Wood fibre boards laid on top of the rafters cover the whole roof with a continuous insulating layer and at the same time work as a second layer of protection against rain beneath the roof tiles. That is why this is the preferred way to insulate a pitched roof in Germany.
Why insulation between the rafters alone is not enough
In a conventional pitched roof the insulation goes between the rafters, while the rafters themselves remain uninsulated. Timber conducts heat about 3.5 times better than the insulation: λ ≈ 0.13 W/mK against 0.036 W/mK for the flexible insulation batts naturheld FLEX.
Rafters 8 cm wide at 80 cm centres take up about 10% of the roof area. Through these strips heat escapes in winter and enters in summer. These are the thermal bridges — and the thicker the insulation between the rafters, the larger the share of the losses that passes through them.
A continuous layer above the rafters
A wood fibre board laid on top of the rafters covers both the insulation and the timber with one and the same layer. The boards are joined with tongue and groove, so there are no open joints, and the rafters stay on the warm side of the construction.
Example for a roof with 8/20 cm rafters at 80 cm centres and 20 cm of naturheld FLEX between them:
| Construction | U, W/m²K |
|---|---|
| FLEX 200 mm between the rafters only | ≈ 0.21 |
| FLEX 200 mm + naturheld 140, 60 mm above the rafters | ≈ 0.15 |
Six centimetres of board reduce the heat losses by about a quarter. The values are indicative, calculated with the roof and wall calculator, where you can enter your own dimensions.
In summer the same layer protects the attic rooms from overheating. The boards are dense (140–180 kg/m³) and have a high heat capacity (2100 J/kgK), so they absorb the heat of the day and release it back outside at night.
A second layer of protection against water
Roof tiles are not completely tight: driving rain, drifting snow or a single broken tile let water in beneath the roof covering. The board beneath them drains it to the gutter before it reaches the insulation and the rafters.
The risk of water penetration is reduced for four reasons:
- Tongue-and-groove joint. The boards overlap like roof tiles and the water runs off over the joint, with no taped overlaps.
- Rigid substrate. Unlike a membrane, the board does not sag between the rafters, does not flap in the wind and does not tear during installation.
- Water-repellent core. The fibres are treated with paraffin through the full thickness, not just on the surface.
- Drying to the outside. The board is vapour-permeable (μ = 3; 60 mm give sd ≈ 0.18 m). Moisture that has got into the construction from inside escapes to the outside instead of condensing on the rafters.
The naturheld 140 tongue-and-groove boards (60–180 mm) and naturheld 220 are tested as a rain-tight underlay (UDP-A) under the rules of the German roofers' association ZVDH, at pitches from 15°. During construction they also serve as a temporary covering: naturheld 140 withstands up to 4 weeks of exposure, naturheld 180 — up to 12.
The installation system
The roof is built up of six layers, from inside to outside, each one more vapour-permeable than the one before.
| Layer | Material | Role |
|---|---|---|
| 1. Internal lining | Plasterboard or timber | Finish towards the room |
| 2. Vapour barrier | Membrane with taped overlaps | Airtight layer on the inside |
| 3. Between the rafters | naturheld FLEX | Main thermal insulation |
| 4. Above the rafters | naturheld 140 or 180, tongue and groove | Continuous insulation and rain-tight underlay |
| 5. Counter battens | Timber, min. 40/60 mm | Press the board down, leave a ventilated gap |
| 6. Battens and roof covering | Roof tiles or sheet metal | Main protection against rain |
Laying the boards follows five steps:
- Start at the eaves. The first row rests on a support board of the same height as the insulation board, which takes the pressure of the rows above it.
- Tongue upwards. The rows run parallel to the eaves, with the tongue towards the ridge and the groove facing down, so that water does not collect in the joint.
- Staggered joints. The end joints are offset from row to row, with no cross joints. A joint may fall between two rafters, but not two joints one above the other in the same bay.
- Counter battens and system screws. A counter batten is placed over every rafter and screwed through the board into the rafter with double-threaded screws. The screws at 60° carry the weight of the roof and the snow, and those at 90° the wind suction. Their number is determined by a structural engineer.
- Sealing the details. The ridge, valleys, chimneys and roof windows are taped with system tape over a primer. Damaged areas and gaps are filled before the tiles are laid.
When an old roof is renovated, the system is installed entirely from outside. The naturheld LDB 0.02 membrane is laid over and between the rafters as the airtight layer, followed by FLEX and a board of at least 35 mm. The rooms under the roof are not opened up and the house remains habitable.
When the board alone is not enough
The board is a second line of defence, not waterproofing, and it has three limits.
- Pitch below 15°. Below this pitch a roofing underlay is laid on top of the board (vapour-permeable, e.g. naturheld LDB 0.02 with sd 0.02 m, for pitches from 10°), and a sealing tape for the screws goes under the counter battens.
- Exposure time. The tiles are laid within the permitted time: 4 weeks for naturheld 140 and 12 for naturheld 180.
- Airtightness on the inside. The board does not replace the vapour barrier. Without an airtight layer on the inside, warm humid air from the rooms gets into the insulation.
Why this is the preferred solution in Germany
In Germany the standards, the subsidies and trade practice all point to one and the same construction.
- It is written into the roofers' rules. The ZVDH rulebook, in force since April 2024, lists the rain-tight underlay of wood fibre board as an additional measure of class 3, for pitches above 14°. The roofer is working to the rule, not to an exception.
- It meets the energy requirements. The GEG act requires U ≤ 0.24 W/m²K when a roof is renovated, and the BEG state subsidy requires U ≤ 0.14. Old rafters are rarely deep enough for that much insulation, so the missing thickness is added above them.
- It protects the timber without chemicals. Under DIN 68800-2 a construction is open to diffusion when the outer layer has sd ≤ 0.3 m. The rafters can then be built in without chemical preservative treatment. A 60 mm board has sd ≈ 0.18 m.
- It is renovated from outside. When the tiles are being replaced anyway, the insulation is laid from above and the furnished attic rooms are left untouched.
- It protects against summer overheating. Attic storeys in Germany are very commonly lived in, and a dense wood fibre board slows the entry of heat better than lightweight insulation materials.
The same arguments apply to Bulgaria, where summers are hotter and attic rooms overheat fastest. If you are planning a new roof or replacing the tiles, send us the area and the rafter dimensions — we will propose thicknesses and quantities.