Engineered Timber Products: LVL, Glulam and I-Joists — Specification, Spans and Building Control

Quick Answer: Engineered timber products — LVL, glulam, and I-joists — are manufactured to consistent structural grades and can carry significantly longer spans than equivalent sawn timber. I-joists must not be notched in the flanges or drilled through the flanges at any point; web holes are permitted only within engineer-specified limits. Building Control will accept manufacturer span tables for standard applications, with structural calculations required for non-standard loadings.

Summary

Engineered timber has largely replaced solid sawn timber for floor joists and primary beams in modern residential and commercial construction. The three main product families — Laminated Veneer Lumber (LVL), glued laminated timber (glulam), and I-joists — each have different applications, span capabilities, and installation requirements. Understanding the differences helps builders and carpenters specify the right product, avoid costly over-specification, and satisfy Building Control requirements without commissioning unnecessary structural calculations.

The appeal of engineered timber over sawn timber is consistency: the manufacturing process eliminates the natural defects (knots, splits, slope of grain) that limit sawn timber strength, producing a reliable, dimensionally stable product with a known strength class. This allows longer spans in tighter floor depths, reducing the overall building height or allowing deeper insulation above a structural floor.

All engineered timber must be installed strictly to manufacturer's instructions and any accompanying engineer's design. The products are highly optimised — they perform exactly as calculated when installed correctly, and significantly worse than calculated when cut, notched, or loaded in ways the design did not anticipate.

Key Facts

Quick Reference Table

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Product Typical Span Range Typical Depth Range Common Use Strength Class
I-joist (TJI) 4.5m–8.0m 150mm–406mm Floor and roof joists Equiv. C24+
Posi-Joist 4.0m–7.5m 200mm–400mm Floor joists (services) Equiv. C24+
LVL (ridge/beam) 4.0m–9.0m 150mm–400mm Beams, lintels, ridges LVL-33/36/45
Glulam (GL24h) Up to 20m+ 90mm–1000mm+ Beams, columns, frames GL24h
Glulam (GL28h) Up to 20m+ 90mm–1000mm+ Heavy structural beams GL28h

Detailed Guidance

LVL: Uses, Grades and Specification

Laminated Veneer Lumber is produced by slicing logs into 3mm veneers, drying, and gluing them with a structural adhesive (phenol-formaldehyde typically) with grain directions parallel. The resulting billet is cut to length, producing a product that is stiffer and stronger than C24 timber of the same cross-section.

Common LVL applications:

UK brands include Finnforest (Finnjoist/Kerto LVL), Metsä Wood, and Steico. Grades typically run from LVL-33 (33 N/mm² characteristic bending strength) to LVL-45. Most residential applications use LVL-33 or LVL-36.

LVL is manufactured to BS EN 14374 and must be CE marked (or UKCA marked post-Brexit). The manufacturer's span tables cover most standard residential applications; structural calculations are required for non-standard loadings, multiple point loads, or spans exceeding the table limits.

Glulam: Strength Classes, Standards and Applications

Glued laminated timber (glulam) is manufactured from lengths of C24 (or higher grade) timber, finger-jointed into long laminations, dried to ≤15% MC, face-glued under pressure, and planed to finished dimensions. The result is a large-section timber beam of consistent quality that can be produced in lengths to 30m or more.

Strength classes (BS EN 14080:2013):

Glulam is dimensionally stable but hygroscopic — it will move with moisture changes. Specifying the correct service class is important:

For curved glulam (arches, portal frames), the minimum radius is set by the lamination thickness — typically 30× the lamination thickness as a minimum, with 40–50× preferred to avoid lamination stress during bending.

Building Control will generally accept manufacturer-certified span tables for standard beams. For anything outside standard tables — point loads from steelwork, combined bending and axial loads, eccentric connections — structural calculations from a chartered engineer are required.

I-Joists: Types, Span Tables and Critical Installation Rules

I-joists have become the standard floor joist product in new-build UK housing. Their high strength-to-weight ratio, dimensional stability, and ability to span 6m+ without intermediate support makes them ideal for open-plan layouts.

The cross-section consists of:

TJI I-joists (Weyerhaeuser): the dominant UK brand; specified by depth (e.g., TJI/150, TJI/250, TJI/360) and series (110, 210, 230, 360, 560) with increasing section sizes.

Posi-Joist: uses a galvanised steel tube or flat bar web in a triangulated pattern, leaving large clear spaces through which pipes, cables, and ducts can pass without any drilling. This is a significant advantage for open-web systems in heavily serviced floors.

Critical installation rules:

  1. No notching or drilling of flanges — the flanges carry bending stress; any reduction in their cross-section significantly reduces the moment capacity. Even a nail hole through a flange can cause a stress concentration crack. This rule is absolute.

  2. Web holes — permitted within engineer-specified parameters only. Standard guidance:

    • Circular holes only (never square or rectangular cuts)
    • Maximum hole diameter: 40% of web depth
    • Holes must be in the middle third of the span (not within 150mm of a joist support)
    • Holes must be at mid-depth of the web (±15mm)
    • Minimum spacing between holes: 1.5× hole diameter
    • No holes within 300mm of concentrated loads

  3. End blocking — solid timber blocking must be installed at the end bearing of each I-joist to prevent web buckling under bearing loads. Blocking details are provided in the manufacturer's installation guide.

  4. Bearing lengths — minimum 89mm on timber plate; 150mm on masonry. Using a pressure-treated timber wall plate for masonry bearing is mandatory.

  5. Squash blocks — solid timber blocks installed vertically within the I-joist depth at point load locations (partition walls, columns) to transfer the load through the section without crushing the web.

  6. Lateral restraint — I-joists must be prevented from rolling at supports; solid blocking or herringbone strutting at mid-span for joists over 3m

Acoustic and Thermal Performance

For floors between dwellings, engineered timber floors must achieve the Building Regulations Part E acoustic requirements (airborne sound ≥45dB Rw+Ctr; impact sound ≤62dB Ln,w). Manufacturer's tested systems (e.g., Fermacell + resilient bar ceiling + acoustic quilt) should be specified to ensure compliance — individual component performance does not guarantee a compliant assembly.

I-joist floors have a poor inherent vibration performance (walking vibration), which must be checked using the BS EN 1995-1-1 vibration criterion. For open-plan floors over 6m, the deflection and vibration limits often govern the design rather than strength. Posi-joist and EcoJoist systems with concrete topping (flowing screed) improve vibration performance.

For thermal floors over unheated spaces (garages, voids), the I-joist depth provides excellent depth for insulation. Mineral wool, PIR boards, or spray foam can be installed between the joists. A vapour control layer must be positioned correctly (warm side of insulation) and the void must be ventilated to prevent condensation risk.

Building Control Submission Requirements

For standard applications within a manufacturer's published span tables, Building Control will typically accept the span table as sufficient evidence. The submission should include:

For non-standard applications — multiple point loads, concentrated loads from steelwork, spans exceeding table limits, or unusual connection details — structural calculations prepared by a chartered engineer (CEng MIStructE, CEng MICE) are required.

The engineer's calculations must be submitted to Building Control before work commences on the floor structure. Retrospective acceptance is possible but risks additional expense if modifications are required after installation.

Frequently Asked Questions

Can I use an I-joist to replace a solid timber joist of the same depth?

Not without checking the span table. I-joists have different section properties from solid timber — they are typically deeper for the same span, or span further at the same depth. A direct depth-for-depth swap is not reliable; always use the manufacturer's span table for the specific product and loading.

My plumber needs to run a 110mm soil pipe through the floor — is that possible?

Possibly, but not through the flange. In a Posi-Joist or EcoJoist system, the open web allows large pipes to pass through without cutting. In a solid web I-joist (TJI), a 110mm hole may be too large for standard web hole rules — a structural check is needed. Reposition the soil stack or switch to an open-web system if the pipe must pass through multiple joists.

Does glulam need to be treated for moisture protection?

Glulam in Service Class 1 (dry internal use) does not require preservative treatment. For Service Class 2 or 3, discuss the exposure conditions with the manufacturer. Some manufacturers offer pre-treated glulam; alternatively, coatings or cladding can protect exposed members.

What's the maximum span for a TJI I-joist in a domestic floor?

This depends on the series, depth, spacing, and load. A TJI/360 at 475mm depth, 400mm centres, under a standard residential load (1.5 kN/m² live + 0.5 kN/m² dead) can span approximately 7.5–8.0m. Always refer to the current TJI span tables from Weyerhaeuser or your structural timber supplier.

Do engineered timber products need UKCA or CE marking?

Yes — since the UK adopted its own conformity marking system post-Brexit, engineered structural timber products for use in England, Scotland, and Wales must carry UKCA marking (or CE marking under transitional arrangements). Products without conformity marking should not be used in regulated construction.

Regulations & Standards