Who this guide is for
- Self-builders comparing upper floor options early in a design
- Owners planning an extension or a new storey and reading a structural proposal
- Anyone who has been told a floor type is fixed and wants to know what that fixes
- Owners of an existing building trying to identify what their floor is
- Anyone coordinating services who has arrived after the structure was chosen
Five ways to make the same plane
A joisted timber floor is repeated members with a deck above and a lining below. A composite metal deck floor is profiled sheet with concrete cast onto it, spanning between beams. A precast plank floor is factory-made units laid side by side and tied. A mass timber panel floor is a solid plate. A mezzanine is an intermediate floor inserted into a volume whose supports were sized for something else.
They differ most obviously in material, and that is the least useful difference to reason from. What separates them in practice is where the empty space is, because that is what the rest of the building competes for.
Where the services are going to run
A joisted floor encloses a routed zone within its own depth. That is the horizontal route services are routinely pushed into once the structure is up, which is why this part of the assembly takes most of the pressure from later work. A composite deck and a precast plank floor move the services out of the structural depth entirely and hang them beneath the soffit. A solid panel removes the service zone the joisted floor depends on.
None of those is better. They are different negotiations. A floor with services inside its depth invites the coordination problem into the structure; a floor with services below it invites it into the ceiling void and the room height.
- Inside the structural depth, between the members
- Beneath the soffit, hung from the deck above
- Within voids formed in the units themselves, where the manufacturer permits it
- In a battened or suspended zone added below a solid plate
Structural depth is contested space
In a joisted floor the same dimension has to satisfy the span, leave room for services and give the ceiling something to fix to, and each party wants it for a different purpose. Deepen it for stiffness and the room below loses height; keep it shallow and pipes have to go through the members rather than past them, which turns a plumbing decision into a structural one.
A composite deck answers the same intermediate floor need with a shallower depth, with services hung below rather than within. That is a genuine trade, not a free improvement: what is gained in structural depth is spent in the ceiling zone beneath it.
What is heard through it
The library is consistent on this and worth repeating exactly: what a floor achieves acoustically is a property of the complete construction including its ceiling, its fixings and the paths running round its edges, and it is judged by a qualified acoustic professional. No entry states a performance, and neither does this guide.
What the choice does settle is the shape of the problem. In a joisted floor, stiffness and the deck fixings govern how far a footfall spreads, while the ceiling and the way it is fixed govern what is heard below, so improving one alone leaves the other untouched. In a dense floor the question moves to what is laid over it and what is hung beneath it.
What the floor asks of the walls beneath
An upper floor is rarely only a floor. Where joists are built in or hung, the floor is also restraining the wall it bears on, so the connection serves the wall as much as the floor. A tied precast plank floor and a concrete-topped metal deck are usually relied on as stiff plates, gathering horizontal force over their area and delivering it into the walls, frame or cores.
That is why bearing and tying are described in the library as stability details rather than support details, and why an alteration at a floor edge can reach a long way past the floor.
How alterable it will be later
This is where the five separate most sharply. In a joisted floor an opening is a structural design carried out by trimming, and the members alongside pick up load they did not have before. In a precast plank floor, openings are settled with the manufacturer before the units are made, because the steel runs the length of each unit. In a composite deck, penetrations are located before the deck is set out, because cutting the sheet interrupts something the finished slab relies on.
In a mass timber structure the deadline is earlier still: openings, service holes, recesses and connection preparation are part of the manufacturing information, so a hole that was not drawn becomes a cut into a load-carrying element after the fact.
The decision closes at different moments
Reading the five together, the useful question is not which floor is best but when each one stops being changeable. Ask that of any proposal and you learn what has to be coordinated, by whom, and by what date — which is usually the information a project turns out to have been missing.
Comparing upper floor structures
- 1Ask where services are intended to run: within the depth, beneath the soffit, or in an added zone.
- 2Establish what structural depth is being assumed and what that leaves for the room below.
- 3Ask what this floor is relied on to do for the stability of the structure at that level.
- 4Ask how the floor connects into the walls or frame it bears on.
- 5Establish where openings are needed, and by when their positions have to be fixed.
- 6Ask what may be drilled or fixed into the floor after completion, and on whose information.
- 7Ask whether the ceiling arrangement is doing an acoustic job as well as a visual one.
- 8Establish whether stiffness has been considered alongside strength for the intended use.
- 9Ask what access is planned to the services once the ceiling below is closed.
- 10Ask what will be recorded so a later designer can assess an alteration.
- 11Where units are made in a factory, ask when the manufacturing information has to be complete.
- 12Ask what happens to the programme if a service route changes after that date.
Common mistakes to avoid
- Comparing upper floors on span alone, when the practical differences are about depth, services and later alterability.
- Assuming a shallower structure is a gain, when the depth is often recovered in the ceiling zone beneath it.
- Treating services in the floor depth as a fitting matter, when whether they pass between members or through them is a structural question.
- Believing a ceiling is decoration, when in most floors it is part of what the construction is relied on to do.
- Reading a floor as only a floor, when a tied deck is commonly gathering horizontal force at that level.
- Leaving opening positions open past the date a factory needs them, in a floor made from manufactured units.
- Assuming an existing floor can be identified by looking at the ceiling, when the soffit reveals very little.
When to involve a professional
- Which floor structure suits a building is a structural determination made by a qualified engineer against the loads, spans and supports of that building.
- Acoustic and fire behaviour are properties of complete tested constructions including their ceilings and junctions, and neither is stated here.
- What may be cut, drilled or fixed into a manufactured floor unit rests on the manufacturer's information and the structural designer, not on general reference.
- Nothing here states a span, a depth, a member size or a performance, and none can be inferred from it.
Frequently asked questions
Questions readers ask about this topic
Which upper floor structure is quietest?
The library will not rank them, and neither will this guide. What is heard through a floor is a property of the complete construction, its ceiling and the flanking paths around it, and it is assessed by a qualified acoustic professional rather than predicted from the structure alone.
Can I run pipes through any floor structure?
No. A joisted floor may allow services within its depth subject to what the member type permits, while precast units and composite decks generally move distribution below the soffit. What is permissible in each case is defined by the designer and the manufacturer.
Why does the joist type matter so much?
Because it changes the answer completely. Some engineered members carry designated service openings and are compromised by a cut elsewhere, while a solid member behaves differently again. The member type therefore has to be known before the first pipe is run.
What makes a floor act as a plate?
Being tied. A precast plank floor becomes a plate when its joints are grouted and its units tied to each other and the structure; a metal deck floor becomes one when the concrete has gained strength and its edges are connected. Until then the design intent is not present.
How do I find out what my existing floor is?
From the records rather than from inspection. A soffit gives no indication of profile, gauge or lap in a composite deck, and nothing on a precast unit reveals where its steel runs. The information made when the floor was designed is usually the only place it survives.
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