Many flat roofs in the Netherlands lie under a layer of gravel, and most owners do not know why. Sometimes there is a good reason, sometimes it was simply done that way because that was the norm at the time.
The distinction is worth making, because gravel has a downside you only notice when something is wrong.
What gravel is for
Ballast. With a loose-laid system, one that is not bonded or mechanically fixed, the gravel holds the covering in place against wind uplift. Then it is not an extra but a structural part.
UV protection. Some membranes are not UV-stable and have to be covered. Gravel is then the cheapest way to do that.
Temperature damping. Gravel absorbs heat and keeps the covering underneath cooler, which limits expansion and shrinkage.
Fire resistance. On some buildings a layer of gravel is used as protection against flying sparks.
“Gravel on a roof is fine, until you have to look for something. Then it takes half a day before you reach the covering.”
Fouad, owner of Astrodak Service
What it is not for
It does not make a roof more watertight, and it does not extend the life of a well-protected top layer. With a torched bitumen top layer with mineral slate chippings, the UV protection is already in the covering. With EPDM, the rubber itself is UV-stable. In those cases gravel is mainly weight.
It hides problems. Under gravel you cannot see cracks, blisters or a seam that has come loose. A roof can age unnoticed for years.
It makes leak detection expensive. With a leak, the gravel has to be removed first, barrow by barrow, and then put back.
It traps dirt. Leaves and silt settle between the gravel and keep moisture against the covering. At the outlet, that causes blockages.
It is heavy. A five-centimetre layer of gravel over a roof of a decent size is a serious load for an older structure.
Loose-laid system
gravel
structurally needed
Torched top layer
gravel
usually unnecessary
Not sure about the right approach?
I would rather come and look at your roof with my colleagues first than come up with a solution from a distance. During the free inspection you hear what is really needed, even if that is less than you thought.
Can you remove the gravel?
Only if you are sure the covering is not loose-laid and is itself UV-resistant. With a loose-laid system, removing the gravel removes the only fixing, and the covering can blow off in the first strong storm. That is not a theoretical risk.
If you do not know which system is there, have that established first. It can be seen in ten minutes by checking at one spot how the covering is attached to the substrate.
What we usually recommend
When replacing a roof, we often choose a bonded or mechanically fixed system with a self-protected top layer, and therefore no gravel. That makes the roof lighter, easier to inspect and easier to maintain. If anything is wrong in the future, you can see it and it can be checked in an hour.
If there is gravel now and the roof still has years left, you do not need to do anything. It is a good idea, though, to have the outlets and edges checked every year, because that is exactly where gravel blocks the view. See
the roof maintenance checklist and the
page about flat roofs.
How to tell which system you have
At one spot by the edge, push the gravel aside and look at how the covering is attached to the substrate. If it is loose and you can lift it, it is a loose-laid system and the gravel holds it in place. If it is stuck to the substrate, it is bonded or torched and the gravel has another function.
That difference decides whether you can remove the gravel. If in doubt, it is a ten-minute question for a roofer, and it is worth knowing before you are standing on your roof with a shovel.
Frequently asked questions about gravel on a flat roof
