Overlaying a tiled roof with steel sheets is asked about regularly, usually because stripping and disposing of the existing covering is the expensive part of the job. It can be done, but it is not simply a matter of screwing sheets over what is already there, and on some roofs it is the wrong decision.

The structural question comes first

You are adding load to a structure that was designed for a different covering. Steel sheets are light, which helps, but the counter-battens or new framework needed to create a fixing plane add weight too, and you are keeping the old covering in place as well. On a roof that is already showing signs of movement, this is not the moment to add anything.

If there is any doubt about the condition of the rafters, wall plates or supporting structure, that needs a structural opinion before the job is priced, not after.

You need a proper fixing plane

Tiles are uneven by nature, so sheets cannot be fixed through them directly. A framework of counter-battens or purlins is set out over the existing roof to give a flat, correctly spaced surface to fix into. Getting this right determines whether the finished roof is flat and whether the fixings land where they should.

Ventilation and condensation

Sandwiching the old covering between the structure and a new steel skin creates a void, and that void needs to breathe. Trapped moisture with nowhere to go will find the timber. Either ventilate the void properly or use a build-up that manages moisture deliberately, such as anti-condensation sheets.

Details and edges

The roof will now sit higher than it did, which affects verges, eaves, abutments and anywhere the roof meets another surface. Gutters may need repositioning. Flashings and trims have to suit the new build-up rather than the old one, and these junctions are where an overlay most often fails.

Pitch

A roof designed for tiles usually has ample pitch for profiled sheet, which is helpful. If the pitch is shallow, end laps need to be longer and sealed, as covered in our guide to overlapping roof sheets.

When to strip instead

If the existing covering is failing, if there is any doubt about the structure, or if the roof has suffered water damage, overlaying hides problems rather than solving them. Stripping costs more up front and is very often cheaper over the life of the roof.

For sheets cut to your exact sizes, send us the measurements.