Over-rafter roof — ceiling junction

Function of the junction

Over-rafter system — termPIR AL insulation (item 07) laid on top of the rafters (item 09), above the timber structure. Rafters remain exposed from inside (an aesthetic feature typical of alpine and modernist homes).

Key advantage vs under-rafter system:

• No timber thermal bridge — rafters are located in the warm zone (below insulation), they do not penetrate the warm envelope.
• Full panel surface — faster installation, joints only between panels (not between rafters).
• Homogeneous insulation layer — easier to achieve passive house standard.

Cost: the over-rafter system requires longer screws fixing the counter-battens (item 04) through the full thickness of insulation into the rafters — Ø8 screws, 200-260 mm long (depending on panel thickness).

Critical installation aspects

• Over-rafter thermal insulation (item 07) — laid on top of the rafters + wind barrier (item 05). Panel joints staggered (like brickwork), full support beneath every joint.
• Counter-batten screws (item 04) — Ø8 mm, length min. PIR thickness + 80 mm. Driven into the rafter (not into PIR only!). Spacing every 600 mm.
• Aluminium tape (item 06) on all panel joints + at the interface with the wind barrier.
• Edge under the collar beam (item 08) — insulation reaches the outer edge of the collar beam. The ceiling under the collar beam has a second insulation layer (item 10) ensuring continuity of insulation.
• Ceiling insulation under collar beam (item 10) — typically 50-100 mm termPIR AL in the unused space above the ceiling. Connected to over-rafter insulation with aluminium tape.

Over-rafter vs under-rafter — when to use which

Aspect	Over-rafter (this)	Under-rafter (see)
Insulation location	above rafters	between + below rafters
Thermal bridges	none	minimal (with under-rafter add-on)
Interior aesthetics	rafters exposed	rafters concealed
Roof height	+20-30 cm	unchanged
Required fasteners	Ø8 × 260 mm	standard nails
Cost	+30-40%	baseline
Typical use	new builds, energy-efficient	renovations, refurbishments

Used in the over-rafter termPIR pitched roof system.

Documentation

termPIR catalogue — Single-Family Houses, p. 17 — over-rafter pitched roof, ceiling junction.

Components in this junction