Torch-On Felt Systems: SBS vs APP Modified Bitumen, Layer Build-Up, Safety and NFRC Guidance

Summary

Torch-on modified bitumen felt replaced traditional oxidised bitumen felt from the late 1980s onward. Modified bitumen sheets have significantly better performance than the old "ruberoid" or "glass fibre" felts they replaced: they are more flexible, more UV resistant, and more resistant to thermal cycling and fatigue cracking. However, torch-on application requires skill, correct equipment, and strict safety protocols — the propane torch reaches temperatures exceeding 1,000°C.

The UK market uses two main types of modified bitumen: SBS (styrene-butadiene-styrene, a thermoplastic elastomer modifier) and APP (atactic polypropylene, a thermoplastic modifier). SBS is dominant in the UK because of its superior cold-weather flexibility — important in a climate with frequent freeze-thaw cycles. APP performs better in very high temperature conditions (Middle East, Mediterranean) but is less forgiving in cold conditions.

NFRC CoP 1 (built-up roofing) is the primary technical standard. It covers material specifications, layer build-up, lapping dimensions, detailing at upstands, and fire safety during hot works. Insurers and warranty providers require compliance with CoP 1 for product warranties to be valid.

Key Facts

• SBS modifier — styrene-butadiene-styrene rubber modifier; gives the bitumen elastomeric properties (stretches and recovers); excellent cold-weather flexibility; the most common modifier in UK flat roofing
• APP modifier — atactic polypropylene; gives the bitumen plastomeric (stiff, high-temperature) properties; less flexible at low temperatures; more common in southern Europe
• Torch temperature — propane torch at the application head reaches 1,000°C+; the membrane surface must be heated to just below its softening point (typically 200–250°C for APP, 150–200°C for SBS) — a visual indicator is a slight sheen appearing on the membrane surface
• Application speed — rolling the membrane sheet forward while heating from beneath; if too slow = overheating, fire risk; too fast = insufficient bond; experienced installers develop a feel for correct speed
• Layer build-up — minimum two layers for domestic flat roofing: underlay (polyester or glass fibre reinforced, bonded to primed substrate) + cap sheet (polyester reinforced, mineral aggregate surface for UV protection)
• Three-layer systems — recommended for trafficked or exposed roofs; additional base layer between the underlay and cap sheet
• Lapping — minimum 75mm side laps; minimum 100mm end laps; both laps must be fully torched and pressed flat to ensure complete bond
• Primer — bituminous primer applied to the clean, dry substrate (plywood, concrete) before the underlay; improves adhesion and seals the substrate; allow primer to dry to tack-free before torching
• Fire safety — hot works permit required for torch-on work near combustible elements; fire watcher with extinguisher during work; check combustible voids, rafter ends, and fascias after completion; 1-hour fire watch after work stops
• Upstand minimum height — 150mm from the roof surface to the top of the upstand; the felt layers must continue up the full upstand height and be terminated by a metal flashing
• Mineral surface cap sheet — slate, ceramic, or mineral granules bonded to the surface of the cap sheet; provides UV protection for the bitumen binder; also provides some slip resistance for foot access
• Self-adhesive alternatives — cold-bonded torch-free alternatives using peel-and-stick modified bitumen sheets are available for areas where hot works are prohibited (fire-sensitive environments)

Quick Reference Table

Detailed Guidance

Material Specification

Modified bitumen sheets are manufactured by saturating a reinforcement carrier (polyester fleece or glass fibre mat) with modified bitumen compound and applying the modifier (SBS or APP) to control performance characteristics. The standard sheet dimensions are:

• Width: 1,000mm
• Length: 8,000–10,000mm (roll)
• Thickness: 3–4mm (underlay); 4–5mm (cap sheet)
• Mass: approximately 4–5 kg/m²

Carrier type matters:

• Polyester (PY) reinforcement: higher tear strength; recommended for areas with thermal movement, upstands, and details; more dimensionally stable
• Glass fibre (G) reinforcement: stiffer; lower elongation; used in base layers; may crack if the substrate moves significantly

For domestic flat roofs, specify polyester-reinforced sheets throughout.

Surface finishes:

• Underlay: sand-finished (fine sand prevents sheets from bonding to each other in the roll); sometimes PE film-finished
• Cap sheet: mineral surface (slate granules or ceramic granules); available in various colours (grey, green, red, terracotta); mineral cap sheet provides UV protection without additional treatment

Installation: Underlay (Base Layer)

1. Substrate preparation: Plywood or OSB deck must be primed with bituminous primer; apply by brush or roller; allow to dry to tack-free (typically 30–60 minutes at 15°C). Concrete decks are also primed. Substrate must be clean, dry, and any projecting fixings driven flush.

2. Unroll the underlay: Begin at the lowest point of the roof (eaves or outlet). Unroll approximately 500mm of the sheet, torch the underside until a sheen appears (not until the surface blisters or smokes — this indicates overheating), and press to the primed substrate.

3. Continue rolling forward: As the underlay is pressed and bonded, continue torching and rolling forward. Two-person technique is standard: one person handles the torch, the other guides the roll. At approximately 1m distance, use the torch to progressively heat the underside of the roll as it unrolls.

4. Side and end laps: Position sheets with minimum 75mm side laps. Torch the lap zone fully before pressing; any unbonded areas in laps are the primary source of water ingress. Use a seam roller to consolidate all laps immediately after torching.

5. Stagger end laps: End laps (where two rolls meet end-to-end) must be staggered by minimum 500mm from the end laps in adjacent sheets — never align end laps across adjacent rolls.

Installation: Cap Sheet

The cap sheet is installed with the same technique as the underlay, but with attention to:

• Offset from underlay joints: Cap sheet laps must be offset from underlay laps by minimum half the lap width (37.5mm minimum, 50mm preferred). Never align cap sheet laps over underlay laps — this creates a double weakness.
• Mineral surface: When torching the cap sheet, torch the underside only. Do not torch the mineral surface — this damages the granule bond and discolours the surface.
• Cap sheet detailing at edges: The cap sheet must extend and bond over all upstands, edge details, and terminations. At eaves, the cap sheet (or a separate edge strip of cap sheet) should overhang the fascia by approximately 30–50mm to form a drip edge.

Upstand and Detailing

Upstands are torch-on's most failure-prone zone. The sheet transitions from horizontal (field area) to vertical (upstand face) at a corner — this corner must be handled correctly:

1. The underlay layer continues up the upstand face in a single piece, or an additional strip is torched over the corner from the field, overlapping the wall face and floor by minimum 150mm in each direction
2. The cap sheet similarly is taken up the wall face
3. The junction at the internal angle (where the flat roof meets the wall) must be reinforced with an additional strip of felt torched in place (a "fillet" or angle piece)
4. Metal flashing (typically Code 4 lead) is dressed over the top of the upturned felt and into a chase in the masonry; the chase is sealed with non-hardening mastic

Where the roof is against a lower wall (such as an extension against the main house), the flashing must be stepped or soakers-and-cover-flashing where brick courses are irregular.

Fire Safety and Hot Works

Torch-on roofing is classified as hot works (a fire risk activity). Failure to observe hot works safety has caused numerous building fires and insurance losses.

Hot works permit: Before starting work, a hot works permit must be completed. Most contractors use the FPA (Fire Protection Association) standard hot works permit. The permit identifies: the work location, the ignition sources, the combustible materials present, the precautions taken, and the fire watcher responsible.

Fire watcher requirements:

• A dedicated person (not the torcher) acts as fire watcher during all torch work
• The fire watcher has a CO₂ or dry powder extinguisher at hand
• The fire watcher monitors for smoke and flame in all accessible areas — particularly: underneath the deck (through any accessible joist space), at the eaves fascia and soffit, at any void adjacent to the work area
• After work stops, a minimum 1-hour fire watch must be maintained — many fires ignite from smouldering in a hidden void after the torch has been removed
• Check accessible voids with a thermal imaging camera after work if available

Combustible risk areas:

• Timber fascias and barge boards: the torch can ignite timber through the deck if the fascia is immediately below the deck edge
• Rafter ends at eaves: heat conducted through the deck can ignite exposed rafter ends
• Felt under the deck: some older roofs have sarking felt; torching the deck from above can heat this to ignition temperature
• Roof lights: plastic rooflights immediately adjacent to torch work can melt or ignite

Cold bond alternative: In fire-sensitive environments (occupied premises, close to flammable materials, where hot works permits cannot be obtained), cold-bond peel-and-stick modified bitumen sheets provide equivalent performance without flame. These use a high-performance adhesive backing and are equally compliant with NFRC CoP 1 specifications.

Defects and Their Causes

Blistering (raised bubbles under the cap sheet surface):

• Cause: moisture trapped between layers during installation (rain, dew, or wet substrate); damp primer; insufficient primer dry time
• Fix: cut out blisters, dry the area, and patch with matching material; address root cause

Lap failures (water ingress at side or end laps):

• Cause: insufficient torch heat at the lap; contamination (dust, rain) on the lap surface; lap width less than 75mm
• Fix: torch and re-bond the failed lap; overlay with an additional strip of cap sheet if the lap zone is too wide to re-bond

Cracking at upstand corners:

• Cause: thermal movement at the internal corner, no fillet detail, or APP modified bitumen in cold UK conditions
• Fix: torch a fillet strip into the corner; use SBS-modified felt for upstands in cold climates

Frequently Asked Questions

Can I apply torch-on felt in winter?

Yes. SBS modified bitumen remains flexible and bondable at low temperatures — as low as −5°C ambient for torch application (the torch heat itself is not temperature-dependent). However, the substrate must be dry and above freezing, the primer must be cured, and the sheets must be stored in a warm location before use (cold sheets are stiffer and harder to handle). Torch work is not advisable in wind exceeding Beaufort 5 (fresh breeze) — the flame is affected and wind chill dries primer too rapidly.

What is the life expectancy of a torch-on roof?

A correctly installed SBS two-layer torch-on system has an expected service life of 20–30 years. The limiting factor is UV degradation of the mineral cap sheet surface, which can cause the surface bitumen to harden and crack over time. Regular inspection (annual) and prompt repair of any isolated failures will extend the system life. Some manufacturers offer warranties of 10–15 years for correctly installed systems.

Does torch-on roofing need building control sign-off?

As with all flat roof replacement work, re-roofing with torch-on felt is notifiable under Building Regulations where the scope includes changes to insulation (triggering Part L), a complete structural re-roofing (Part A, C), or other notifiable changes. Like-for-like felt replacement may not require notification in all circumstances — check with your local Building Control or use a competent person scheme.

My customer has had a fire on a torch-on job — what are the liability implications?

Hot works fires can result in significant insurance claims and liability for the contractor. Adherence to the hot works permit procedure (FPA documentation), deployment of a fire watcher, and a post-completion fire watch are the key risk mitigations. If a hot works permit was not completed, or a fire watcher was not deployed, insurers may decline to cover the claim, and professional liability exposure is significant. Always complete the FPA hot works permit, even on small domestic jobs.

Regulations & Standards

• NFRC CoP 1 — built-up roofing; the primary standard for torch-on modified bitumen installation in the UK

• Building Regulations Approved Document C — weather resistance

• Building Regulations Approved Document L (2021) — thermal performance of the insulation below the membrane

• Building Regulations Approved Document B — fire performance; torch-on work requires compliance with BROOF classification

• BS EN 13707 — flexible sheets for waterproofing; reinforced bitumen sheets for roof waterproofing; defines the product standards that SBS/APP sheets must meet

• FPA (Fire Protection Association) Hot Works Guidance — hot works permit and fire watcher requirements; not a statutory instrument but widely required by insurance policies

• NFRC: CoP 1 Built-up roofing — primary technical reference

• Fire Protection Association: Hot works guidance — hot works permit system

• BSI: BS EN 13707 — reinforced bitumen sheet standard

• GOV.UK: Approved Document C — weather resistance requirements

• nfrc flat roofing standards — NFRC CoP 1 framework governing torch-on installation

• warm flat roof detail — insulation and deck build-up below the torch-on felt

• flat roof inspection survey — inspection criteria for existing torch-on felt roofs

• flat roof repair vs replacement — when torch-on patch repair is viable vs full replacement