A roofing underlay is chosen by four things: what it is laid on, the pitch of the roof, how long it will stay uncovered and what the roof covering is. It has to be vapour-permeable — with a low Sd value — so that moisture from the insulation can escape outwards. The weight alone does not decide the choice.
All membranes with prices and technical data are on the Roofing underlays page. This article explains how to choose between them and how they are installed.
Do you need a membrane at all?
If there is a tongue and groove naturheld wood fibre board above the rafters and the pitch is at least 15°, the board itself is the rain-tight underlay and a membrane is optional. How this works is described in Insulation above the rafters. A membrane is needed when:
- the pitch is between 10° and 15° — naturheld LDB 0.02 is laid on the board;
- there is no board above the rafters — the membrane is laid over the rafters, on the insulation or on timber boarding;
- the roof is renovated from outside without opening up the inside — naturheld LDB 0.02 over and between the rafters with a board of at least 35 mm on top (see The system);
- the pitch is below 10° or the covering is metal on timber boarding — an OMEGA membrane made for that case is chosen (see below).
Sd value: how much vapour the membrane lets through
The Sd value is the thickness of still air that resists vapour as much as the membrane does. The lower it is, the more easily moisture escapes. Our roofing underlays have Sd values from 0.02 to 0.25 m. For comparison, the vapour barrier on the inside of the roof is much tighter — AIRSTOP DIVA has an Sd of 0.5–30 m. That is why the two membranes are not interchangeable: the vapour barrier goes inside, the roofing underlay outside (see Vapour barriers).
Weight, outdoor exposure and pitch
Our 10 roofing underlays weigh from 145 to 455 g/m². The lightest is OMEGA LIGHT. More important than the weight is how long the membrane may stay uncovered until the roof covering goes on, and from what pitch it can be used. The table is sorted by weight; the data come from the manufacturers’ technical data sheets.
| Membrane | Weight | Sd value | Outdoor exposure | When |
|---|---|---|---|---|
| OMEGA LIGHT | 145 (± 5) g/m² | 0.025 (+ 0.035 / - 0.01) m | 4 weeks | Lightweight; for ventilated pitched roofs and closed ventilated facades. |
| naturheld LDB 0.02 | 170 g/m² | 0.02 m | up to 12 weeks | On naturheld boards at a pitch below 15° or without a board above the rafters; for renovation from outside — over and between the rafters. Minimum pitch 10°. |
| OMEGA 180 | 180 (± 10) g/m² | 0.025 (+ 0.035 / - 0.01) m | 4 weeks | Three-layer; non-slip and tear-resistant. |
| OMEGA MONO 200 | 200 g/m² | 0.15 (± 0.1) m | 12 weeks | Monolithic; directly on insulation or timber boarding, nail-tight (ÖNORM B 3647). |
| OMEGA MONO 230 SK DUO | 230 (± 10) g/m² | 0.15 (+0.05) m | 12 weeks | Monolithic; increased UV resistance and tightness against driving rain. |
| OM 240 ND SK DUO | 240 g/m² | 0.15 (+0.05) m | 3 months | Directly on insulation or timber boarding; tested for tightness when pierced by nails and screws (ETA 25/0019). |
| OMEGA UDOs 330 | 330 (+ 5%) g/m² | 0.18 (± 0.04) m | 5 months | Weldable. From 1.5° with welded seams, from 5° with SK DUO adhesive strips. |
| OMEGA MONOTOP 330 SK DUO | 330 g/m² | 0.12 (+0.05 / -0.04) m | 6 months | Two integrated adhesive strips; for pitches from 5°. |
| OMEGA DRAINY PLUS | 450 (± 5 %) g/m² | 0.02 m | 2 months | Below metal roofing on timber boarding; with a drainage mesh. |
| OMEGA UDOs 330 TopGrip | 455 g/m² | 0.25 m | 12 months | Weldable, with a non-slip surface. From 1.5° with welded seams, from 5° with SK DUO adhesive strips. |
How to choose — case by case
- naturheld board above the rafters, pitch 10–15°: naturheld LDB 0.02.
- Directly on the insulation or timber boarding: OMEGA MONO 200, OMEGA MONO 230 SK DUO, OM 240 ND SK DUO or OMEGA UDOs 330 — all are for direct laying. OM 240 ND is tested for tightness when pierced by nails and screws.
- A lightweight membrane: OMEGA LIGHT — for ventilated pitched roofs; OMEGA 180 — three-layer, non-slip and tear-resistant.
- Low pitch: OMEGA MONOTOP 330 SK DUO — from 5°; OMEGA UDOs 330 and UDOs 330 TopGrip — from 5° with the adhesive strips and from 1.5° with welded seams.
- The roof will stay uncovered for a long time: OMEGA UDOs 330 TopGrip — up to 12 months of outdoor exposure.
- Metal roofing on timber boarding: OMEGA DRAINY PLUS — with a drainage mesh that protects the membrane and provides ventilation below the metal.
Installing a roofing underlay
- The base. The membrane is laid on the wood fibre boards, on the insulation, over the rafters or on timber boarding — depending on its type (see the table).
- The overlaps. With SK DUO membranes the overlaps are bonded with the two integrated adhesive strips. Without strips — with the OMEGA QUILLI sealing adhesive (applied from +7 °C). OMEGA UDOs 330 is welded with OMEGA QSM liquid or bonded with OMEGA N55.
- Below the counter battens. The screws and nails through the membrane are sealed with a tape below every counter batten: bitumen OMEGA NDB for pitches from 1.5° or PE foam PE NDB for pitches from 5° (the tapes are applied from +5 °C). OMEGA QUILLI, which foams slightly and seals around the nails, can also be used.
- Penetrations. Pipes through the roof are sealed with OMEGA Alu-Butyl grommets (the pleated FRGD for pitches up to 45°), and with UDOs 330 — with the UDOs grommets. A ventilation stub is available for venting through the membrane.
- Counter battens, battens and covering. The counter battens, battens and the tiles or metal go on top — before the time the membrane may stay uncovered runs out.
All tapes, adhesives and grommets are in the Accessories for roofing underlays section.
Common mistakes
- A board above the rafters at a pitch below 15° without a membrane. Below 15° the board alone is not enough — naturheld LDB 0.02 is laid on top.
- The membrane stays uncovered too long. Depending on the type this is from 4 weeks to 12 months — see the “Outdoor exposure” column.
- No tape below the counter battens. Every screw through the membrane is a hole; the tape or the adhesive closes it.
- A roofing underlay instead of a vapour barrier on the inside. The roofing underlay is vapour-permeable and goes outside; the inside needs a vapour barrier.
Questions
What does the Sd value of a roofing underlay mean?
The Sd value shows how strongly the membrane resists water vapour, expressed as the thickness of a still air layer with the same resistance. naturheld LDB 0.02 has Sd 0.02 m — it resists vapour like 2 cm of air. The lower the Sd value, the more easily moisture from the insulation escapes outwards. The roofing underlays we offer have Sd values between 0.02 and 0.25 m; a vapour barrier on the inside has a much higher value — AIRSTOP DIVA, for example, 0.5–30 m.
Does the weight in g/m² matter?
It is not the only criterion. Our roofing underlays weigh from 145 to 455 g/m². How long they may stay uncovered differs — from 4 weeks for OMEGA LIGHT (145 g/m²) to 12 months for OMEGA UDOs 330 TopGrip (455 g/m²) — but what decides the choice is the roof pitch, what the membrane is laid on and the roof covering.
Is a roofing underlay needed on top of a wood fibre board?
With a tongue and groove naturheld board above the rafters and a pitch of at least 15°, the board itself is the rain-tight underlay and a membrane is optional. At 10–15°, or without a board above the rafters, a vapour-permeable membrane such as naturheld LDB 0.02 is laid on top.
From what pitch can a roofing underlay be used?
naturheld LDB 0.02 from 10°; OMEGA MONOTOP 330 SK DUO from 5°; OMEGA UDOs 330 and TopGrip from 5° with adhesive strips and from 1.5° with welded seams.
How much does a roofing underlay cost?
Between €2.35 and €13.93 per m² incl. VAT, depending on the membrane (the lowest price of each product). The price of each membrane is on its own page and in the table on the roofing underlays page.