How the two differ in kind
An I-joist puts its material where bending needs it — top and bottom flanges far apart — and puts almost nothing in between, because the web's only job is to carry shear and hold the flanges in position. That is extremely efficient: a deep I-joist spans a long way for a fraction of the timber a solid member would need, and a floor of them at close centres is light, straight and dimensionally stable.
The price is that the section has no redundancy. The FLANGES are the member: cut one, even slightly, and the beam is finished. The web is thin engineered board, so a concentrated load or a bearing crushes it unless a stiffener is fitted — and manufacturers mark where holes may be cut and how large, because a hole in the wrong place is not a small reduction, it is a shear failure waiting for a load.
A glulam beam is solid through its depth. It carries a point load anywhere along its length without a detail, it can be notched within the ordinary timber rules, it can be curved in manufacture, and it is a member you can leave exposed and expect to look like a beam. In fire it chars at a predictable rate and keeps a cool, full-strength core, which an I-joist's thin web does not.
The factors that actually differ
Show
| Glulam beam | Engineered I-joist | |
|---|---|---|
| Where the material is | Through the section. Solid, redundant, and tolerant of ordinary carpentry. | In the flanges. The web is thin board doing a different job, and the section has no spare anywhere. |
| Span for the weight | Good, and heavy. A glulam spanning a long way is a lifting operation. | Excellent. This is what the product exists for — long spans at close centres, carried by hand. |
| Point loads | Anywhere along the span, without a special detail. | Need a designed detail — a squash block or web stiffener — because the thin web will not carry a concentrated load on its own. |
| Bearings | Ordinary timber bearing rules apply. | A stated minimum bearing length AND usually a web stiffener at each end, because the web crushes before the flanges do. |
| Services through it | Holes within the ordinary timber limits — middle of the depth, away from supports, no notches near bearings. | Generous, and only where the manufacturer says. Pre-marked knockouts, permitted hole sizes by position, and the flange NEVER cut. |
| Fire | Chars at a predictable rate with a cool core, so a residual section can be calculated. This is why exposed glulam achieves ratings that exposed steel cannot. | The thin web burns through quickly. Exposed I-joist floors are a recognised hazard, which is why they are protected by their ceiling and why removing that ceiling matters. |
| Exposed appearance | A finished product. Appearance grades exist and it is meant to be seen. | Not intended to be seen, and generally not permitted to be exposed in fire terms. |
| Shape of the cost | Dearer per member, fewer members. Suits a frame of beams. | Cheap per square metre of floor at close centres. Suits a field of repeating joists. |
Which one, and when
Choose glulam beam when…
- The load arrives at points — a beam under a wall, a trimmer around an opening, a ridge or a transfer.
- The member will be exposed, and has to look like a beam and behave like one in fire.
- Spans are long with concentrated loads on them, where a flanged section would need a detail at every one.
- The geometry is not straight: a curve, a taper, a non-standard depth.
Choose engineered i-joist when…
- The floor is a field of repeating joists at regular centres over a regular span.
- Services have to cross the floor everywhere, and a web with permitted knockouts saves dropping a ceiling.
- Weight matters for handling: a floor that has to go in by hand, up stairs, without plant.
- Dimensional stability matters — no shrinkage, no crowning, no twist in a floor that will be tiled.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Can I drill an I-joist?
- The web, within the manufacturer's permitted zones and sizes, freely — and this is one of the product's real advantages, because those zones are generous and remove the need to drop a ceiling for services. The FLANGES, never: not a notch, not a nick, not a screw through the edge into the flange from below, not a saw cut to ease a fit. The flanges are the entire bending capacity of the member, and cutting one is not a reduction, it is a failure the floor is now waiting to demonstrate. Every manufacturer publishes a hole chart that varies with depth, span and position along the member, and the honest rule on site is that if the chart is not present, no hole is cut.
- Why does an I-joist need a web stiffener?
- Because the web is thin engineered board and a bearing or a point load applies a concentrated compression to it across a small area. Solid timber has enough material to spread that; an I-joist's web buckles or crushes locally instead, at a load well below anything the flanges are troubled by. A stiffener — a block of board or timber fitted tight between the flanges at the loaded point — carries the load through the depth and puts it into the bearing rather than into the web. They are required at end and intermediate bearings by most manufacturers, and under any point load such as a post or a beam landing on the joist. They are also small, cheap and invisible once the floor is down, which is why they get left out.
- Which spans further?
- For a given depth and a distributed load, the I-joist, usually by a wide margin — that efficiency is the whole product. But the comparison is misleading because they are rarely doing the same job. I-joists span a floor as a field of members at four hundred or six hundred centres; glulam spans as a beam carrying the ends of those members, or a wall, or a roof. The question that matters is not which spans further but where the load arrives: distributed over an area, or concentrated at points. And on long spans, neither is likely to be governed by strength — a light long-span timber floor is usually governed by VIBRATION, which is a frequency-and-mass question rather than a bending one.
- What actually happens to each in a fire?
- Glulam chars from its exposed faces at a roughly predictable rate — a fraction of a millimetre a minute for softwoods — and the char insulates what is behind it, so the core stays near ambient temperature and at full strength. That means a residual section can be calculated, and it is why exposed heavy timber achieves fire resistance periods that unprotected steel of the same capacity cannot. An I-joist behaves completely differently: the web is thin, it burns through quickly, and once it does the flanges have nothing holding them apart. This is why engineered I-joist floors are protected by their ceiling lining, why unprotected ones are a documented hazard to firefighters, and why removing or penetrating that ceiling is a fire-safety change rather than a decorating one.
- Can I mix them in the same floor?
- Routinely, and it is the normal arrangement rather than a compromise: I-joists as the field, glulam or LVL as the trimmers around openings and as the beams the joists hang from. What needs attention is the JUNCTION between them, because the two have different bearing requirements and different depths available for a hanger. Joist hangers are specific products matched to the joist profile and the supporting member, and a hanger designed for solid timber fitted to an I-joist, or one nailed into a flange it should not be nailed into, is a failure at the connection rather than in either member. Manufacturers publish the compatible hangers; substituting on site because the right one did not arrive is the version of this that goes wrong.
- Which is cheaper?
- Per square metre of ordinary floor, the I-joist comfortably — that is why most new timber floors are built from them. Per member for a beam carrying real point loads, the glulam, because achieving the same with a flanged section means a detail at every load and a member that cannot be cut. The comparison that actually decides a project is neither of those: it is the whole floor, including the ceiling that an I-joist floor needs for fire, the stiffeners it needs at bearings, the services that a web knockout saves dropping a ceiling for, and the lifting plant a long glulam needs. Those are project quantities rather than product prices, and they are where the difference lives.