Halls and Large Roofs: Preventing Damage from Snow and Water – Danish Building Guidance
The document Haller og store tage – forebyg skader fra sne og vand is an official guidance issued by the Danish Authority of Social Services and Housing (Bolig- og Planstyrelsen). Published in July 2021, this reference provides technical instructions for preventing structural damage and collapses in halls and buildings with large roof surfaces due to snow accumulation and water buildup. It sits within the framework of the Danish Building Act (Byggeloven) and the Building Regulations 2018 (BR18).
The guidance covers risk assessment procedures, statutory requirements for static documentation, and technical specifications for emergency drainage systems. It specifically addresses buildings with large spans, such as sports halls and exhibition centers, and details 11 specific risk scenarios for snow drifting based on building geometry and surroundings. The scope includes structural elements such as glulam beams (limtræsbjælker), steel frames (stålrammer), and TT-elements, defining the technical deficiencies that often lead to failure under heavy environmental loads.
The following sections outline the legal responsibilities of owners, technical standards for drainage, and procedures for engineering assessments and emergency preparedness.
Legal Requirements and Statutory Responsibility
Under the Danish Building Act (Byggeloven), the building owner is legally responsible for ensuring the safety and maintenance of the structure. This includes a duty to prevent damage and ensure compliance with building legislation.
- Byggeloven § 17, stk. 1: Municipalities may order owners to rectify conditions that do not comply with the law.
- Forsikringsaftaleloven § 52: Owners must take reasonable steps to limit the risk of damage to maintain insurance coverage.
- BR18 Requirements: Construction must follow the calculations and execution rules in Eurocode 1 (DS/EN 1991-1-3) and the Danish national annexes.
- Static Documentation: Buildings in construction class 1 (KK1) and class 2 (KK2) or higher must provide documentation for the load-bearing structures as per BR18 § 489 and § 504.
Risk Assessment for Water and Snow Accumulation
The guidance identifies specific building types that require mandatory investigation by an experienced engineer to avoid collapse from local snow or water loads.
- Buildings with Large Spans: Halls where many people congregate, such as sports and exhibition halls.
- Unusual Geometry: Roofs with double-curved surfaces, complicated floor plans, or sudden changes in roof height.
- Poor Maintenance: Structures where the roof or primary load-bearing elements show signs of degradation.
- Snow Drifting Risks: Areas where snow accumulates due to leeward conditions, such as near side buildings, parallel buildings, or dense vegetation.
Emergency Drains and Water Management
Emergency drains (nødafløb) must function independently of the primary drainage system to prevent water accumulation from cloudbursts or melting snow.
- Maximum Water Level: Drains must be dimensioned so the water level at the primary drain does not exceed 120 mm.
- Dimensioning Standards: Systems should be designed for a 10-year rain event as per DS 432, equivalent to 230 l/(s ha).
- Placement: Emergency drains must be placed close to the primary roof drains and discharge water directly to the outside, never to the sewer.
- Design Variations: The document specifies requirements for internal drains with raised bowls and external gargoyles (udspyere) through the parapet wall.
Common Structural Deficiencies
Failures in large roofs are often linked to specific defects in design or execution that reduce load-bearing capacity.
- Glulam Beams: Insufficient shear capacity at supports, especially where beams are notched or sloped.
- Beam Connections: Weaknesses caused by bolt holes, slots for steel plates, or wood shrinkage in high beams.
- TT-elements: Sensitivity to uneven snow loads if not specifically dimensioned for such conditions.
- Steel Frames: Lack of bracing against lateral-torsional buckling (kipning) or missing web plates in frame corners.
Emergency Preparedness and Snow Removal
When structural capacity is insufficient, building owners must implement an emergency plan (beredskabsplan) and a snow removal plan.
- Action Thresholds: Snow removal must begin when the snow load reaches the characteristic load capacity of the roof.
- Evacuation: Buildings must be evacuated if the snow load exceeds 1.2 times the characteristic load capacity.
- Snow Density: For calculation purposes, the weight of snow is set at 2.5 kN/m³ (250 kg/m³).
- Priority Areas: Plans must identify critical roof zones and prioritize removal to minimize load asymmetry.
Frequently Asked Questions
What is the building owner's responsibility regarding snow loads?
The owner is responsible under the Building Act for ensuring the structure is safe and maintained to prevent collapse from snow or water accumulation.
When should an engineer be consulted for a roof assessment?
An engineer should always be consulted for halls with large spans, buildings with unusual roof shapes, or structures where maintenance has been neglected.
What is the maximum allowed water level on a flat roof?
Emergency drains must ensure that the water level at the roof drain does not exceed 120 mm to avoid overloading the structure.
How should emergency drains be dimensioned?
They must be dimensioned for a 10-year rain event according to DS 432, which is approximately 230 l/(s ha).
What are common risks for glulam beams in halls?
Common risks include insufficient shear capacity at supports and structural weakening from bolt holes or slots in large connections.
What weight should be used for snow in emergency calculations?
The weight of the snow should be set to 2.5 kN/m³, which corresponds to 250 kg/m³.
When is evacuation required due to snow buildup?
Evacuation should be initiated when the snow load exceeds 1.2 times the roof's characteristic load-bearing capacity.
How do climate changes affect roof safety in Denmark?
Higher temperatures during snowstorms can increase snow density and moisture content, leading to higher loads even if total snowfall decreases.
