PY vs G vs PYG SBS Membrane Reinforcement: Differences, Uses & 2025 Standard Update

Quick Answer: PY, G or PYG — Which Reinforcement Do You Need?

PY and G remain the principal reinforcement categories for SBS membrane under the current GB/T 18242-2025 standard, while PYG is a legacy classification from the former GB 18242-2008 standard. PY (polyester felt) is generally preferred where higher elongation and movement accommodation are required — Type II under the former standard required at least 40% elongation at peak. G (glass fiber mat) is designed primarily for tensile strength and dimensional stability, and is typically used where substrate movement is limited. PYG combined polyester with glass-fiber reinforcement in older specifications, mainly for mechanically fastened single-layer roofing. The correct choice depends on substrate movement, installation method, membrane system and the project specification — not on price alone.

What Do PY, G and PYG Mean in SBS Membranes?

Under the current GB/T 18242-2025 standard, PY and G are the principal reinforcement categories. PYG is mainly relevant to legacy GB 18242-2008 specifications and older market terminology.

1. PY — polyester felt. A non-woven polyester reinforcement known for relatively high elongation, toughness and movement accommodation. The fibres interlock in all directions, so the mat stretches, recovers, and absorbs movement without tearing.
2. G — glass fiber mat. A fiberglass-based reinforcement designed primarily for tensile strength and dimensional stability. Compared with polyester reinforcement, it has substantially lower elongation and limited ability to accommodate substrate movement.
3. PYG — glass-reinforced polyester composite (legacy). A reinforcement classification from the former GB 18242-2008 standard that combined polyester with glass-fiber reinforcement to improve tensile strength and dimensional stability while retaining part of polyester’s toughness.

What Changed Under GB/T 18242-2025?

The 2008 edition (GB 18242-2008) was replaced by GB/T 18242-2025, effective 2026-05-01. Three changes matter to buyers:

• Merged standard. SBS (elastomeric) and APP (plastomeric) modified bitumen membranes are now covered by one standard — GB/T 18242-2025 — replacing both GB 18242-2008 and GB 18243-2008.
• PYG removed as a separate category. The composite reinforcement is no longer retained as a standard classification. It may still appear in older specifications, supplier catalogs and legacy products, but it should not be treated as a current standard category.
• PY and G remain. Polyester felt and glass fiber mat stay as the principal reinforcement categories under the current standard.

For buyers this means: if a project references PYG, check whether the specification is based on the former GB 18242-2008 classification, and confirm the applicable product class and test documentation with the manufacturer.

Reinforcement Performance Compared

Table A — Categories under the current and former standard

Reinforcement	GB/T 18242-2025 (current)	GB 18242-2008 (superseded)
PY — Polyester Felt	Standard category	Standard category
G — Glass Fiber Mat	Standard category	Standard category
PYG — Composite	Not retained as a separate category	Standard category — mechanically fastened single-layer roofing

Table B — Historical Type II performance requirements (GB 18242-2008)

Historical reference only. These values relate to the former GB 18242-2008 classification and should not be treated as the current GB/T 18242-2025 specification. Current limits should be confirmed from GB/T 18242-2025 and the manufacturer’s TDS.

Property (Type II)	PY (Polyester)	G (Glass Fiber)	PYG (Composite)
Peak tensile — max peak (N/50mm)	≥800 (900+ factory tested)	≥500	≥900 (second peak ≥800)
Elongation at max peak	≥40%	No requirement (near zero)	—
Elongation at second peak	—	—	≥15%
Dimensional change after heat aging	≤0.7%	—	≤0.3%
Nail shank tear strength (N)	—	—	≥300
Standard thicknesses	3 / 4 / 5mm	3 / 4mm	5mm
Typical role	Primary waterproofing on moving structures	Base layer of multi-layer systems	Mechanically fixed single layer

Three details in the historical table are easy to miss and worth knowing:

• Glass fiber had no elongation requirement. The former standard did not even test it — a G membrane is expected to stay put, not stretch. Where the substrate can move significantly, G is generally less suitable.
• PYG was tested at two peaks. During a tensile test the glass grid breaks first (second peak), then the polyester felt carries on. The second-peak requirement of ≥800 N/50mm is what keeps the sheet intact after the grid fails.
• Thickness was restricted by carrier. Under GB 18242-2008, G was available in 3mm and 4mm only, and PYG in 5mm only. Thickness and carrier are linked by the applicable standard, not free variables.

How to Choose SBS Reinforcement by Application

Where movement accommodation matters. PY is commonly evaluated first for foundations, tunnels and other structures where settlement, cracking or differential movement may occur. Its elongation allows the membrane to bridge cracks and absorb differential movement without rupturing.

Where dimensional stability matters. G may be suitable for stable substrates and specific multi-layer roofing systems where high elongation is not the primary design requirement. It is often the economical base-layer option, which is never exposed.

Legacy mechanically fastened systems. PYG appears mainly in legacy specifications associated with mechanically fastened single-layer roofing under the former GB 18242-2008 classification. For current projects, confirm the applicable standard and classification with the manufacturer.

Final reinforcement selection should follow the project specification, waterproofing system design, substrate condition and applicable local standard.

Three Common Misconceptions

1. “G is simply a cheaper version of PY.” G is not a downgrade — it is a different design. PY favors elongation and movement accommodation; G favors dimensional stability. Choosing G where significant substrate movement is expected is a specification risk, not a cost saving.
2. “PYG is still a standard reinforcement category.” Not under GB/T 18242-2025. PYG belongs mainly to the former GB 18242-2008 classification and may still appear in legacy specifications or supplier terminology.
3. “Thickness matters more than reinforcement.” Thickness and reinforcement are separate specification variables, and both affect membrane performance. Their available combinations may also be restricted by the applicable product standard.

What to Specify When Buying

A complete SBS specification names more than a thickness. Confirm: reinforcement type, applicable standard, performance classification, nominal thickness, surface finish, carrier basis weight where relevant, tensile and elongation data, low-temperature flexibility, heat resistance, batch test report, TDS, and intended application or installation method.

Never compare two SBS quotations based on thickness alone. A 4mm membrane with a different reinforcement, grade, surface finish or test performance is not necessarily an equivalent product.

When you compare suppliers, request the TDS and batch test report with every quotation, and check that the tested values match the reinforcement, grade and standard you specified. See our SBS price factors and B2B purchasing guide for the full comparison checklist.

For the material decision between SBS and APP, see SBS vs APP Modified Bitumen Membranes. For thickness selection and system design, see our 3mm vs 4mm vs 5mm SBS Membrane guide.

Our SBS Waterproofing Membrane product page lists the available reinforcement options, surfaces and thicknesses with representative Type II test values. Request the corresponding TDS and batch test report before specification or procurement.

Frequently Asked Questions

What is the difference between PY, G and PYG reinforcement in SBS membrane?

PY (polyester felt) is an elastic non-woven carrier with relatively high elongation and movement accommodation, so it is generally preferred where the substrate can move or settle. G (glass fiber mat) is a fiberglass-based reinforcement focused on tensile strength and dimensional stability, with substantially lower elongation. PYG (glass-reinforced polyester) is a legacy classification from the former GB 18242-2008 standard that combined polyester with glass-fiber reinforcement, mainly for mechanically fastened single-layer roofing. Under the current GB/T 18242-2025 standard, PY and G are the principal reinforcement categories. Confirm the carrier, applicable standard and performance grade in every quotation.

Is PYG still included in GB/T 18242-2025?

No. PYG was included in the former GB 18242-2008 classification, but it is no longer retained as a separate reinforcement category in GB/T 18242-2025, which took effect on 2026-05-01. PYG may still appear in older project specifications, supplier catalogs and legacy product terminology. If a project references PYG, confirm whether the specification is based on the former classification, and verify the applicable product class and test data with the manufacturer before ordering.

Which SBS reinforcement is better for moving substrates?

PY is generally preferred where higher elongation and movement accommodation are required — under the former GB 18242-2008 classification, PY Type II required at least 40% elongation at peak. G reinforcement has substantially lower elongation and is generally less suitable where significant crack bridging or substrate movement is expected. Final selection should still follow the waterproofing system design, substrate condition and project specification, not the reinforcement type alone.

Can G SBS membrane be used on moving substrates?

G reinforcement has substantially lower elongation than polyester reinforcement, so it is generally less suitable where significant crack bridging or substrate movement is expected. G belongs on stable substrates, typically as a base layer in multi-layer roofing systems where thermal movement is minimal. If the structure is subject to settlement, thermal cycling or seismic activity, evaluate PY reinforcement instead, and confirm the choice against the project specification before ordering.

Does SBS reinforcement affect membrane price?

Yes — reinforcement is one of the main price drivers in an SBS quotation. Polyester and composite carriers generally cost more than glass fiber because the raw materials and processing are more expensive, and the same thickness in a different carrier is a different product at a different price. Always compare quotations on identical specifications: carrier (PY, G or PYG), thickness, performance grade, surface finish and applicable standard. Request the TDS and batch test report to verify actual test performance before comparing unit prices.

Technical References

• GB/T 18242-2025《弹性体/塑性体改性沥青防水卷材》 — current standard, effective 2026-05-01
• GB 18242-2008《弹性体改性沥青防水卷材》 — former standard, superseded
• Tianyuan Waterproof SBS TDS and batch test reports — available on request

Technical review: Tianyuan Waterproof Product Engineering Department.