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Looking up inside a large steel portal-frame building at first light: the bare underside of a profiled metal roof between purlins, rooflights along the ridge, pale daylight.

Application

The underside of a profiled metal roof: the build-up, in sequence.

Steel profiled sheet, purlins, fixings. What is applied to it, in what order, and what makes each step succeed or fail. The page a specifier reads to decide whether the applicator knows what it is doing.

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 it.

The existing sheet is the weather line, and it stays. Closed-cell foam is applied to its underside from below, in passes, following the profile and closing the side laps. The purlin runs through the layer and is still a thermal bridge; the calculation accounts for it rather than ignoring it. Five things in the section, keyed beneath it.

Schematic / not to scaleKeyed 1 to 5 below

A cut sample of profiled steel roof sheet on a purlin, with a closed-cell layer applied to its underside following the profile, cut cleanly to show the section.
The sectionSheet, layer, purlin
The same section as the drawing, cut through: the sheet on top, the layer following its profile beneath, the purlin running through it.
1 / Existing profiled steel sheet

The weather line, and it stays. We do not replace it and we do not make it watertight.

2 / Prepared underside

Loose coating, corrosion product and contamination removed. Adhesion is decided here, not later.

3 / Closed-cell layer, in passes

Follows the profile and closes the side lap. λ 0.027 W/mK is the NexSeal LE product figure, not a HOLVEM performance figure.

4 / Purlin and fixings

Encapsulated by the application, and still a thermal bridge. The calculation accounts for it rather than ignoring it.

5 / Coating, where specified

B-s2,d0 belongs to the coated NexSeal LE and DC315 system, as tested. Where the coating is not applied, that classification does not carry.

Closed-cell system dataThe building this is written for most: warehouse and distribution

02Project records

The sequence, on real roofs.

HOLVEM's own footage and photographs on profiled metal roofs: continuous passes from beneath, the layer following the profile and wrapping the purlins, and a roof where the rooflights were left exposed. Press play to watch.

Project recordContinuous passes
Enverge NexSeal LE applied in continuous passesReal installation footage.
Project recordRoof underside
Roof underside sprayed in continuous passesClosed cell, from below, the profile followed.
An empty warehouse floor looking towards an open roller-shutter door: the whole profiled steel roof and its purlins sprayed from beneath in a continuous pale closed-cell layer, the rooflights and high-bay lights left exposed.
Project recordRooflights left exposed
Warehouse roof sealed from beneathClosed cell; the rooflights left exposed, the layer stopped at each.

All project records

03The sequence

Five steps. Each one can fail.

A build-up is only as good as the step that was done worst. Step through the five in order. For each, the drawing shows the step as it succeeds and as it fails; choose either. The succeed condition is what HOLVEM verifies as the work proceeds; the fail condition is what a poor applicator leaves behind the layer, where nobody sees it.

STEP 1 · AS IT SUCCEEDSSchematic / not to scale

Each step is checked before the next begins. Where one cannot be met on the day, the work stops and the reason is recorded; it is not sprayed anyway.

04Why this layer, on this sheet

A steel roof gets cold faster than almost anything else in the building.

Why this layer on this sheet. The same roof on a cold, still morning: bare, and with spray foam applied. Switch between the two and read the three marks in each.

Switch between the two

Tap either, or press ← → with the switch in focus.

Three marks on each picture. The run beneath says why.

Looking up inside a steel-framed industrial building on a cold morning: the bare grey underside of a profiled steel roof between purlins, beaded with condensation along every trough, a translucent rooflight strip letting in flat daylight, and a single drop falling from the sheet. The same kind of roof from the floor: the underside of a profiled steel roof with a continuous pale closed-cell layer following every rib and wrapping the purlins, one bay at the left edge still bare grey steel, the rooflights left clear.

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

1The sheet

Thin, cold, wet.

Under a millimetre of steel between the weather and the building, with almost no thermal mass. On a still, cold morning it is the coldest surface inside: below the dew point of the air beneath it, so the air gives up its water onto the sheet.

2The laps and fixings

Every rib, lap and fixing is a path.

Warm, wet air from the floor reaches the cold steel at every joint in the sheet: the side laps, the end laps, and every fixing through it. An insulation that leaves those joints open has left the problem where it was.

3The drip

It looks like a leak. It is not one.

Water forms across the whole field of the sheet and falls under the valleys, onto whatever the building holds. Repairing the roof would not stop it; the roof is not letting anything in.

The short version: a thin steel sheet gets this problem faster than most roofs, and a sprayed layer is the one that follows its shape instead of bridging it. The rest of this page is how that layer goes on properly.

05The rooflights

Sprayed, or left exposed. Your call.

Some customers have the rooflights sprayed as well, for a full airtight seal with no interruption in the layer. Others want them left exposed. Both are real jobs, done differently. Choose either below; the panel shows the idea of it, material for the seal, daylight for the rooflight.

Sprayed, on request

One continuous seal, rooflights included.

Primed first with Bostik Idenden ET-150, then the layer carries over the upstand and the rooflight without a break. The primer protects the foam from UV degradation, and protects the rooflight.

01Prime · ET-15002Spray03One seal

Left exposed

Daylight stays. The layer stops short.

Boxed off in advance by the client, or covered by HOLVEM where possible. The layer runs to a closed, coated stop short of the upstand; the upstand and the rooflight are left as they are.

01Box off or cover02Spray to the stop03Close the edge

TWO ROUTES · CHOOSE ONE BELOW

Every other termination is detailed on the project, before the work starts.

GuttersEavesRidge and vergePenetrationsService supportsPlant basesWall junctions

The layer stops at a coated edge; the detail is drawn per project. A service that will need access is stopped short of, not sprayed over. Step 5 of the sequence above shows a stop as it succeeds and as it fails.

06Which depth fits

Four objectives. Each one sets the depth.

Decide what the roof has to do and the depth follows. Four depths are drawn to one scale below, with what each one is for. The survey confirms the right one on your roof, with the purlins in the calculation.

Four depths, drawn to one scale and measured from the sheet underside. The objective and the calculation set the thickness on your building.

ObjectiveDepthWhat that depth is for
Control condensation25 mmCondensation control on the sheet. The face the air touches sits above the dew point, so nothing forms and nothing drips. Not a U-value and not a materially warmer building.
Retain heat50 mmCondensation control plus useful heat retention. The building holds its heat overnight, so there is less to bring back in the morning. Not a percentage saving and not compliance on its own.
Reach a calculated U-value75 mm and aboveSet by the U-value calculation. A figure to meet for a heated or temperature-controlled building, calculated for this build-up with the purlin bridge in it; the assessor holds the compliance position.
Achieve Euroclass BDepth to suit + DC315The coated system, where the building needs it. DC315 intumescent coating on the cured layer: B-s2,d0 to BS EN 13501-1 belongs to the coated NexSeal LE and DC315 system as tested: 35 to 40 mm of NexSeal LE on 9 mm plasterboard with DC315 at 0.7 mm wet film thickness (BTTG report 27/06638/12/24), confirmed for each project against the fire strategy. Uncoated, the layer carries Class 1 surface spread of flame to BS 476-7, which is not a Euroclass.

Thickness is set by the objective and the calculation, never by the area. Whether a classification is required is the fire strategy's question; HOLVEM answers what the coated system achieves.

Thickness and objectiveFire performance and coatingsWhat it costs: the cost guide

07The survey

Five things we check before anyone talks about depth.

A survey is one visit, from below, on your floor, fitted around your shift pattern. Nothing comes off the roof and nothing stops. It gives us the five things a depth is confirmed against.

  1. The objective

    Condensation control, thermal performance, a calculated U-value or a reaction-to-fire classification. The objective sets the depth, so it comes first.

  2. Substrate and weathertightness

    The condition of the sheet, from below, and whether the roof is weathertight. Active ingress is repaired first, by someone else.

  3. Where the system stops

    Rooflights, vents, ridge, eaves and penetrations, and what each termination needs.

  4. Fire classification

    Whether the fire strategy calls for a classified surface, and therefore whether the coating is specified.

  5. Keeping the floor working

    Bays, clearing, containment and shift pattern. The floor keeps operating while we work above it.

What you get after the survey

Five things in writing, whether or not you go ahead.

  • A specificationWritten for this roof, from what we found, with its assumptions named.
  • The depth, and whySet by the objective and confirmed by calculation for the element as built, bridges included.
  • The condensation-risk positionWhether anything will condense inside the build-up, to BS EN ISO 13788 with the BS 5250 climate.
  • The fire positionWhether your fire strategy calls for a classified surface, and the coated or uncoated build-up that answers it. Confirmed against the strategy, never assumed.
  • A firm priceFor your roof, in writing. What moves it is explained in the cost guide.

Survey, or photographs?Send photographs of the roof

Tell us about the roof. The survey follows.A short brief: the building, what you are seeing, and how to reach you. Sent before 3pm on a working day, we are in touch the same day.

Start a project brief

Project records

Where we have done this before.

All project records
Project recordTJW Grab Hire, BournemouthA 505 m² box-profile roof at 50 mm. The drips onto the lorries stopped.Read the record Project recordA Midlands distribution warehouse75 mm of closed-cell under a corrugated metal roof that leaked air and ran with condensation.Read the record

08Straight answers

Ask us the hard ones.

The questions people with a metal roof actually ask on the call, answered the way the specialist answers them. A question we have not answered? Ask it on the call. No obligation.

Can this save me from replacing the roof?
Often, yes. Where the sheets are sound, closed-cell foam bonds to the underside and seals the laps and fixings in place, which is why a roof replacement gets deferred rather than booked. But foam is not a structural repair. It will not rescue a roof that has already failed, and it will not hold up sheets that have gone. That is what the condition survey establishes, before anything is recommended.
How much does metal roof spray foam insulation cost?
It depends on the depth, and the depth depends on what you are trying to fix: 25 mm is the condensation control specification, 50 mm controls condensation and retains useful heat, and above 75 mm the depth is set by the U-value you are targeting. Access, substrate condition and preparation move it as well. What moves the price is set out in the cost guide; send the building in a project brief and you get a real figure for your roof.
How do you prove the system was installed correctly?
Applied thickness measured and recorded, coverage logged, all compiled into a QA evidence pack handed over at completion. You hold the evidence, not our word.
Why don't you recommend the same thickness for every building?
Buildings don't share heat-loss profiles, moisture risks or targets. Condensation control may need 25 mm; a contract-specified U-value needed 140 mm. The thickness is the output of analysis. A standardised number would mean the analysis never happened.
Isn't spray foam a problem for surveyors and lenders?
Those concerns overwhelmingly involve residential lofts and undocumented installs. Our commercial work is the opposite case: BBA-certified products, written specification from condensation risk analysis and U-value calculation, QA evidence pack at handover. The building file answers the surveyor's question before it's asked.

The next step

Your building.
Your brief.
A specialist.

You may already know the thickness, the system or the U-value; if so, we price to it and confirm it is the right specification. If not, tell us what you do know about the building and the roof. “Not sure” is a useful starting point.

Start a project brief

Or call . Enquiries received before 3pm on a working day are contacted the same day. After 3pm, the next working day. A commitment to contact, not a quotation, a survey or a price.

Photographs of the roof first? Upload photographs of the building · about five minutes, nothing else needed.