How the two differ in kind
Look at the two members end-on and the whole comparison is already visible. A piece of dimensional lumber is a rectangle of wood: strong everywhere, including in the middle of its depth where bending stress is close to nothing, which is a polite way of saying most of it is along for the ride. An I-joist puts flanges of laminated veneer or graded sawn timber at the top and bottom, where the bending work happens, and joins them with a thin structural panel web that does nothing but carry shear and hold the flanges apart. Take the same weight of wood, move it to the edges, and it goes further. That is the entire trick, and every advantage and every weakness on this page descends from it.
The consequence that decides most jobs is reach. Solid joists run out of span at a distance set by the depth of stock the yard actually carries, and when the room is wider than that, the alternative to an I-joist is not a cheaper joist — it is a cheaper joist plus a beam across the middle, plus something to hold the beam up, plus a footing under that, plus a wall or a post standing in a room where nobody wanted one. That is the real ledger, and it is why an open-plan floor and a floor divided by walls can reach opposite answers with the same materials priced the same way. Where the span genuinely fits ordinary stock, the argument narrows to fit and finish rather than possibility, and solid lumber gets much harder to dismiss.
The second consequence is what the member does after it is installed. An I-joist leaves the factory at a controlled moisture content, at a depth that is the same on every piece in the pack, and it stays there — no crown to read, nothing to plane off, no shrinkage pulling the deck away from the joist to make the squeak that appears in the second winter. Solid lumber arrives wetter than the house it is going into, then gives that water up over the first heating season and shrinks across the grain doing it: the ceiling below cracks at the joints, nails pop, and a floor built dead flat settles into something slightly less flat. And the third consequence is the one worth being blunt about, because it is a life-safety difference rather than a quality one: a thin web burns through in a fraction of the time a solid joist takes to char down to failure, which is why the residential code asks for a protective membrane under engineered floor framing and exempts deep solid-sawn joists from the same requirement. So before comparing anything else, answer three questions — how far must this floor span without help underneath, will the underside ever be finished or sprinklered, and does the design have to stay changeable after the order goes in?
The factors that actually differ
| Engineered wood I-joists | Solid-sawn dimensional lumber joists | |
|---|---|---|
| How far it reaches before something has to hold it up | Goes well past what sawn stock can do at the same depth, so a floor plate can cross a room in one piece and the walls below can be partitions rather than structure. | Stops at a distance set by the deepest stock the yard carries. Past that the floor needs a mid-span beam, posts and a footing — a structural line through the plan, not just a longer joist. |
| What the member does after it is fitted | Manufactured to a moisture content and a depth, and it holds both. No crown to read, every piece identical, and the deck stays where it was glued. | Dries in service and shrinks across the grain doing it. Crowns vary piece to piece and get read and installed crown-up; the ceiling below shows the movement at its joints. |
| Getting services across the floor | The web is designed to be holed. Knockouts and a hole chart give large openings in defined zones, so ducts and waste pipes pass through the floor instead of below it — and the flange is untouchable, forever, by anyone. | Bored and notched under the ordinary solid-timber rules: holes to about a third of the depth, kept off the edges, notches banned from the middle third of the span. Anything larger than that goes underneath in a bulkhead. |
| Behaviour in a fire | The web is thin, and once it burns through the joist is two flanges with nothing between them. Unprotected engineered floors fail markedly sooner, which is why a gypsum or panel membrane below is a code requirement rather than a finish choice. | Chars from the outside in and keeps a load-carrying core for longer, which is the reason deep solid-sawn joists are exempted from that same membrane rule. |
| Where the money sits | Material-heavy and committed at order: a priced package with a layout drawing, a rim board and specified hangers, not just joist metres. Labour is light — no sorting, no crowning, uniform depth, services through the webs. | Material-cheap and bought from stock, with the cost pushed into labour and into the plan: sorting and crowning, bridging, and the beam-and-post line that a long span forces. That last cost scales with the shape of the building, not with its floor area. |
| Buying it | Ordered from a supplier against a layout, with a lead time, and the geometry effectively closes when the order goes in. A joist that arrives short is a re-order, not a walk to the pile. | In stock, everywhere, in every length, today. The floor can start this morning and absorb whatever the building turns out to be once the walls are open. |
| Ends, edges and point loads | Unforgiving and prescriptive: minimum bearing, blocking panels or an engineered rim board to stop rotation, web stiffeners where the details call for them, and squash blocks anywhere a post or wall above lands, because the web will not take vertical crushing. | Tolerant. The rim is the same stock as the joists, a bearing wall above lands on solid wood, and the failure mode of a rough detail is usually a squeak rather than a call to an engineer. |
| On site before the deck goes down | Light per metre and easy for one person to carry, and precisely because it is light, deep and thin it rolls. Bracing goes on as the run goes up; an unbraced I-joist floor is genuinely dangerous to walk on. | Heavier to lift and shorter, but stable the moment it is nailed at both ends. Standing on a bare joist run is ordinary carpentry. |
| Changing it later | A conversation, not an afternoon. New openings need engineered headers and hangers to the manufacturer's details, reinforcement means adding a matching member, and a plumber who notches a flange has destroyed the joist rather than weakened it. | Sistered with another length of the same stock, trimmed for a new opening with headers any carpenter can size from a table, and repaired by anyone who can buy wood. |
Which one, and when
Choose engineered wood i-joists when…
- The span is longer than sawn stock will cross, and the alternative is a beam, a post and a footing standing where the plan wanted open floor.
- Large services have to cross the joists rather than run under them — the web hole chart is worth the whole premium on its own.
- The ceiling below is being finished or sprinklered anyway, so the membrane the code asks for is a layer you were paying for regardless.
- Flatness matters to what goes on top: tile, a levelling topping, or a long run of finish that will show every crown and every millimetre of shrinkage.
Choose solid-sawn dimensional lumber joists when…
- The span fits comfortably within ordinary stock, so the engineered member's headline advantage is buying reach you do not need.
- The underside is staying open — a garage, a workshop, an unfinished basement — where an unprotected engineered floor is the wrong answer and a membrane is a cost with no other purpose.
- The work is small, remedial or improvised: a few joists in an old house, a repair, a floor whose dimensions will be discovered rather than drawn.
- Nobody on this job has fitted an engineered floor before, and the bearing, blocking and squash-block details would be learned on the job at the floor's expense.
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
- Which is cheaper?
- They are expensive in different shapes, which is why no figure appears here. Solid lumber is the cheaper material and it is cheap everywhere, from stock, with no lead time and no minimum order — but its cost keeps going after the delivery, into sorting and crowning, into bridging, into remedial work when the floor dries out, and above all into whatever has to be built underneath when the span exceeds what the stock will do. That last item is the one that flips the comparison, because it is not a joist cost at all: it is a beam, its posts, its footing, and a line through the plan. I-joists move the cost forward into a priced package that includes the rim board and the hangers, and take it out of the hours: no sorting, no crowning, one depth, and services through the webs instead of a bulkhead. The comparison that misleads people is joist against joist. Price the two floors whole — including the structure under them, the membrane where the code asks for one, and the day rate for the hours each one takes — and get a real quote for the engineered package, because it prices the actual layout rather than a guess at it.
- Do these two calculators size the joist?
- Neither of them, and it is worth being exact about what each does. The I-joist tool applies a span-to-depth rule of thumb and returns a preliminary depth; it says so in its own confidence note, it knows nothing about your loads, your spacing or your deflection limit, and the number that governs is the one in the manufacturer's code-approved span table for the specific series you are buying. The lumber tool is a quantity aggregator, not a structural check: it counts members from a length and a spacing and multiplies by the span to give the lineal timber to order. It is set up for ceiling joists, but the counting arithmetic is identical for a floor deck. Use the first to see whether a depth is roughly plausible before an engineer or a span table confirms it, and the second to turn a settled layout into a delivery. Neither checks vibration, which on a long-span floor is the criterion most likely to govern.
- Are I-joist floors really more dangerous in a fire?
- Unprotected, yes, and it is not a small margin. The web is thin, so once fire reaches it the joist loses the thing holding its flanges apart, and engineered assemblies have collapsed in tests in a fraction of the time solid-sawn framing took — which is precisely why fire services argued for the rule and why the residential code now asks for a gypsum or wood-panel membrane on the underside of engineered floor framing, with an exception where the floor is sprinklered and an allowance for small unprotected areas. Deep solid-sawn joists are exempted from that requirement because they char rather than perforate. Two honest qualifications. First, once the ceiling below is finished to that standard the comparison narrows considerably; the danger case is the open, unprotected floor over a basement or garage. Second, exemptions and details vary by jurisdiction and by code edition, so confirm what applies where you are building rather than taking the shape of the rule from a web page.
- Can I mix I-joists and solid lumber in the same floor?
- It is done routinely, and it works when the differences are designed for rather than discovered. Solid members earn their place at short bays, around openings, at cantilevers the manufacturer's details do not cover, and wherever a point load has to come down through the floor. Three things have to be watched. Depths must be matched so the deck lands flat, and nominal depths do not line up neatly between the two systems. The solid members will shrink and the engineered ones will not, so a joint between them can telegraph a ridge or a dip into the finish above within a year. And the two have different stiffness, so a floor framed half in each can feel like two floors under foot even when both satisfy the same table. Where they meet, the connection and the load path through it deserve to be drawn rather than improvised.
- Can I cut holes in an I-joist wherever I like?
- In the web, within the chart; in the flange, never. Every manufacturer publishes a hole chart specific to the depth, the series and the position along the span, and it governs the size, the spacing between holes and the distance from a bearing — larger holes have to sit nearer mid-span, where shear is lowest, and the zone close to a support is essentially off limits. Most webs also come with small pre-scored knockouts for cables and pipes that need no reference to anything. The flange is a different matter entirely: cutting, notching or drilling it removes the material doing the bending work, and there is no version of that repair which is a piece of timber and some screws. Solid joists are far more forgiving here but not lawless — holes to about a third of the depth, kept clear of the top and bottom edges, and no notch in the middle third of the span. If a duct will not fit either rule, the honest answers are a deeper floor, a different route, or a bulkhead.
- My engineered floor meets the span table and still bounces. Why?
- Because the table you passed was probably about strength, and bounce is about stiffness and frequency. A long, light, deep floor can be comfortably strong and still move under a footfall in a way that feels wrong, and this is where the I-joist's own advantages work against it: less mass than a solid-sawn floor of the same reach, and very little damping. Manufacturers publish this honestly — most span tables carry a separate, shorter set of spans for a stiffer floor, and the gap between the two columns is the size of the problem. The levers, in the order they actually work: more depth, which raises stiffness fast; closer spacing; a properly glued and screwed deck so the floor acts as one thing; ends that genuinely do not move, since a soft bearing or a poorly nailed hanger adds straight into what you feel at mid-span; and mass or a topping, which changes the character of the movement rather than removing it. Fixing this after the ceiling is on is far more expensive than choosing a deeper joist at the layout stage.