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A dark profiled metal roof in full sun, heat shimmer above the ridge and a bright rooflight, seen across an industrial estate.

Building problem

Summer overheating: the roof that loses heat in January collects it in July.

A bare metal sheet in full sun gets far hotter than the air around it, and it radiates that heat straight down onto the people, stock and animals beneath. The same thin sheet that lets heat out all winter lets the sun in all summer. This page is why, what it costs, and what a layer on the underside changes.

  • Metal and corrugated roofs
  • One layer, both seasons
  • Nationwide coverage

01What you see

Hottest under the roof, in the afternoon, on the top pallet.

Summer overheating follows the sun, not the air temperature. It is worst under dark sheets and rooflights, on mezzanines and top pallets, in the afternoon, and it fades quickly after sunset because a thin sheet stores nothing.

Looking up from a mezzanine at the underside of a dark profiled metal roof in strong afternoon sun, glare from a rooflight, a fan on a stand doing little.
Follows the sunUnder the sheet in July
  • Follows the sun, not the thermometer. The air in the shade may be pleasant while the space under the sheet is not. What you feel is radiant heat from above.
  • Worst high up. Mezzanines, top pallets and roof-level plant sit closest to the hot underside.
  • Gone by evening. A millimetre of steel holds no heat. When the sun goes the sheet cools within the hour, and in winter the same lack of mass works against you.

Not the outside temperature.Not fixed by moving the air.The sheet absorbs the sun and radiates it down. Change what the room sees.

02Why it happens

Solar gain through the sheet, then radiant heat from it.

Sun on a dark sheet is absorbed and becomes heat. The sheet, a millimetre thick, conducts it straight through, and its hot underside radiates into the room like a low-hanging heater. Shade-air temperature tells you little; the radiant heat is what you feel on your head.

Switch between the two states

A summer afternoon. The sun heats the sheet well above the air temperature, the heat conducts through a millimetre of steel, and the hot underside radiates down onto everything and everyone beneath it.

Schematic, not to scaleArrows show radiant heat, not measured temperatures.

03What it costs you

Heat stress, spoiled stock, and cooling bought to fight the roof.

An overheated building costs in the afternoons: people slow down, temperature-sensitive stock and animals suffer, and fans, cooling and open doors are bought to fight a roof that goes on radiating.

  1. The people

    Afternoons, mezzanines

    Workplace regulations require a reasonable indoor temperature, and radiant heat from the roof is what makes a space unreasonable. Productivity falls before anyone complains; the mezzanine empties first.

  2. The stock and animals

    Top pallets, pens

    Chocolate, chilled goods, pharmaceuticals, feed and livestock all take the radiant load from above. Write-offs and welfare problems that follow the weather forecast.

  3. The cooling

    Fans, doors, plant

    Fans move hot air, open doors bring in dust and security risk, and cooling plant runs against a roof that is radiating into the space. All of it is bought to fight the roof, every summer.

04What usually gets tried

Fans move the air. The roof is radiating onto you regardless.

Each usual remedy works on the air, and radiant heat does not care about the air. Fans and open doors help a little at the cost of dust and security; cooling plant is sized to fight the roof. One usual remedy does work on the sheet: a pale, reflective coating on top of it.

  1. Fans and open doors

    Moving air helps sweat evaporate and that is all. The hot underside radiates onto people and stock just the same, and open doors bring dust, birds and a security problem.

  2. Cooling plant

    Sized against a roof that radiates all afternoon, it runs hard and costs every hour. It treats the symptom in the air while the source stays above it.

  3. A pale, reflective roof coating

    Applied to the top of a sound roof, a pale silicone coating absorbs far less sun, so the sheet does not get as hot in the first place. It is a genuine lever on the source, and it waterproofs the roof at the same time. It is a topside job on a roof that is sound.

    Roof coatings and waterproofing

05What fixes it

A layer beneath the sheet. A pale coating above it where the roof is sound.

Closed-cell spray foam applied to the underside from inside the building, in passes, following the profile, so the room sees a face close to its own temperature. Where the roof is sound and summer gain is the main problem, a pale silicone coating on the top face cuts what the sheet absorbs. Together they work on the source from both sides.

  1. The layer, from below

    Bonded to the underside across every crown and trough, sealing the laps and fixings. The same layer that stops condensation and holds winter heat stops the hot sheet radiating into the room.

    The application, step by step

  2. The coating, from above

    A pale silicone coating over a sound roof, including asbestos-cement, reflects sun the dark sheet used to absorb and waterproofs the laps and fixings. Repairs to the roof come first.

    Roof coatings and waterproofing

  3. Ventilation sized afterwards

    With the radiant load removed, the fans and vents the building already has do their proper job: moving air that is no longer being heated from above.

06How HOLVEM checks

The sheet, its colour and what sits beneath it are recorded before a depth is named.

Overheating is a radiant problem, so HOLVEM looks at the sheet, its colour, its orientation and its rooflights, and at what the space beneath it holds. A vapour-closed layer also changes how the roof dries in summer, so the moisture risk is checked before the depth is confirmed.

  1. Photographs, then a survey from below

    Two photographs with a postcode tell a specialist the roof type. The survey records the sheet and its colour, the rooflights, the orientation, and what sits closest to the roof.

    Survey, or photographs?

  2. Condensation risk analysis

    The build-up is assessed to BS EN ISO 13788 and, where inconclusive, with a dynamic model to BS EN 15026 that runs through summers as well as winters. The result decides the depth.

    How the analysis is done

  3. Applied thickness recorded

    The depth is checked and recorded as the work proceeds and compiled into the completion pack you hold at handover.

07From our own jobs

The layer on a corrugated roof, from below.

Footage and a photograph from HOLVEM's own projects. Everything here carries the label it has earned.

Project recordCorrugated roof
Corrugated roof sprayed in continuous passesReal installation footage from an agricultural building.
A corrugated metal roof on timber rafters sprayed from beneath in a pale closed-cell layer, polythene masking below.
Project recordCorrugated on timber
Corrugated metal roof on timber raftersThe layer follows the corrugations between the rafters; the masking protects the room.

Written records, with their figures and the basis each one rests on, live on the project pages.All project records

Your next step

Your building.
Your brief.
A specialist.

You do not need to know the depth or the system. Tell us what the building holds, where it is hottest and when; if something is uncertain, say so. Someone who does this every day reads it and comes back to you.

Start a project brief About five minutes. Photographs help; nothing else is needed.

Or pick up the phone

Telephone . Before 3pm on a working day, you are contacted the same day. After 3pm, the next working day. A commitment to contact, not a quotation.

Two photographs first? The roof from outside in sun, and the underside from the floor. Send photographs of the roof