Timber joint group
Timber joint group (JD1 to JD6) classifies timber species by density and fastener-holding capacity under AS 1720.1. Governs screw, bolt, and nail capacity.
Ask Chalkline about this →A timber joint group is a classification assigned to a timber species under AS 1720.1-2010, indicating its density and fastener-holding capacity. Joint groups run from JD1 (highest density, best fastener grip) to JD6 (lowest density, least fastener grip) for seasoned timber, and J1 to J6 for unseasoned timber. The group determines the characteristic capacity values for screws, bolts, nails, and coach screws from the relevant tables in AS 1720.1.
Common Australian residential timbers: MGP12 and MGP10 pine (seasoned softwood) falls in joint group JD4 to JD5. Dressed hardwoods (e.g. spotted gum, blackbutt) are typically JD1 to JD2. LVL and glulam are assigned a joint group by the manufacturer based on test data.
In practice, the joint group governs two design parameters that chippies and engineers need to know: the minimum screw penetration into the main member (7D for JD1/JD2 up to 12D for JD5/JD6) and the characteristic withdrawal capacity per mm of penetration (higher for dense timber). Using the wrong joint group for a species results in either an over-designed (expensive) or under-designed (unsafe) connection.
Also known as: joint group, timber classification group (AS 1720).
Category: design / structural.
Related
- Structural screws for timber construction
- Pine framing grades
- Bolts (M-series metric) for residential construction
See also
- Screws and fasteners for residential construction: overview
- Durability class (glossary)
- Tolerance (glossary)
Last updated: 2026-05-10. Verified: 2026-05-10. Quarterly review for AS 1720.1 currency.