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2026-11-11T08:52:01

Timber Durability Class and Strength Class I–V: Teak, Bangkirai, Meranti

木材耐久等级与强度等级I–V：柚木、巴劳木、柳安木、樟木

Timber durability class and strength class are two separate ratings that buyers often mix up when ordering door frames, roof framing or flooring in Indonesia. The durability class (I–V) describes resistance to termites, insects and moisture, while the strength class (I–V) is set by density, bending strength and compressive strength. Teak, for example, is durability class I but strength class II, whereas camphor wood (kamper) is strong (class I/II) but only durability class III and not termite-resistant. For structural design, SNI 7973:2013 now uses grade codes E5–E25 in place of PKKI 1961.

Key Facts

What timber durability classes I to V mean

The Forest Products Research Institute (LPHH) at Gunung Batu, Bogor, grades Indonesian timber by usage level, durability and strength. Prijasambada's Timber Construction lecture notes summarise its tables. Durability is set by the wood's resistance to termites, insects and other small pests, and is divided into five classes.

Class I timber is expected to last 8 years in damp ground, 20 years in an unprotected location kept from water ingress, and indefinitely under cover; it is not eaten by termites or powder-post beetles. Class II lasts 5 and 15 years under the same conditions. Class III lasts 3 years in damp ground and 10 years unprotected, and is eaten by termites fairly quickly. Classes IV and V have very short lives in damp ground and are eaten by termites very quickly.

Strength class and density

The strength class is set by air-dry density, bending strength and compressive strength. Class I has a density above 0.90, bending strength above 1,100 kg/cm² and compressive strength above 650 kg/cm². Class II has a density of 0.60–0.90, class III 0.40–0.60, class IV 0.30–0.40 and class V below 0.30. Heavy timber is therefore usually strong but not necessarily durable; the two numbers must be read together.

Durability and strength of popular species

According to the same notes, teak is durability class I and strength class II with a density of 0.67, and is known for stability because it shrinks and swells little. Merbau is durability I and strength I (density 0.90–1.00), but iron in contact with it rusts quickly because of its acidic sap. Ulin (belian) is durability I and strength I (0.90–1.20). Bangkirai is durability II and strength I (0.80–1.10). Kamper is durability III and strength I/II (0.70–0.90): resistant to powder-post beetles but not termites, and unsuitable for unprotected construction. Kruing is durability III and strength II/III. Meranti is durability IV and strength III/IV (0.50–0.80), easily attacked by termites and prone to large movement.

Choosing timber for frames, roof structures and floors

LPHH also gives usage levels. Levels I and II are for heavy, unprotected construction in contact with damp ground, such as teak, merbau, bangkirai and belian. Level III is for heavy construction under cover, such as kamper and kruing. Level IV is for light construction under cover, such as meranti. Level V is for temporary work only.

In renovation practice, external door frames, terrace decking and pergolas exposed to rain are best made from level I or II timber. Roof framing and interior frames under cover can use level III timber, while low-durability wood such as meranti suits light or temporary work unless it has been preserved. Ask for a clear species name in every quote, because one species often has many regional names. The homeX team can help compare specifications from several suppliers.

SNI 7973 E grades and moisture content

For structural calculations, Indonesia's National Standardization Agency issued SNI 7973:2013, a modified adoption of the 2012 National Design Specification for Wood Construction, to replace NI-5 PKKI 1961. It uses grade codes E5 to E25 based on modulus of elasticity. For example, E25 has a reference modulus of 25,000 MPa and a reference bending strength of 26.0 MPa, while E10 has 10,000 MPa and 7.9 MPa. Durability is not part of these grade codes, so both still need checking.

Moisture content matters too. PKKI grades timber as quality A at 12–18% air-dry moisture (15% on average) and quality B below 30%. SNI 7973:2013 sets reference design values for dry service with moisture content not above 19%; above that, values are multiplied by a wet service factor, for example 0.9 for modulus of elasticity. Check moisture with a meter when timber arrives on site.

timber durability class: related articles and products

FAQ

Which woods are class I in Indonesia? Under the LPHH classification cited by Prijasambada, teak, merbau, ulin (belian) and resak are durability class I. For strength class I, examples are merbau, ulin and bangkirai.

What is the difference between durability class and strength class? Durability class measures resistance to termites, insects and moisture; strength class is set by air-dry density, bending and compressive strength. Teak, for example, is durability I but strength II.

Is kamper suitable for external door frames? Kamper is durability class III, not termite-resistant and unsuitable for unprotected construction, so it is better used inside the building.

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Sources: Prijasambada: Timber Construction ch. III, Wood Species and Groups in Indonesia (Universitas Persada Indonesia YAI); BSN: SNI 7973:2013 Design Specification for Wood Construction (Klinik Konstruksi DIY)