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The underside of a corrugated sheet roof inside a UK agricultural building, the sinuous profile running away over timber purlins in soft morning daylight.

Application

The underside of a corrugated roof: sealed from beneath, corrugation by corrugation.

Sinusoidal steel, box profile or fibre-cement, the layer follows the wave of the sheet instead of bridging it, closes the laps and fixings, and stops the drip that looks like a leak.

Start a project brief What it costs: the cost guide
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Is this your roof?

01The build-up

The sheet stays. The layer follows every wave of it.

A corrugated sheet has more surface, more laps and more fixings per square metre than any flat roof, and every one of them is a path for warm, wet air to reach cold metal. Switch the drawing between the bare roof and the sprayed roof; the three marks and the reading beneath follow.

Two states. Switch between them; the marks and the reading follow.

Swipe across to see the whole section.

BARE SHEET · A COLD MORNINGSection, schematic, not to scale. Three marks in each state; the cards beneath read them.

Cold sheet · below the dew pointThe dew point · water formsAbove the dew point · dry

1The sheet

Thin, cold, and wet by morning.

A corrugated sheet has almost no thermal mass. On a still, cold morning it is the coldest surface in the building: below the dew point of the air beneath it, so that air gives up its water onto the metal.

2Laps and fixings

The wave multiplies the joints.

Every side lap, end lap and fixing is a path for warm, wet air to the cold sheet. An insulation that bridges the corrugations leaves those joints open behind it.

3The drip

It looks like a leak. It is not one.

Water forms across the whole sheet, runs to the troughs and falls. Repairing the roof would not stop it; nothing is getting in.

The short version: a corrugated sheet gets condensation faster than most roofs, and a sprayed layer is the one that follows its shape instead of bridging it. A board cannot. The rest of this page is how the layer goes on properly.

02Project record

The sequence, on a real corrugated roof.

Installation footage from HOLVEM's own projects. This item is a project record, played on request; nothing on this page that is not real footage carries that label.

Project recordCorrugated roof, from beneath
Corrugated roof sprayed in continuous passes. Closed cell, from below, the corrugation followed.

Continuous passes, the wave followed.

The applicator works along the bay, each pass overlapping the last, the layer taking the shape of the sheet as it cures. What you see is the face the building will have from then on.

All project records

03The order of work

Five steps, from below, in order.

The work is done from inside the building, from the floor or from access equipment, bay by bay. Each step is checked before the next begins; where one cannot be met on the day, the work stops and the reason is recorded.

  1. Substrate condition

    The sheet is inspected from below: profile, coating, corrosion, moisture and whether it leaks. A leaking roof is repaired first, by someone else. The layer does not make a roof watertight.

    HOLVEM checksSound, dry and weathertight before any foam goes on.

  2. Preparation and protection

    Loose coating, corrosion product, dust and contamination come off the underside. Rooflights, lights, cables and the floor beneath are masked or covered. Adhesion is decided here.

    HOLVEM checksClean, dry, keyed surface; everything beneath protected.

  3. Passes and depth

    Closed-cell foam is sprayed in passes, following each corrugation and closing the side laps. Depth is built up to the specified figure and measured as the work proceeds.

    HOLVEM checksApplied thickness measured and recorded, bay by bay.

  4. Coating, where specified

    Where a reaction-to-fire classification is required, an intumescent coating is applied over the cured foam. Where it is not specified, the foam is left as the finished face.

    HOLVEM checksCoverage logged where a coating is specified.

  5. Terminations

    Eaves, ridge, gutters, penetrations and rooflight upstands are detailed per project. The layer stops at a closed, coated edge, never feathered out or left open.

    HOLVEM checksEvery stop closed and detailed.

A close detail of the underside of a bare corrugated sheet on a timber purlin, the wave of the profile and a row of fixings visible, pale daylight along the ribs.
BeforeThe bare sheet, from below
The substrate as it is inspected: the underside of the corrugation, its fixings and its purlin. Everything the layer will follow is visible from here.

Every step is checked before the next. The evidence is compiled into a QA record handed over at completion: thickness measured, coverage logged.

04Preparation

Before the first pass: what the roof has to be.

Most of what decides a good result happens before the gun is switched on. Three things HOLVEM establishes on a corrugated roof, and what happens if one of them is not met.

Weathertight

Leaks are repaired first.

Foam is not a roof repair. Slipped sheets, failed laps and holed fixings are put right by a roofer before the layer goes on. Spraying over a leak traps the water where nobody can see it.

Clean and dry

Adhesion is decided here.

Corrosion product, loose paint, dust and oil come off the underside, and the sheet is dry at the time of application. A wet or dusty sheet fails the bond, and a failed bond fails the seal.

Protected beneath

The building keeps working.

Masking, sheeting and containment protect stock, plant, lighting and the floor. On a working building the bays are sequenced around the shift pattern rather than the other way round.

Survey, or photographs?How HOLVEM works: we understand your project before we price it

05Thickness and objective

The depth is set by what the roof has to do.

Three objectives, three depths. Each one is a direction for the survey to confirm on your roof, with the purlin bridge in the calculation. What it costs is on the cost guide, not here.

25 mmCondensation control

Keep the inside face above the dew point.

Enough to stop the drip on most corrugated roofs: the face the air touches is the foam, warm and dry, and the laps and fixings are closed. The objective where the building is unheated or lightly used.

50 mmHeat retention

Hold useful heat overnight.

Controls condensation and keeps warmth in the building, so there is less to bring back in the morning.

75 mm and aboveA calculated U-value

A figure to meet, and to prove.

Set by the U-value the project has to reach, calculated for this roof as built, purlins included. Specified per project and confirmed by the calculation, not by a rule of thumb.

The underside of a corrugated roof with a continuous cream closed-cell layer following every corrugation, the purlins wrapped, raking daylight showing the even finish.
AfterThe layer, following the wave
The finished face: continuous, following the corrugation, the purlins inside it. This is the surface the air in the building touches from now on.

Thickness and objectiveWhat it costs: the cost guide

06Where it suits

The buildings that carry a corrugated roof.

Corrugated sheet is the roof of the British farm and of a great many older industrial sheds. Three building pages written for it.

Building

Agricultural buildings

Livestock, machinery and grain under corrugated steel or fibre-cement: the condensation drip onto stock and feed is the reason most farmers call.

Read the building page

Building

Industrial units and factories

Older single-skin sheds still working hard. The layer stops the drip onto plant and product and keeps the heating bill in the building.

Read the building page

Building

Workshops and garages

Small, cold, and damp by morning. A corrugated roof sealed from beneath makes the space usable all year.

Read the building page

Every building type HOLVEM works on

07What HOLVEM checks

Three things checked before anyone talks about depth.

HOLVEM does not quote a depth from a photograph. The roof is understood first: how it behaves in winter, whether fire performance is required, and what U-value it must reach.

Condensation risk analysis

Will the inside face stay above the dew point?

A condensation risk analysis to BS EN ISO 13788 for the roof as built, with the internal conditions the building actually runs at. Where the simple method is inconclusive, HOLVEM runs a dynamic simulation to BS EN 15026.

Fire

Is a reaction-to-fire classification required?

Where it is, the coated system (closed-cell foam with an intumescent coating) is specified, classified B-s2,d0 in the build-up tested. Where it is not, the foam is left uncoated. The requirement is established before the depth.

U-value

What figure does the roof have to reach?

Calculated to BS EN ISO 6946 for this build-up, purlins and fixings included, not read off the foam's own conductivity. The calculation is yours to keep.

Condensation risk analysisU-values and Approved Document LFire performance and coatings

08Straight answers

Ask us the hard ones.

The questions people with a corrugated roof actually ask on the call, answered the way the specialist answers them.

Does spray foam work on fibre-cement corrugated sheet as well as steel?
Yes, and the sequence is the same, with one difference: a fibre-cement or asbestos-cement sheet goes through a condition survey gate first, because a brittle or friable sheet changes the job. The encapsulation page covers that substrate.
Will it stop my roof dripping?
If the drip is condensation, yes: the layer puts the face the air touches above the dew point, and the laps and fixings are closed. If the drip is a leak, no: the roof is repaired first, and the layer goes on afterwards. Telling the two apart is the first thing the survey does.
Can this save me from replacing the roof?
Often. Where the sheets are sound, the foam bonds to the underside and seals the laps and fixings in place, and the sheet also stiffens, so flexing and wind noise reduce. But foam is not a structural repair and will not hold up sheets that have gone.
What about the rooflights?
Your choice. Some clients have them sprayed for a full airtight seal, primed first with Bostik Idenden ET-150 to protect the foam and the rooflight from UV. Others want the daylight kept, so the rooflights are boxed off or covered and the layer stops short at a closed, coated edge.
How do you prove it was done properly?
Applied thickness is measured and recorded, coverage is logged, and the evidence is compiled into a QA record handed over at completion. You hold the evidence, not our word.

Project records

Where we have done this before.

All project records
Project recordA Midlands distribution warehouse75 mm of closed-cell under a corrugated metal roof that leaked air and ran with condensation.Read the record Project recordA Nissen hut for Rolls-Royce Motor Cars50 mm of closed-cell to the curve, finished with DC315 coating.Read the record

The next step

Your roof.
Your brief.
A specialist.

If you already know the thickness or U-value you need, we price to it and confirm it is right; if not, we work it out with you. Tell us what the roof is, what it does in winter and what you want it to do. Photographs of the underside are the most useful thing you can send.

Start a project brief Send photographs first

Or call . Enquiries received before 3pm on a working day are contacted the same day. After 3pm, the next working day. We understand your project before we price it.