecting the steel beneath) — it's a well-proven, widely available option but has a shorter service life than the alternatives in aggressive environments once the zinc coating is depleted. Zincalume (an aluminium-zinc alloy coating, roughly 55% aluminium) generally outperforms plain galvanised in most environments because the aluminium content forms a more stable, longer-lasting protective barrier, though it can perform differently than galvanised in certain aggressive industrial or marine-adjacent chemical exposures, so checking manufacturer guidance for the specific exposure is still worthwhile. Colour-coated (pre-painted) sheet starts from a galvanised or Zincalume base and adds a factory-applied paint system on top, which provides both the aesthetic colour and an additional protective barrier over the metallic coating — the paint system's own quality and thickness meaningfully affects service life independent of the metallic coating underneath, so a cheap paint system over a good metallic coating can still under-perform.

Question 2

How do I choose the right roofing sheet material for a coastal or industrial environment?

Accepted Answer

Coastal environments (commonly defined as within a few kilometres of the sea, with the risk increasing closer to the shoreline) expose roofing to airborne salt, which accelerates corrosion significantly compared to an inland site — standard galvanised steel can corrode within a fraction of its normal inland service life in a genuinely coastal location, making Zincalume with a quality paint system, or in the most aggressive coastal exposures aluminium sheet, the more appropriate choice despite the higher upfront cost. Industrial or urban-pollution environments expose roofing to airborne chemical contaminants (sulphur compounds, industrial emissions) that attack coatings differently than salt does, and typically call for a polyester-coated or, for more aggressive exposure, a PVDF-coated steel system specifically rated for chemical exposure resistance. Confirming the site's actual environmental classification — coastal distance, industrial proximity, general pollution level — before specifying sheet material is worth the effort, since choosing the wrong coating system for the environment is an expensive mistake that often isn't visible until years after installation.

Question 3

What does BMT mean and why does it matter more than total sheet thickness?

Accepted Answer

BMT (Base Metal Thickness) is the thickness of the underlying steel itself, before any metallic or paint coating is applied — this is the figure that actually determines the sheet's structural strength, dent resistance, and wind uplift capacity, since coatings add negligible structural contribution regardless of how they affect the total measured thickness. Some suppliers quote total coated thickness rather than BMT, and total thickness is typically a small amount greater than BMT (the exact difference depends on coating type and thickness) — always confirm BMT specifically in a purchase order or specification, not just 'thickness,' since ordering based on total thickness when the specification actually required a minimum BMT can result in a sheet that's technically thinner in the structurally relevant dimension than intended. Minimum BMT for residential roofing is commonly around 0.42mm, with 0.47mm recommended for better dent resistance and wind performance — sheets thinner than this (sometimes marketed around 0.35mm) are intended for secondary structures, not main roofs.

Question 4

How is purlin spacing determined for a given sheet type and thickness?

Accepted Answer

Purlin spacing (the maximum span a roofing sheet can bridge between structural supports without excessive deflection) is determined by the sheet manufacturer's published load-span table, which accounts for the sheet's profile, BMT, and the design wind and any snow load for the specific location — it is not a fixed universal number, and using a generic 'typical' spacing figure without checking the actual manufacturer's table for the specific sheet being used risks specifying a span the sheet can't actually support. As a general pattern, corrugated (sinusoidal) profile sheets typically support purlin spans in a roughly 1.2 to 1.5 meter range in standard residential conditions, while trapezoidal (box-rib) profiles with deeper ribs can span further — sometimes 1.5 to 3.0 meters depending on rib depth and BMT — making trapezoidal profiles a common choice where wider purlin spacing is structurally or economically desirable. Exceeding the manufacturer's specified span for the sheet causes visible deflection, water ponding at the low points between purlins, and over time permanent deformation and fatigue cracking at the profile crests.

Question 5

What overlap should I use for side laps and end laps, and does roof pitch change it?

Accepted Answer

Side lap (the overlap between adjacent sheets across the roof width) is typically a minimum of one full corrugation for corrugated profiles or one rib for trapezoidal profiles, commonly translating to roughly 40 to 80mm of physical overlap depending on the specific profile's corrugation or rib pitch — using less than a full corrugation risks a gap that wind-driven rain can bridge. End lap (where sheets join end-to-end along the slope, needed for rafter lengths longer than a single available sheet length) is more pitch-sensitive: a steeper roof (above roughly 15°) can generally use a smaller end lap, commonly around 150mm, because water runs off faster and has less opportunity to be driven backward under the lap by wind, while a lower-pitch roof (10-15°) typically needs around 200mm, and a very low-pitch roof (below 10°) often needs 250mm or more — some very low pitches fall below what a given sheet profile is rated for entirely, requiring either a different profile or a different roofing system. Always confirm both figures against the sheet manufacturer's specific guidance for the profile and pitch being used, rather than applying one fixed number across all pitches.

Question 6

Why do screws need to be fixed at the crest of the corrugation, not the valley?

Accepted Answer

Fixing at the crest (the raised peak of the profile) keeps the screw's sealing washer compressed against a dry, load-bearing part of the sheet that water naturally runs away from, while fixing in the valley (the low point, where water actually flows during rain) places the screw exactly where water pools and pressure can work through the seal over time even with a correctly compressed washer — this is one of the most common and avoidable roofing installation errors, and a common cause of slow, hard-to-trace leaks appearing well after installation, concentrated around whichever screws were placed in valleys rather than crests. This applies to fixing into purlins along the sheet's length; at side laps, additional stitching screws (fixing the overlapping sheets to each other between purlins) may use a different pattern per the manufacturer's guidance, but the crest-not-valley principle for structural fixing into purlins holds regardless of profile type.

Question 7

How do I choose between corrugated, trapezoidal, and concealed-fix roofing profiles?

Accepted Answer

Corrugated (sinusoidal wave) profile is the most widely available and generally most economical option, well suited to standard pitched residential and light commercial roofing, but has a comparatively shorter maximum purlin span than deeper-ribbed profiles. Trapezoidal (box-rib) profile has deeper, more structurally efficient ribs that allow longer purlin spans, making it a common choice for larger commercial and industrial roofs where wider purlin spacing reduces structural cost, though it's typically somewhat more expensive per sheet than basic corrugated. Concealed-fix (clip-fix) systems have no visible through-sheet fasteners — panels clip together and are fixed via a separate clip system — commonly used on low-pitch and architectural applications where visible screw heads are undesirable or where the lower pitch benefits from having no screw penetrations through the weathering surface itself, though the system is generally more expensive and more installation-sensitive than screw-fixed profiles. Match profile choice to the roof's pitch, span requirement, budget, and whether visible fasteners matter aesthetically, not on cost per sheet alone.

Material	Coating	Advantage	Consideration
Galvanised steel	Zinc hot-dip coating, sacrificial corrosion protection	Well-proven, widely available, economical	Shorter service life than Zincalume in most environments once coating depletes
Zincalume (aluminium-zinc alloy)	~55% aluminium-zinc alloy coating	Generally longer service life than galvanised in most environments	Some aggressive chemical exposures need checking against manufacturer guidance
Colour-coated (pre-painted)	Galvanised or Zincalume base plus factory paint system	Aesthetic colour, added protective barrier over metallic coating	Paint system quality/thickness meaningfully affects actual service life
Aluminium	Solid aluminium sheet, no separate coating to fail	Excellent for aggressive marine/coastal exposure	Higher cost; softer than steel, more prone to denting

Environment	Recommended Material	Notes
Inland, low pollution	Zincalume or prepainted steel	25–40 year typical service life; standard galvanised also viable in genuinely low-corrosion inland areas
Coastal (within a few km of the sea)	Zincalume with quality paint system, or aluminium in the most aggressive exposure	Standard galvanised steel can corrode within a fraction of its inland service life in genuine coastal exposure
Industrial / urban pollution	Polyester-coated or PVDF-coated steel	Chemical/sulphur exposure attacks coatings differently than salt — needs a coating rated for chemical resistance

Profile	Typical Purlin Span	Notes
Corrugated (sinusoidal)	1.2–1.5 m	Standard residential and light commercial roofing; widely available
Trapezoidal (box-rib), shallow rib	1.5–2.2 m	Common mid-range commercial application
Trapezoidal (box-rib), deep rib	2.2–3.0 m	Larger commercial/industrial spans where wider purlin spacing reduces structural cost

Roof Pitch	Typical End Lap	Why
Above 15° pitch	~150 mm minimum	Steeper pitch sheds water faster; less opportunity for wind-driven backflow under the lap
10–15° pitch	~200 mm minimum	Lower pitch needs more lap length as a safety margin against wind-driven rain
Below 10° pitch	250 mm or more, or a different roofing system entirely	Some profiles are not rated for very low pitch at all — confirm against the manufacturer's minimum pitch rating

Region	Relevant Standards
United States	ASTM A653 (galvanised steel sheet) and ASTM A792 (aluminium-zinc alloy coated steel sheet) cover coating specifications; local building codes reference wind uplift and fastening requirements
Europe / UK	BS EN 508-1 covers profiled steel sheet for roofing; BS EN 10346 covers continuously coated steel sheet coating classes
India	IS 277 covers galvanised steel sheet; IS 14246 covers Zincalume-type aluminium-zinc alloy coated steel sheet
Australia / New Zealand	AS 1397 covers steel sheet and strip metallic coatings, including Zincalume and galvanised classifications
General guidance	Always follow the specific sheet manufacturer's load-span table, minimum pitch rating, and fixing schedule for the exact product being installed — general figures in this guide are a planning reference, not a substitute for the manufacturer's own published data

Roofing Sheet Types & Installation Guide

Sheet Material Comparison

Choosing Material by Environment

BMT and Purlin Spacing

Overlap by Roof Pitch

Fixing: Crest, Not Valley

Crest fixing (correct)

Valley fixing (incorrect)

Common Mistakes

Specifying Standard Galvanised Steel in a Genuine Coastal Environment

Ordering by Total Thickness Instead of Confirming BMT

Using a Generic Purlin Spacing Figure Instead of the Manufacturer's Load-Span Table

Fixing Screws in the Valley Instead of the Crest

Using an Undersized End Lap for a Low-Pitch Roof

Mismatching Ridge, Hip, or Valley Accessories to the Sheet Profile

Relevant Standards and References

Final Verdict

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