Construction - Roofs

Low-Sloped Roofing Drains

• Drains should be placed at mid-span between structural supports.
• Roofs deflect under weight, making the mid-span the lowest point.
• Ponding is more likely in long spans due to structural deflection, leading to a cycle where the weight of water causes more deflection.
• The code allows roofs with at least a 2% slope or $(\frac{1}{4})$ inch per foot.
• Flatter roofs are permitted if over-structured to account for ponding.

Green Roofs

• Intensive green roofs (soil directly on the roof) may be flat for water retention.
• Extensive green roofs (plants on trays) require a minimum 2% slope.
• Slopes steeper than 10 degrees need precautions to prevent erosion.
• Green roofs are generally not suitable beyond 45 degrees.

Intentionally Ponding Roofs: Blue Roofs

• In areas like New York City, roofs are designed to intentionally pond water to manage stormwater runoff.
• Water is either stored on the roof or in containers inside the building and released slowly.
• Drains should be cleaned periodically to avoid ponding and potential catastrophic collapse.

Roofs vs. Walls

• Roofs are the critical horizontal span requiring more structure.
• Roofs are typically thicker than walls due to their horizontal span.
• Roofs are exposed to more sunshine, rain, and UV radiation.
• Roofs experience a wider temperature range than walls.
• Roofs often have a shorter lifespan than walls and require replacement multiple times.

Roofs as the Dentist

• Roofs require more maintenance and design attention.

Roof Slope Calculation

• To find the percentage of any slope, divide the vertical distance by the horizontal distance.
• For example, a 2/12 roof: $\frac{2}{12} = 0.17$, so the slope is 17%.

Types of Roofs

• Low-slope roofs (often called flat roofs) have a slope less than 2/12 (17%).
• Steep roofs have a slope greater than 2/12 (17%).
• Slopes shallower than $(\frac{1}{4})$ inch per foot are generally too flat without special structural precautions.

Steep vs Low Slope Roofs

• Steep Roofs:
  • Use gravity to shed water quickly.
  • Employ overlapping small units like shakes, shingles, or tiles.
  • Units can be easily replaced and can flex with the roof's movement.
  • Roofs are often visible.
• Low Slope Roofs:
  • Have slow drainage, so small errors can cause ponding or leaking.
  • Require flawlessly watertight membranes.
  • Are susceptible to blistering, flapping, or cracking due to water vapor or wind.
  • Can cover buildings of any horizontal scale.
  • Have simpler geometry, accommodating complex wall designs.
  • Can be planted (green roofs) or occupied (roof terraces).

Creating Slope in Low-Slope Roofs

• Slope can be achieved by: sloping the structure, tapering the structure, or tapering the insulation. All three options have varying benefits.
• Tapered insulation is common but can create divots that lead to ponding or mosquito breeding grounds.

Low-Slope Roof Surfaces: Families of Materials

• Bituminous sheets (asphalt-based)
• Rolled-out plastic membranes
• Sprayed-on waterproof layers

Membranes

• Membranes require a smooth substrate, such as gypsum or wood fiber sheets. Also known as substrate boards.
• The roof must be dry before membrane installation to prevent trapped moisture.

Preventing Roof Collapse

• Standing water on roofs is heavy and poses a risk of collapse.
• To manage this, secondary systems are put in place, which incorporate backup drains or overflow scuppers with separate piping.
• Backup drains should be no more than two inches above the primary drain.
• Backup drains should connect to a separate piping system and trigger an alarm.
• Overflow scuppers allow water to pour out of the building, signaling a potential issue.

Roof Drain Components

• Strainer (typically twice the area of the drain pipe)
• Hub or sump
• Clamp to secure the membrane to the roof deck
• Drain pipe

Low-Pitch Roof System Families

• Bituminous: Asphalt-based material.
• Single-ply: Rolled-on plastic membrane.
• Fluid-applied: Chemicals mopped on.

The Three Families in Detail

• Bituminous
  • ModBit
  • Multiple Layers of Overlapping Felt
• Single Ply
  • EPDM falls into this family
• Fluid Applied
  • Chemicals, sealants, or glues are mopped or sprayed on

Bituminous Roofing Characteristics

• Made from asphalt with overlapping layers.
• Felt layers are damaged by sun exposure and require protection.
• Protection methods include gravel ballast or aluminum coating.
• From bottom to top: roof deck, rigid insulation, asphalt, cover board, alternating layers of asphalt and felt.
• Modified bitumen (ModBit) is a modern version with greater elasticity and temperature resistance.

Single-Ply Roofing Characteristics

• EPDM is a type of single-ply roof. Plastic in sheet form that is unrolled and heat welded or glued at the seams.
• Metal sheets ca be used.

Fluid-Applied Roofing Characteristics

• Sprayed or mopped on seamlessly.
• Self-flashing.
• Multiple components like gauze and plaster.

Areas of Concern on Roofs

• Seams and penetrations (for mechanical equipment, pipes, plumbing).
• Differential movement due to expansion and contraction.
• Edges, seams, and holes require the most attention.

Common Roof Penetrations

• Plumbing vent pipes
• Roof hatches
• Exhaust fans
• Mechanical penthouses
• Roof access hatches
• Power and water lines to HVAC equipment.
• Roof drains and overflow drains.

Roof Penetration Details: Leak Likelihood

• Pitch pans are the most likely to leak.

Methods for Addressing Vertical Penetrations

• Pitch Pan: Metal pan around penetration filled with pourable sealant. Prone to leaks.
• Cone-Shaped Boot: Uses a clamp and tapered design to mechanically divert water.
• Cricket: Uses a mini-pitched element with a ridge to divert water around the penetration.

Accounting for Shrinkage and Expansion

• Building separation joints account for building expansion and seismic activity.
• Expansion joints accommodate shrinkage and expansion of the roof membrane.

Creating a Roof Detail

• Roof membrane should extend up the parapet wall for a better seal.
• A cant strip allows for a more moderate bending radius (two 45 degree bends instead of one 90 degree turn) of the membrane.
• Seal the membrane to the wall with fabric and mastic.
• Use counter flashing and flashing to protect the wall cavity.
• Top the assembly with coping (e.g., stone coping attached with stainless steel pins).
• Counter flashing allows roof membrane replacement without removing the coping and main flashing.

Vapor Control and Topsider Roof Vents

• An air and vapor barrier prevents moisture from reaching the cold membrane.
• Topsider roof vents allow moisture to escape from the space between the roof membrane and insulation.

Insulating a Roof

• Methods below roof deck
  • Vented Roofs
    • Insulation, and air moving in and out of a ventilated cavity.
    • Controls Condensation
    • Prolongs the life of the roof surface, because air is on both sides of the surface and they have the same temperatures.
    • Tiny leaks have a chance to dry out and are therefore mitigated
    • Baffles are used in rafter bays to maintain a ventilation.
    • Baffles allow air to move from the soffit to the ridge, even with insulation present
  • Spray Foam
    • Seals to the structure
    • Works as a vapor diffusion barrier and and air barrier
• Methods above roof deck
  • Outboard insulation
    • Roof structure, vapor barrier, rigid insulation, membrane.
    • Structure, membrane, insulation
    • This helps to keep membrane warm

Albedo & Emissivity

• A roof that controls heat gain from the sun must have both a high albedo and high emissivity
• Albedo is a term for solar reflectance
• High albedo roofs appear white, silver or shiny in color
• A good target for a reflectivity spec is 0.65, meaning the assembly reflects 65 percent of the visible light.
• Emissivity is a term for how a roof sheds heat.
• High emissivity roofs are better able to re-radiate the heat thet have absorbed to the sky
• White pavers or white gravel ballast can be used.
• Gravel ballasts can thermally outperform even green roofs.

Metal Roof Seams

• How these are sealed depends on whether a roof is standing seem or not. Also is affected by the presence of wood batons, which can restrict the connection.
• Metal can work with both low- and steep roofs.

Roofing - Steep Roofs: Shingles & Shakes

• Roof shakes are thicker than roof shingles.
• Shingles are typically tapered and overlapping, and staggered in plan.
• If using wood, it ought to be rot-resistant.
• A method for prolonging the lifespan of a roof is to keep the temperatures of both sides the same.
  • Skip Sheathing
  • Tangled Fiber Methods
• A layer of Asphalt Saturation Felt paper ought to be unrolled such that each layer overlaps the one below it, in order to account for which direction water will flow.
• Self Selling Strips can be used to help with insulation.
• Any place where a vertical surface extends beyond the roof is a risk for leaks, and ought to be flashed.