Best Practice for Measuring an Industrial Roof for Coating Works
This guide explains how to measure an industrial metal roof for coating works, allowing contractors to estimate roof areas, laps, gutters and details when preparing refurbishment quotations.
When pricing or estimating the cost of an industrial roof coating project, the quality of the measurements matters.
A roof area taken from above is a useful starting point, but on its own it is rarely enough. To prepare a realistic quotation, it is also important to understand the condition of the roof, for example, the extent of corrosion, whether mid-laps require sealing, whether any previous treatments are present, and whether the roof has actually been inspected.
This guide explains how to measure an industrial metal roof for coating works when preparing a contractor quotation, including roof area correction, corrosion measurement and mid-lap sealing requirements.
It is useful for professional specifiers and roof coating contractors preparing tender documentation and specifications for budget costing and material pricing estimation.
Key Information Required for a Roof Coating Quotation
- Overall roof area in square metres
- Profile and pitch correction (typically up to 20%)
- Total length and width of edge corrosion
- Total length and width of mid-lap corrosion
- Whether mid-laps require sealing
- Sealing method (HP Butyl Tape or sealant)
- Whether existing cut edge treatments are present
- Whether measurements are from Google Earth or physical inspection
Accurate roof measurements allow coating contractors to prepare realistic quotations and help ensure the correct refurbishment strategy is selected for the roof.
Why accurate roof measurement matters
For coating purposes, the plan size of the roof is only part of the picture.
An accurate quotation may also depend on:
- the corrected roof area once profile and pitch are allowed for
- the total length of edge corrosion
- the total length of mid-lap corrosion
- the treatment band widths required
- whether mid-laps are to be sealed
- whether old cut edge repairs are already present
- whether the information has come from Google Earth or from a physical inspection
The more complete the information, the more reliable your tender pricing is likely to be.
Step 1: Measure the overall roof area
Start by measuring the overall roof size in square metres.
This is often done using roof plans, scaled drawings, or online tools such as Google Earth. These usually give the flat plan area only. On a profiled steel roof, the true coated area will normally be greater because the sheets are corrugated and the roof is pitched.
Allow for profile and pitch correction
For quotation purposes, the roof area should normally include a profile and pitch allowance of up to 20%.
This helps account for the extra surface area created by:
- the roof profile
- the pitch of the roof
- the corrugations and troughs in the sheeting
Example:
If the plan area is 1,000m², the coating area may be treated as up to 1,200m² for pricing and material calculations.
Step 2: Measure edge corrosion lengths and band widths
Next, identify and measure any edge corrosion requiring treatment.
For pricing purposes, it is not enough to say that edge corrosion is present. The quotation will normally depend on both the total length and the width of the affected area.
What should be recorded
For each area of edge corrosion, note:
- the location
- the total linear metres
- the average treatment band width
Example:
North slope eaves line: 42 linear metres at approximately 100mm band width.
Step 3: Measure mid-lap corrosion lengths and band widths
Mid-lap corrosion should be measured separately from edge corrosion.
This usually appears along the side laps of roof sheets and may show as rust tracking, coating failure or deterioration along the lap line.
For quotation purposes record:
- the total linear metres affected
- the average treatment band width
Important: edge corrosion and mid-lap corrosion should always be measured separately.
Step 4: Confirm whether mid-laps need sealing
Not every roof requires mid-lap sealing, so this should be confirmed rather than assumed.
Where sealing is required, this should be identified clearly at quotation stage.
Record the sealing method
If sealing is required, note whether the proposed method is:
- HP Butyl Tape
- Sealant
What should be recorded
For each affected area record:
- whether sealing is required
- the total length of lap to be sealed
- the proposed sealing method
Example:
South slope mid-laps: 68 linear metres with a band width of 100mm to be sealed using HP Butyl Tape.
Step 5: Check whether an existing cut edge treatment is already present
If a previous cut edge corrosion treatment has already been applied, this can affect both pricing and suitability for overcoating.
Older treatments may be:
- cracked
- brittle
- poorly adhered
- debonded
- trapping moisture
- masking deterioration of the sheet edge
- hiding a much bigger defect
In some cases, what appears acceptable from aerial imagery may prove problematic when inspected more closely.
Record whether
- no previous treatment is visible
- a previous treatment is visible
- it is unknown because the roof has not been inspected closely
Where an existing treatment is present, an on-site survey may sometimes be necessary before a reliable quotation or tender document can be provided.
Step 6: State how the measurements were obtained
This should always be made clear.
There is a significant difference between measurements taken from aerial imagery and those taken following a physical roof inspection.
Remote measurements
Example wording:
“Areas and lengths are approximate and based on Google Earth imagery or drawings only. No physical roof inspection has yet been carried out.”
Physical roof inspection
Example wording:
“Measurements and observations taken following a physical roof inspection.”
Being clear about this helps set realistic expectations for pricing accuracy and also helps contractors and contract administrators avoid ambiguity.
Step 7: Be clear about what cannot be confirmed remotely
Remote measurement can be useful for early budgeting, but it has limits.
Google Earth, roof plans and aerial imagery cannot always confirm:
- the true severity of corrosion
- whether previous treatments are sound or failing
- whether roof sheets are thinning or deteriorating
- whether laps are open
- whether sealing is appropriate
- whether local repairs may be required
Where these points cannot be confirmed, it is sensible to allow for the possibility that an on-site survey may be required before final pricing or specification is agreed.
Checklist before requesting a quotation
Before requesting a quotation for roof coating works or material costing, try to provide the following information.
Roof area
- overall roof size in m²
- include profile and pitch correction (up to 20%)
Edge corrosion
- total linear metres
- average band widths
Mid-lap corrosion
- total linear metres
- average band widths
Mid-lap sealing
- whether sealing is required
- length to be sealed
- whether HP Butyl Tape or Sealant is proposed
Existing treatment
- whether any previous cut edge corrosion treatment is already present
Inspection method
- whether measurements are based on Google Earth, drawings, drone imagery or a physical inspection
Survey requirement
- whether the roof can be priced reliably from the available information
- or whether an on-site inspection is likely to be required.
Practical guidance
For early budget pricing, approximate measurements from online tools may sometimes be enough.
For firm quotations, particularly where corrosion is significant or previous repairs are visible, a physical inspection is often the better approach.
The aim is simple: gather enough information at the start so the quotation reflects the actual condition of the roof, not just the plan size of the building.
Accurate roof measurements allow appropriate refurbishment systems to be identified and material quantities to be estimated, enabling contractors to obtain reliable pricing for systems such as Liquasil’s metal roof coating and gutter lining solutions.
If you would like assistance estimating material quantities or obtaining a supply price, send your measurements to our technical team or
call 0121 709 5352 for an instant materials and cost estimate.
Technical Guidance Hub
If corrosion is visible on the roof while measurements are being taken, it may be helpful to review our industrial roof defect guidance which explains common issues such as cut edge corrosion, coating failure and mid-lap deterioration. All of these can significantly affect the pricing of roof coating projects