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IS 4891:1988 is the Indian Standard (BIS) for preferred cut sizes of structural timber - specification. This standard specifies the preferred dimensions (thickness, width, and length) for cut sizes of structural timber. It aims to facilitate standardization, reduce milling wastage, and optimize the availability of structural timber in the market.
preferred cut sizes of structural timber - Specification
Overview
Status
Current
Usage level
Specialized
Domain
Structural Engineering — Building Construction Practices incl. Painting, Varnishing
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! Always specify timber dimensions at standard moisture content (typically 12%) as actual dimensions will vary with shrinkage or swelling.
! Using standard preferred sizes drastically reduces material cost and lead time, as sawmills optimize their log cutting for these dimensions.
! Specified sizes are for sawn timber. If planed or finished timber is required, nominal allowances (usually 2-3 mm per face) must be added during procurement.
EN 1313-1:1997CEN (European Committee for Standardization), Europe
HighCurrent
Round and sawn timber - Permitted deviations and preferred sizes - Part 1: Softwood sawn timber
Specifies preferred sizes and tolerances for structural softwood, directly comparable to IS 4891's scope.
PS 20-20NIST (National Institute of Standards and Technology) / ALSC (American Lumber Standard Committee), USA
MediumCurrent
American Softwood Lumber Standard
Establishes standard nominal and minimum-dressed sizes for various softwood lumber species.
AS 1748.1-2011Standards Australia, Australia
MediumCurrent
Timber — Sawn and milled products — Part 1: Product specification
Covers requirements for sawn and milled timber, including dimensions and tolerances, though broader in scope.
JAS 600MAFF (Ministry of Agriculture, Forestry and Fisheries), Japan
MediumCurrent
Japanese Agricultural Standard for Sawn Lumber
Defines grading, dimensions, and tolerances for sawn lumber used in Japan, based on metric sizes.
Key Differences
≠IS 4891 specifies actual metric dimensions (e.g., 50 mm x 100 mm). The US standard (PS 20) uses a nominal imperial system where the actual finished size is smaller (e.g., a '2x4' is actually 1.5" x 3.5" or 38 mm x 89 mm).
≠The reference moisture content (MC) at which dimensions are specified differs. EN 1313-1 uses a reference MC of 20%, while PS 20 provides different dimensions for 'green' (>19% MC) and 'dry' (≤19% MC) lumber. IS 4891 does not explicitly state a reference MC for its size tables.
≠Permissible tolerances on dimensions vary significantly. For a dimension ≤ 100 mm, IS 4891 allows ±3 mm, whereas EN 1313-1 (Tolerance Class 1) specifies a tighter range of -1 mm to +3 mm.
≠The specific preferred sizes differ. For example, IS 4891 includes 30 mm and 60 mm as preferred thicknesses, which are not common preferred sizes in EN 1313-1, which instead lists 32 mm, 44 mm, and 63 mm.
Key Similarities
≈The primary objective of IS 4891 and its international counterparts is to rationalize and standardize sawn timber sizes to simplify design, ordering, and construction processes.
≈Like most modern international standards (e.g., EN, AS, JAS), IS 4891 is based on the metric system (millimeters), facilitating easier comparison and international trade compared to imperial-based systems.
≈All standards acknowledge the inherent variability in sawing timber and provide tables of permissible deviations or tolerances for width, thickness, and length.
≈IS 4891 groups sizes by intended use (e.g., Beams, Columns, Battens). Other standards also implicitly or explicitly define size ranges that are typically used for similar structural applications.
Parameter Comparison
Parameter
IS Value
International
Source
Preferred Thickness (Small Section)
40 mm, 50 mm
38 mm, 50 mm
EN 1313-1
Preferred Width (Medium Section)
100 mm, 125 mm, 150 mm
100 mm, 125 mm, 150 mm
EN 1313-1
Tolerance on Thickness/Width (for size ≤ 100 mm)
±3.0 mm
-1.0 mm to +3.0 mm (Tolerance Class 1)
EN 1313-1
Tolerance on Thickness/Width (for size > 100 mm)
+6.0 mm / -3.0 mm
-2.0 mm to +4.0 mm (Tolerance Class 1)
EN 1313-1
Reference Moisture Content for Dimensions
Not explicitly stated (practice implies 12%)
20%
EN 1313-1
System of Dimensioning
Actual metric sizes (e.g., 50 mm)
Nominal imperial with actual size (e.g., '2 inch' is 1.5 inch / 38 mm)
PS 20-20
Example of a common beam size
100 mm x 200 mm
89 mm x 184 mm (Nominal '4x8' in dry condition)
PS 20-20
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Minimum preferred length1.5 m
Maximum preferred length6.0 m
Length increments0.3 m
Moisture content for standard dimensions12 percent
Tolerance for width/thickness < 100mm+3 mm, -1 mm
Tolerance for width/thickness >= 100mm+6 mm, -2 mm
Tables & Referenced Sections
Key Tables
Table 1 - Preferred Cross-sectional Dimensions for Structural Timber