Essential Principles of Roof Design and Drainage

Pitched Roof Drainage and Protection

Gutter Slopes: Under and above eaves min. 0.5%, above the cornice 0.66%, inter-roof and behind the parapet wall 1%. Expansion of gutters (galvanized, copper, titanium zinc) has a maximum length of 15 m.

Galvanic Corrosion: This occurs when two different metals are in electrical contact in the presence of an electrolyte (like water). In slopes, when rainwater runs over or stays between dissimilar metals, such as aluminum flashing and copper gutters, the less noble metal (anodic) corrodes faster, while the more noble metal (cathodic) is protected.

Measures Against Snow Falling

• Snow guards (catchers).
• Statically prepared structures.
• Prevention of ice dams and icicles at the eaves via good thermal insulation (double-skin, triple-skin).
• Heating cables inserted into the eaves.

Drainage of Flat Roofs

Roof classifications: Flat roof slope ≤ 5° (5° = 8.75%), Pitched roof 5°–45°, Steep roof ≥ 45°. The allowed minimum slope of a flat roof is 3% (2%). At a lower slope, there is a risk of ponding water.

Advantages

• Reduces the overall height of the building.
• Possibility of roofing irregular floor plans.
• Purposeful use of roof areas.
• Easy accessibility.
• Saving of materials and less labor-intensive.

Disadvantages

• Requires professional design of roof layer composition and thermal technical assessment.
• Strict adherence to technological principles during implementation.
• Inspection of internal layers without damaging the roof composition is not possible.
• Characteristic places (parapet connections, penetrations, drains) are prone to failure.
• Failures are difficult to detect and laboriously repaired.

Drainage Methods

1. Outside the layout (drainage into gutters).
2. Into the building layout (drainage of roof drains).

Systems:

• Gravity system: Lower drainage capacity.
• Vacuum system: Higher drainage capacity, ideal for large roof areas.

Drains must be placed at least 500 mm (preferably 1.0 m) from parapet walls and max 15 m apart.

Single-Skin Flat Roofs

A roof structure that separates the protected interior environment from the exterior with a single roof layer. The classic order is:

1. Waterproofing layer (upper strip).
2. Waterproofing layer (lower strip).
3. Thermal insulation in the slope (EPS, XPS).
4. Vapor barrier.
5. Supporting structure.

Inverted, Green, and Walkable Roofs

The inverted roof places the insulation above the waterproofing to protect it; the insulation must be water-resistant. The classic roof places the insulation below, leaving the waterproofing more exposed.

Inverted Roof

Advantages: Quick installation, waterproofing layer is protected against UV radiation and weather, and it eliminates the possibility of water vapor condensation.

Green Roof Layers

1. Vegetation (grass, shrubs, trees).
2. Substrate.
3. Filter fleece.
4. Drainage layer (washed gravel 8/16, mat with fibers, EPS drainage elements).
5. Geotextile (approx. 140 g/m²).
6. Styrodur® C.
7. Root protection layer.
8. Roof waterproofing.
9. Reinforced concrete slab.

Walkable Roofing

• Operational Layer: Screed, tiles glued to screed, on pads, or in a gravel bed.
• Separation Layer: PE foil, asphalt strip type A.
• Waterproofing: 2x asphalt strip S, or PVC foil.
• Thermal Insulation: XPS, foam glass, or EPS with a distribution board.
• Vapor Barrier: Safety waterproofing in the slope.
• Slope Layer.
• Ceiling construction.

Ventilated Double-Skin Flat Roofs

Also known as a cold roof, this structure separates the interior from the exterior with two roof layers and an air gap. The air cavity can be non-accessible, accessible, or walkable.

Upper Skin

Provides waterproofing. We must eliminate the risk of condensation on its underside. Components include: Waterproofing layer, thermal insulation, trapezoidal sheet metal, lattice truss, and a ventilated air gap.

Ventilated Air Gap

Vents water vapor to the exterior. The intake opening should be lower, and the exhaust opening higher (at least 100 mm). Distance should be lower than 18 m, with a minimum width of 50 mm.

Lower Skin

Provides thermal insulation and ensures vapor/air tightness. Components include: Diffusion-permeable foil, thermal insulation between lower chords of lattice trusses, lightweight vapor barrier, and a suspended soffit with thermal insulation.

Heavy vs. Lightweight Systems

• Heavy double-skin: Load-bearing structure is a massive ceiling (e.g., reinforced concrete). Advantages: Good airtightness. Disadvantages: Expensive and complex.
• Lightweight double-skin: Lower skin is a lightweight suspended soffit. Advantages: Covers large spans, economical, flexible interior space, quick. Disadvantages: Requires upper layer insulation, complex ventilation design, and potential thermal bridges.

Pitched Roofs and Structural Elements

Pitched roofs have a slope of 5° < α ≤ 45°.

Purlins and Replacements

Purlins transfer loads from rafters to vertical load-bearing structures (walls, columns). Replacements for roof windows, chimneys, or skylights consist of a shortened rafter supported on an auxiliary perpendicular element.