How the two differ in kind
The first thing to notice is that these two floors are manufactured in different places, and almost everything else on this page is that fact working its way outward. A CLT floor is a factory product: panels cut on a machine to a model, with the openings, the hanger pockets and the service holes already in them, delivered numbered and in erection sequence, screwed down, and structurally complete the hour they land. A concrete floor does not exist until it is built in place. What arrives is ingredients and temporary works — falsework, formwork, reinforcement, a pour — and the great majority of the days on a slab go into building the thing that shapes it, which is then stripped off and hired again for the floor above.
That difference sets where the money is committed and where the risk sits. Timber's cost is decided at order, months out, and it is largely material: a manufactured panel costs what a manufactured panel costs, and it does not get much cheaper because you are buying the ninth identical floor. Concrete's cost is labour, plant and time, sitting on top of a commodity material that is cheap everywhere — and it genuinely does fall with repetition, because a set of forms and a crew that have already built this floor plate five times build the sixth faster than anyone new to it can. Site follows the same split. Timber is a small crew, a crane and a short, dry, predictable cycle; concrete is a larger crew, a wet trade and a cure clock, with backpropping running down through the floors below because a slab reaches its design strength on the concrete's schedule rather than the programme's.
Then comes the weight, which is the strongest argument on the timber side and the reason this is not merely a comparison about speed. Softwood is roughly a fifth as dense as normal-weight concrete; a CLT deck has to be considerably thicker to span the same distance and still lands at a fraction of the slab's self-weight. That discount compounds all the way down the building — smaller columns, less transfer structure, smaller foundations, and less seismic mass for the lateral system to restrain. Where what sits underneath has a capacity you cannot increase, that alone can decide the scheme. But mass is not only a load to be carried; it is also what makes a floor quiet and still, and this is where honesty is required. At ordinary office and residential spans a CLT floor is usually governed by vibration and by impact sound rather than by bending strength — a light, stiff, lightly damped deck is close to the textbook recipe for a footfall problem, and a bare panel does not have the mass that separating-floor acoustic criteria are effectively asking for. The fixes are well understood and none of them is free: a resilient layer and a topping, sometimes a ceiling below, sometimes a concrete topping that buys mass and a diaphragm at once. Each returns a share of the weight, and some return the wet trade that timber was chosen to avoid. So the decision does not turn on the material. It turns on four questions — how severe the acoustic and vibration criteria genuinely are for this use, whether the grid can be drawn to panel widths and then repeated, whether the design can be frozen when a factory needs it rather than when a concrete frame would have needed it, and whether anyone within reach of this site has built one before.
The factors that actually differ
| CLT / mass timber floor panels | Reinforced concrete slab | |
|---|---|---|
| Where the floor is manufactured | In a factory, to a machine file, weeks before it arrives. Panels land cut, numbered and in sequence; the deck is a completed structural element the hour it is screwed down. | On site, from ingredients. Falsework, formwork, steel and a pour — and most of the days go into temporary works that are stripped off afterwards and used again on the floor above. |
| What paces the floor cycle | The crane. A plate goes up in picks, and the trades above can follow as soon as the deck is fixed and the edge protection is up. There is nothing to wait for. | The cure. Strike times, backpropping down through the floors below, and a slab that gains strength on its own schedule. What makes it fast is repetition — the same forms and the same crew turning out one floor after another. |
| Self-weight, and what has to carry it | A fraction of the slab's, even allowing for a thicker deck. The saving compounds downward into columns, transfers, foundations and the seismic mass the lateral system restrains. | Heavy, and the frame below is sized for it. On good ground with nothing constraining the substructure, that weight costs little; over a basement, on poor ground, or on an existing structure with a fixed spare capacity it is frequently the thing that stops the scheme. |
| What actually governs the design | Vibration and deflection, usually, long before bending strength — and there is no span/depth table that exempts you from checking, so the footfall assessment is a method to run against a chosen comfort criterion rather than a depth to look up. | Strength and deflection, with a code shortcut that removes the deflection calculation altogether: ACI 318's minimum thickness table sets a depth from span and support condition alone, span/20 simply supported through span/28 both ends continuous. Mass and damping mean footfall is rarely the argument — a long, slender post-tensioned floor is the exception where it becomes one. |
| Impact sound between floors | The hard problem, and the one that most often eats the advantages. A bare panel has neither the mass nor the damping that separating-floor criteria want, so the answer is a build-up — resilient layer, topping, sometimes a ceiling below — which returns weight, can return a wet trade, and can quietly delete the exposed soffit that was half the reason for choosing timber. | Easier, but a bare slab does not pass either — mass solves half the problem, not the test. Weight is what stops the low-frequency end and buys airborne performance outright, so the slab starts far closer to the criterion; the tapping-machine end still needs a resilient layer or a floating screed, because a hard surface rigidly connected to the structure transmits impacts whatever it weighs. The difference is how much build-up it takes to get there and how much of the design it disturbs, not whether any is needed. |
| Span, depth and what the soffit looks like | Panels are all but always designed to span one way, and past a practical span the deck gets uneconomically thick. Beyond it the floor grows glulam beams beneath it, ribbed cassettes, or a tighter grid — and downstands become part of the architecture whether they were meant to be or not. | Reaches further at a shallower depth, and post-tensioning reaches further again with a flat soffit and no downstands at all — which timber has no equivalent for. A flat plate is also the easiest soffit there is to run services beneath. |
| Changing it once the design is set | Expensive, and the deadline is early. Every penetration, pocket and riser has to be in the model at the freeze, which lands months before most trades expect to be asked. A late hole through a finished soffit is a torn edge, unsealed end grain and an engineering question about which layers it just interrupted. | Forgiving in both directions. The design keeps moving until the pour is booked, and a missed box-out afterwards is a cored hole with a scanner run over it first. Post-tensioned slabs are the loud exception — coring one is a survey-and-permission exercise, not an afternoon. |
| Weather during construction | Water is a programme risk with a visible signature. An open deck is a tray, end grain wicks, and a wet fortnight that would leave no trace on concrete leaves marks on a ceiling people will later stand under. Drainage, protection and a fast close-in are real line items. | Rain is an inconvenience. Cold and heat are the genuine constraints, and placing in a hard frost or a heatwave needs measures — but no amount of rain shows on the finished frame. |
| Who can build it and how confidently it prices | A short list: fewer fabricators, fewer erectors who have done it, frequently one supplier holding a large share of the job, and a wide spread between tender returns. Design-and-supply bundling helps buildability and concentrates the risk in one place. | Everyone. The deepest subcontract market in construction, priced from first principles by every estimator in the room, with no single-source exposure and a benchmark for what it ought to cost that the whole team already carries in their heads. |
Which one, and when
Choose clt / mass timber floor panels when…
- What is under the floor has a capacity you cannot increase — an existing structure, a retained basement, poor ground, or a high seismic demand. The weight argument is doing the deciding, and it is a strong one.
- The site is tight and the programme is the constraint: no room for a pumping or batching operation, deliveries booked into a live street, and a floor cycle counted in days by a small crew.
- The soffit is meant to be seen, and deleting the ceiling package is part of the business case rather than a bonus.
- The design can genuinely be frozen when the factory needs it, and the grid can be drawn to panel widths and then repeated.
- Embodied carbon is a stated project requirement with an assessment attached, not a preference expressed in a design statement.
Choose reinforced concrete slab when…
- Long spans wanted at shallow depth with a flat soffit and no downstands — post-tensioned concrete does something timber has no answer to.
- Dwellings, hotels or anything where separating-floor impact sound is a pass-or-fail test on completion. Both floors need a resilient build-up to pass one, but mass is the cheapest half of that performance and the slab already has it — the margin is wider and the remedy, if a test comes back short, is smaller.
- The design will keep moving: an unlet building, a fit-out nobody has drawn, penetrations still being argued over at tender.
- Many identical floor plates. Formwork is a cost that falls with every reuse, and nothing about a manufactured panel falls the same way.
- Nobody within reach has erected mass timber, and this job has to be competitively priced and built by people who have done it before.
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 floor is cheaper?
- They are expensive in different shapes, which is exactly why no figure belongs on this page. Concrete's cost is site labour, plant and temporary works sitting on a cheap commodity material, and a large part of it falls with repetition — formwork and falsework are bought or hired once and amortised across every identical floor plate, so a tall building with a repeating plan is where concrete is at its most competitive. Mass timber's cost is a manufactured product, committed at order, tracking panel area closely and barely falling at all on the ninth repeat of the same floor. The trap is comparing the two frame packages alone, because that systematically flatters concrete: a meaningful share of timber's saving never appears in the frame quote at all. It shows up in preliminaries — fewer weeks of crane, scaffold, site management and site accommodation — and in an earlier date on which the building starts earning. Comparing programme alone flatters timber for the mirror-image reason. The two lines most often missing from an early timber comparison are the acoustic build-up and the weather protection, and neither is small. Price the whole scheme both ways, including the substructure, because that is where the weight discount actually cashes out.
- Do these two calculators design the floor?
- No, and it is worth being precise about what each one does. The CLT tool divides floor area by the area of one panel and rounds up, and it says in its own notes that it takes no account of cut waste at edges or openings — it is a procurement and layout sanity check, useful for asking whether a grid tiles sensibly into the panel sizes your supplier actually presses. The slab tool returns the ACI 318 minimum thickness that lets you skip an explicit deflection calculation, for normal-weight concrete with Grade 60 reinforcement; it is not a strength design, it does not know your loads, and lightweight concrete or another bar grade needs the code's adjustment factor. Neither tool checks vibration, which is the criterion most likely to govern the timber floor, and neither knows anything about your fire strategy, your diaphragm or your connections. Use them to test whether an option is roughly sane before an engineer prices the work of confirming it.
- Doesn't a concrete topping on CLT make this a false choice?
- It makes it a three-way choice, and the hybrid deck is often the honest answer rather than a fudge. A topping buys the two things a bare panel is short of — mass for impact sound and footfall, and a straightforward diaphragm — while also giving a flat, level finish and a service zone. What it costs is a share of the weight saving and the return of a wet trade with a cure attached, which is precisely the thing the timber option was chosen to remove. What it does not cost you is the exposed ceiling: the topping goes on top, so the timber below is still the soffit people see. Whether it acts compositely with the panel is a separate decision — connectors or notched shear keys buy real stiffness and let the floor be shallower, at the price of tying the two materials together permanently and complicating anything you might want to do to that floor later. A plain non-composite topping is simpler and is just mass. Price the hybrid as its own option early, because discovering it late usually means discovering it after the grid was set for something else.
- Why do mass timber tenders come back so far apart?
- Two reasons, and only one of them is market depth. The first is genuinely that fewer firms have done it, so you are pricing against a shorter list with less competitive pressure and a wider range of real experience — one return may carry a contingency for unfamiliarity that another does not need. The second is more within your control and does more damage: a building designed as a concrete frame and converted to timber late almost always prices badly, because the column grid, the floor-to-floor, the core positions and the riser strategy were all set for a slab. Panel widths want a say in the grid. The factory wants the model finished far earlier than a concrete job would demand. The erector wants a plate that repeats and openings that fall within panels rather than across joints. Timber tendered against a concrete-shaped design is timber priced at its worst, and the resulting number is often taken as proof the material is expensive when it is really proof the decision was made too late.
- Is the carbon case as strong as it is made to sound?
- The upfront embodied carbon of a mass timber frame is genuinely lower than an equivalent concrete one, and that part is well supported. The caveats matter if you intend to make a claim in public. The frame is not the building: cores, foundations, any topping and usually the substructure remain concrete, so the whole-building difference is always smaller than the frame comparison implies. How biogenic carbon is treated — whether the carbon stored in the timber is counted as a credit, and what end-of-life assumption sits behind that — moves headline figures substantially and is a methodology choice rather than a property of the building, so two honest assessments of the same scheme can disagree. Sourcing and certification affect the credibility of the claim independently of the arithmetic. And concrete is not standing still: mix design and cement replacement are where its own reductions are coming from, and a low-carbon mix specified deliberately narrows the gap. If carbon is a planning condition or a contractual target, get the assessment done on this scheme with the methodology your assessor will actually use, rather than on the materials in the abstract.
- What about adding storeys to an existing building?
- This is the one case in the comparison where it stops being close. An existing frame and its foundations have a spare capacity that is fixed and was decided decades ago, and a light floor is often what converts a vertical extension from impossible into buildable — where the same extension in concrete would need strengthening carried down through an occupied building to the ground, which is disruptive, slow and frequently the cost that kills the scheme outright. The caveats are real and should not be skipped: the lateral system has to accept the extra height as well as the extra mass, the existing structure needs surveying and testing rather than trusting to record drawings, and putting a light structure on top of a heavy one changes the building's dynamic behaviour in ways worth checking rather than assuming. Fire strategy and means of escape for the taller building are their own conversation. But of every scenario on this page, this is the one where self-weight decides on its own and the rest of the factors are detail.