Professional insulation of flat roofs with EPS foam:
detailed technical guide
Insulation and technical parameters for flat roofs:
In Lithuania, flat roofs are becoming an increasingly popular solution for private house construction due to their modern aesthetics and functionality. A flat or overlapping roof is defined as a roof with a pitch between 0.7° and 7° (1.2% – 12.3%). This solution allows: a more rational use of the building volume, the creation of additional space (terraces, green roofs), the easier integration of solar energy systems, and the reduction of wind loads on the structure.
Technical parameters and properties of thermal insulation materials
EPS foam types and their characteristics:
Foam type | Compressive strength (kPa) | Thermal conductivity λD (W/mK) | Recommended use |
EPS 80 | ≥80 | 0,037-0,038 | For the base layer of insulation |
EPS 100 | ≥100 | 0,035-0,036 | For slope shaping, higher loads |
EPS 80N (grey) | ≥80 | 0,031-0,032 | For more efficient insulation, 20% thinner |
EPS 100N (grey) | ≥100 | 0,030-0,031 | For slope forming, higher loads |
Important technical parameters:
- Water absorption: <2% by volume (WL(T)2)
- Dimensional stability: ±0.2% (DS(N)2)
- Fire resistance: class E according to EN 13501-1
- Long-term compressive stress: ≤2% deformation over 50 years
- Frost-heat resistance: >150 cycles without change in properties
Insulation of flat roofs, structural layers and their functions
A properly installed flat roof system consists of the following layers (from bottom to top):
Supporting structure: reinforced concrete slab, reinforced concrete slab, profiled sheet,
Vapour barrier layer: 200mkr vapour barrier film, one layer of virgin bitumen adhered (often can act as a temporary roof during winter if temporary bays are installed), self-adhering vapour barrier (also acts as a temporary roof if properly installed). Very often customers install the roof themselves and do not carefully polish the seams.
Main thermal insulation layer (EPS 80 or EPS 80N, 150-350 mm)
Slope-forming layer (EPS 100 or EPS 100N, variable thickness)
Waterproofing coating(bituminous, EPDM, PVC)
A protective ballast layer (pebbles, green roof system – optional) is often recommended when the roof is visible through the window.
Technical solutions for slope formation
Principles and mathematical calculations of slope formation:
Slope (%) | Slope (°) | Change in 1 year | Change over 10 years | Recommended use |
1,0% | 0,57° | 1,0 cm | 10 cm | Minimum permissible slope |
2,0% | 1,15° | 2,0 cm | 20 cm | Standard recommended |
2,5% | 1,43° | 2,5 cm | 25 cm | Recommended for bituminous coatings |
3,5% | 2,00° | 3,5 cm | 35 cm | In areas of intense rainfall |
5,0% | 2,86° | 5,0 cm | 50 cm | For complex roof geometries |
Water drainage systems:
Internal system – water is directed into bays in the roof plane:
- Lower gradient required (1.5-2.5%)
- Number of bays: 1 bay for lateral connection-150-200 m² roof area, parapet bay collects approx. 60-70 m2 water
- Hidden gutter system is available, I always recommend installing a heated cable to the floor
External system – water is diverted into gutters:
- Higher gradient required (2.5-3.5%)
- Gutter dimensions: 125-150 mm wide
- Number of gutters: 1 pc. / 70-100 m² roof area
Installation technology and requirements
Substrate preparation:
- Substrate moisture: ≤4%
- Surface smoothness: ±5 mm over a 2 m length
- Substrate temperature during installation: >+5°C
Methods of fixing thermal insulation:
- Mechanical fixing:
- Number of studs: 4-8 pcs./m² (depending on wind zone)
- Stud length: thermal insulation thickness + min. 35 mm screwed into the substrate
- Stud type: telescopic studs with metal core, corrosion resistance class C3 (you can read more about it here)
- Gluing:
- Polyurethane adhesive: 150-250 g/m²
- Adhesive strip spacing: every 25-30 cm
Installation rules for panels:
- Seam overlap between layers: min. 200 mm
- Maximum panel spacing: 10 mm
- Arrangement of the panels: in “checkerboard” order
- Deformation joints: every 15-20 m in both directions
Energy efficiency and cost-effectiveness
Heat transfer coefficients by energy performance class:
Energy efficiency class | Required U-value (W/m²K) | Recommended EPS thickness (mm) |
A+ | ≤0,10 | 300-350 |
A++ | ≤0,09 | 350-400 |
A+++ | ≤0,08 | 400-450 |
Additional technical solutions and options
Special applications:
- Green roofs – additional load: 80-150 kg/m² (requires EPS 150-200)
- Solar installation – fixing point load: 15-25 kg/fixing point
- Terraces in service – required compressive strength: ≥150 kPa
Other solutions:
- Integrated rainwater harvesting systems
- Combined thermal insulation systems with PIR panels, the system is glued when installing the EPDM coating as the EPDM glue dissolves EPS
- Manufactured EPS parapet elements
Quality assurance and guarantees
- Roof structure guarantee: 10-15 years
- Foam insulation guarantee: 25-30 years
- Recommended periodic maintenance: 1-2 times a year
- Certified materials according to EN 13163
- Compliance with STR 2.05.02:2008 “Structures of buildings. Roofs” requirements
A properly designed and installed flat roof system with foam thermal insulation will ensure the long-term energy efficiency, structural reliability and comfort of the building over its lifetime.