Why Do They Put Rocks on Roofs?

Why Do They Put Rocks on Roofs

Why Do They Put Rocks on Roofs? It’s common to see loose rocks, gravel, or stone ballast covering commercial flat roofs. But why are rocks placed on top of roofing membranes? What purpose does this coarse rocky layer serve?

There are several practical reasons roofing professionals apply protective layers of rocks or stone ballast to flat commercial roofs. 

The stones shield the waterproofing membrane below from damage, provide ballast and fire resistance, and help regulate roof temperature.

Understanding the benefits rocks bring to flat roofing systems gives insight into this smart roofing practice.

Key Reasons Rocks Are Used on Flat Roofs

  • Protect the roofing membrane from weathering, UV rays, hail, and foot traffic.
  • Hold roofing components in place as ballast against wind uplift
  • Improve fire rating by covering combustible materials
  • Regulate temperature by reflecting sunlight and insulating
  • Hide and protect the waterproofing membrane for an attractive finish

Covering flat roofs with rocks or gravel is an integral part of the roof assembly on many buildings.

How Rocks Protect the Roofing Membrane

Flat roofs are covered by a continuous waterproofing membrane that keeps the interior dry. This membrane may contain materials like PVC, TPO, EPDM rubber, or modified bitumen.

The membrane is strong but still vulnerable to damage from weather, sun exposure, falling debris, and foot traffic. Loose lay rocks or gravel act as a protective barrier shielding the membrane.

Weathering Damage

Exposure to rain, hail, snow, and temperature swings can degrade roofing membranes over time. A top layer of rocks shelters the membrane from these elements.

Water runoff washing over bare membranes erodes surfaces. But rocks break up and slow runoff, mitigating this abrasion.

Hail can dent and puncture uncovered membranes, especially softer ones like TPO (thermoplastic polyolefin). A rock ballast layer absorbs hailstone impacts before they reach the membrane.

UV Degradation

Ultraviolet rays from the sun degrade and accelerate the aging of roofing materials. This leads to problems like surface cracking and discoloration.

Rocks deflect and scatter sunlight, shielding membranes below from intense UV exposure. This significantly slows UV damage and deterioration.

Foot Traffic Wear

On roofs with equipment that require maintenance, foot traffic can grind down and puncture membranes. Gravel protects against this abrasive wear and tear.

The loose rocks also provide traction, preventing slips and falls, which could further damage the roof. They cushion and distribute concentrated loads.

Overall, a gravel, crushed stone, or river rock layer shields vulnerable roofing membranes from environmental factors that reduce lifespan.

rocks on roof

Ballast to Hold Down Roofing

Another vital purpose of rock ballast is to use its weight to provide ballast resistance, securing the roofing assembly.

The gravel counteracts upward wind forces trying to lift the roofing components. It prevents billowing or detachment of the membrane and insulation boards from wind gusts.

Ballast requirements vary based on roof size, location, and building height. A typical gravel ballast depth is 1″-2″ but can be over 3″ for high wind zones. Ballast is calibrated to local wind speeds and roofing system design.

Perimeter areas and corners often require heavier ballast concentrations to resist increased uplift forces. Rooftop equipment and penetrations also need enhanced ballasting to anchor them down securely.

As covered next, ballast rock weight protects against fire hazards and moisture accumulation.

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Fire Resistance Benefits

The mass of gravel or stone roof ballast improves fire resistance in several ways:

Fire Rating

The rock layer shields combustible materials like wood decking and separates them from ignition sources. This may boost the roof assembly’s fire rating by slowing flame spread.

Smothering Embers

Burning embers landing on the gravel are extinguished and smothered during a fire instead of accumulating on bare membranes.

Moisture Retention

By retaining rainwater, the rocks maintain higher moisture content, which resists ignition. In contrast, bare, dry membranes are more vulnerable.

Additional Protection Options

For even better fire safety, specialized stone ballast types like river rock aid drainage, while compact aggregates like Black Beauty absorb heat and shed burning embers.

Temperature Regulation

Rock ballast moderates rooftop temperatures year-round. In summer, it reflects heat and provides shade. During winter, it insulates against frost and cold air.

Cooling Effects

The light color of rock ballast reflects sunlight instead of absorbing it. This reduces heat gain, minimizing the roof’s peak temperatures on hot sunny days.

The mass of gravel also creates an insulating barrier, slowing the transmission of exterior heat into the building.

Winter Insulation

In cold weather, the rocks act as insulation, keeping warmth and winter temperatures out for reduced heating costs.

They also absorb energy from weak winter sunshine and release it slowly to provide subtle warming.

Enhanced Durability and Appearance

Beyond their functional benefits, rocks also enhance durability and provide an attractive finish:

  • Disguises flaws and patches in the waterproofing membrane
  • Hides dirt buildup, improving appearance over time
  • Provides additional impact protection for membrane durability
  • Prevents roof damage from solar heat concentration at glass edges

So, loose-laid gravel or stone ballast is necessary in roof system performance and longevity.

gravel on roof

Common Ballast Rock Types Used on Roofs

There are many suitable types of stones and aggregates used for ballast. The most common options include:

Ballast TypeDescription
Washed river gravelSmooth rounded river rocks with high drainage capacity
Crushed stoneIrregular chunks and pieces of aggregate with sharper edges
Marble chipsLow dust marble gravel is available in white or colors
Volcanic gravelLightweight porous rocks like scoria or pumice
Recycled roof ballastReused stone gravel cleaned of dirt and roofing dust
Black beauty slagInert blast furnace slag that absorbs heat and sheds embers

Many sizes and gradations are available within these categories, from fines to 2″+ cobblestones. Roofing contractors assess ballast needs and specify appropriate aggregate.

Key Considerations for Rock Ballast Roofs

There are some important considerations when planning and maintaining gravel roofs:

  • Aggregate size matters – smaller rocks shift more quickly in the wind versus larger diameters
  • Angular crushed stone provides better grip versus smooth round rocks
  • Aggregate must be dry before installation – wet gravel gets heavy
  • Base roofing components must withstand ballast-weight loads
  • Inspect annually and replace shifted or depleted ballast
  • Control roof access to avoid displacing rocks
  • Consider a separation layer between gravel and membrane to prevent abrasion
  • Replenish ballast after any maintenance or repairs
  • Remove all ballast before altering the roofing system or membrane

With routine inspection and maintenance, a quality rock ballast roof will provide decades of reliable performance.

Why Rocks Are an Effective Roofing Practice

Rock ballast is a simple, low-cost way to enhance roof protection. The natural stones shield vulnerable membranes, retain heat or cooling, prevent uplift, and improve aesthetics.

Gravel’s flexibility also allows building owners to easily remove and replace ballast to conduct any needed roof repairs or modifications down the road.

Unlike alternative solutions like concrete pavers or walkways, loose-laid rocks involve less installation and maintenance labor, components, and costs. The natural loose-lay approach continues to prove an enduringly effective roofing practice.

So next time you see a commercial roof covered in rocks, know that it is not just a random decoration – the gravel is an integral protective shield extending the roofing system’s life.

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