Whether box profile sheets need an underlay depends almost entirely on what the building is for. There is no single right answer, and specifying a membrane you do not need adds cost and an extra layer to get wrong.
What an underlay is actually for
A membrane beneath the sheet does three things: it catches any water that gets past the covering, it manages condensation forming on the underside of the steel, and it can help with airtightness. Of these, condensation is usually the reason it is specified on a metal roof.
When you probably do not need one
Open or unheated structures with good natural ventilation, such as field shelters, open-fronted carports and log stores, rarely justify it. There is little internal moisture, air moves freely, and any condensation that does form evaporates quickly.
When you do
- Buildings that are heated, even intermittently.
- Anywhere with a moisture source: livestock, washing, vehicles brought in wet, an unsealed floor.
- Spaces where dripping would damage contents, such as workshops with tools or machinery, or anywhere used for storage.
- Habitable or semi-habitable rooms, where comfort and condensation both matter.
The alternative: anti-condensation sheets
For many buildings the simpler answer is to deal with condensation at the sheet rather than adding a separate layer. Our Dripstop sheets carry a membrane bonded to the underside of the steel. It absorbs moisture as it forms and releases it back into the air as conditions warm, so it never reaches the point of dripping.
Because the membrane arrives already fixed to the sheet, there is no separate layer to support, tension or detail at the laps, which removes a whole category of installation error. For a single skin building this is very often the more practical specification.
And if you are insulating anyway
Where an insulated panel is being used, the question largely goes away. The inner face stays close to internal air temperature, so the surface does not reach the dew point in the first place.
Deciding
Work through it in this order: is the building heated, is there a moisture source, and would dripping matter. If the answer to all three is no, single skin is likely to be fine. If any is yes, look at anti-condensation sheets first and a full insulated build-up second.
Tell us about the building and we will point you to the right specification. All sheets are cut to your exact lengths and delivered within seven days.
