Structure and support · Floor Structures
Timber Joisted Upper Floor System
This entry describes an upper floor as a single depth doing structural, service, separation and finish work at the same time, and shows why a decision taken in any one of those roles removes options in the others.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What timber joisted upper floor is
A timber joisted upper floor is the depth between two occupied rooms. Joists span between walls or beams, a deck is fixed over them and a lining is fixed under them, and the space enclosed between carries pipes, cables and sometimes ducts. Everything the floor is asked to do, from support and stiffness to separation and ceiling, happens within that same depth.
Its nearest sibling is the suspended timber ground floor, and the difference is decided by what lies underneath. An upper floor separates occupied space from occupied space, so its void is a service zone and the concerns are deflection, bounce and what is heard through it. A suspended ground floor separates occupied space from open ground, so its void is ventilated and the concern is keeping the timber dry.
The joist product, whether solid timber, an engineered I-joist or a metal-web member, is a variable within this entry rather than a different system. Changing it changes how services pass through the depth and how the deck and ceiling are fixed, but the roles in the assembly and the interfaces around it stay where they are. Where the floor separates one occupancy from another, the resilient layer, the ceiling treatment and the flanking paths are owned by the separating floor system, which describes the same physical floor in its acoustic duty; this entry keeps the span, the deck and the load path.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- joisted intermediate floor
- timber floor deck
- suspended timber intermediate floor
- upper floor deck
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Carry the loading of an occupied floor across a span into the walls or beams supporting it.
- Answer stiffness as well as strength, with what counts as acceptable movement set by a qualified engineer against the intended use.
- Enclose a routed zone for pipes, cables and ducts between the rooms the floor separates.
- Provide a fixing plane above for floor finishes and below for the ceiling of the room beneath.
- Frame openings for stairs, hatches and flues without interrupting the load path around them.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportBearings, hangers and wall plates
The arrangements by which joists reach their supports, whether built into masonry, carried on a plate or hung off hangers. This detail decides how load leaves the floor, and it is also where floors are most often altered without anyone recording it.
- Primary supportFloor joists
The spanning members setting the depth of the whole assembly. Their size and spacing are structural, and they also fix how much room is left for services and how the deck and ceiling boards are supported between them.
- AttachmentStrutting and lateral restraint
The blocking, herringbone strutting and straps stopping joists twisting and tying the floor to the walls it bears on. They turn individual members into a floor acting together, and they take part in restraining the walls themselves.
- SubstrateStructural deck
The boarding or sheet material fixed over the joists. It spreads point loading between members, makes the floor behave as one surface, and its fixing pattern determines much of the noise the floor later makes underfoot.
- Service zoneService route within the depth
The space between deck and ceiling used for pipework, cabling and sometimes ducts. It 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.
- Finish surfaceCeiling lining
The board fixed to the underside, forming the ceiling of the room below. Whether it is fixed directly, on battens or on resilient fixings changes what the construction does and how easily the services above can be reached again.
- Primary supportTrimming to openings
The doubled members and hangers framing stairwells, hatches and flues. They pick up the joists that were cut and carry their load around the opening, which is why an opening is a structural design rather than a carpentry adjustment.
- Jointing and sealingPerimeter and penetration sealing
The sealing where the floor meets the walls and where services pass through the lining. It bears on air movement between storeys and on the sound path around the edges, and it is normally done by whoever is last on site.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The depth is contested. 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.
Notching and drilling is where that contest becomes damage. A joist is not equally strong across its depth or along its length, so where a hole may go is decided by how the member works and not by where the pipe wants to be. Engineered members change the answer completely: some have designated service openings and are ruined by a cut elsewhere, which is why the member type has to be known before the first pipe is run.
Stiffness and sound travel together without being the same thing. Strutting and the deck fixings govern how much the floor moves and how far a footfall spreads through it; the ceiling and the way it is fixed govern what is heard below. Improve the ceiling alone and a lively floor still transmits; stiffen the structure alone and the path through gaps at the perimeter is untouched.
Openings redistribute everything at once. Trimming a stairwell moves load onto the members alongside it, changes where the deck is continuous, and interrupts the ceiling and any sealing line in the same place. That is why a floor opening is a structural, acoustic and detailing question together, and why the trimmers around an old opening are the first thing to look at when a floor feels lively.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Members of these families are commonly encountered as joists and trimmers. Which member answers a span, and what may be cut through it, is determined by the structural designer and the manufacturer's information for that product.
Substrate
These are commonly encountered as the structural deck or as a board layer over it. How a board is fixed, and whether it can receive a particular covering, is set out by the manufacturer rather than inferred from appearance.
Finish surface
Linings and fixing components of these kinds are commonly encountered at the ceiling. What a ceiling achieves depends on the complete tested assembly and the construction around it, which is a matter for a qualified professional.
Thermal layer
Quilts and loose fills of these families are commonly encountered within the joist depth. Whether adding one changes what the floor does, and whether it is being added for heat or for sound, is a design question rather than a product one.
Attachment
Metal families of this kind are commonly encountered in hangers, straps and restraint components. What a given connection requires is set by the structural design and by the component manufacturer's documentation.
Jointing and sealing
Sealants and tapes of these kinds are commonly encountered at perimeters and around penetrations. Which applies where a floor divides storeys, and what any of them is relied on to do, belongs with the designer.
Components
Building components that fill these roles
The discrete elements commonly occupying each role in this assembly. This is a different statement from the material families above: those name what a role is commonly made of, these name the positioned elements that occupy it. Commonly encountered, never recommended.
Edge and termination
Finish surface
Primary support
- Beam →Carries joists where the floor cannot span the full distance.
- Floor Joist →The repeated spanning members forming the floor structure.
- Noggin →Restrains joists against twisting so they act as intended.
- Carriage →Bears at the head onto the structure trimming the stairwell opening, alongside the strings.
- Landing →A timber landing is framed and trimmed in the same way as the floor it adjoins, and carries the flights meeting it.
- Stringer →Bears at the head onto the structure trimming the stairwell opening in the joisted floor.
- Structural Deck →The continuous surface spanning across the joists and distributing load between them.
- Trimmer →Frames stairwells and other openings, carrying the interrupted joists.
- Wall Plate →Spreads joist bearing along the supporting wall where joists are not built into it.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Bearing into masonry walls
Where joists are built in or hung, the floor is also restraining the wall. The connection therefore serves the wall as much as the floor, and altering it, by taking members out to run a duct for instance, has consequences beyond the floor.
Read about Load-Bearing Masonry →Ceiling fixed to the underside
The ceiling below is a system in its own right. Whether it is fixed straight to the joists or on independent or resilient supports changes both what is heard in the room beneath and whether the service zone can be opened later without rebuilding it.
Read about Framed Board Ceiling →Where the floor divides occupancies
Once the space below belongs to a different household, the floor is assessed as a whole construction including its ceiling, its fixings and the paths running round its edges, and a lightweight deck raises different questions from a dense one. That assessment belongs to the separating floor entry, and the two must not be treated alike.
Read about Separating Floor →Coordination with routed services
Pipes and cables in the depth arrive from a different discipline and usually later than the structure. Whether they pass between the members or through them is the single coordination decision that most often turns into structural damage.
Read about Service Void →Where an inserted mezzanine meets this floor
A mezzanine put into a tall space carries its own load, on new columns or on connections made into the host structure, and the question at this floor is whether it lands on the joists, on the walls they bear on, or deliberately clear of both. Where the two are connected, the joisted floor acquires a load path it was not drawn for.
Read about Mezzanine Structure →Floor as part of a framed structure
In a platform frame the floor is built as a storey deck that the walls above are raised from, so it also transfers stability forces. Its edges then belong to the frame's load path rather than being a simple bearing on a wall.
Read about Timber Platform Frame →Structural Insulated Panels
Where a floor lands on a panel wall the plane is interrupted across the floor zone. Structural continuity has to be remade through that zone and the sealing line carried across it, which is one of the few places the joint problem turns from vertical to horizontal.
Read about Structural Insulated Panels →Lateral Stability
A joisted floor is a set of separate members until a deck and its edge fixings make it act as one. Lifting boards, forming an opening or replacing a deck with something laid loose can therefore alter the plate without anyone intending a structural change.
Read about Lateral Stability →Air Barrier System
Where a floor is built into an external wall, the barrier plane on the inner face is interrupted by the floor zone and has to be carried around or through it. The detail has to be resolved before the floor is decked, because afterwards the zone is unreachable.
Read about Air Barrier System →Twin-Leaf Cavity Wall
Joists and their hangers land on the inner leaf, putting a horizontal line of interruption across the room-side face and, where hangers hook over the leaf, into the void itself. Both the air control line and the thermal layer have to be carried past that zone without a break.
Read about Twin-Leaf Cavity Wall →Solid Wall
Joist and plate ends built into the thickness sit in masonry that is periodically damp by design. Their durability depends on that masonry still drying and on air being able to reach them, which is why linings and coatings applied later can affect timber that is never seen.
Read about Solid Wall →Timber Frame Wall
At each floor the wall is interrupted by a rim member and by joists bearing on the frame. Both the insulation and the sealed layer have to be carried across that band, and the timber there will shrink as it dries, which the layers have to tolerate.
Read about Timber Frame Wall →Internal Wall Insulation
Where floor timbers bear in the masonry, the lining stops at the floor and the wall above and below is cooled while the timber stays buried in it. Whether that timber's surroundings become wetter is one of the first questions a survey should raise.
Read about Internal Wall Insulation →Masonry Partition
A masonry wall built on a timber floor applies a line load to members chosen for a distributed one. Whether the floor can take it, and whether the wall sits over a joist, between joists or on something introduced beneath it, is a structural question answered before the wall is set out.
Read about Masonry Partition →Stair Structure
This is the junction at which a stair stops being a stair. The floor entry describes what happens to its own members when they are interrupted; this entry describes what the interruption was made for. The two have to be read together, because the trimmer is simultaneously carrying the floor around the hole and receiving the flight's bearing and outward action.
Read about Stair Structure →
Internal junctions
Junction conditions inside this system
Junctions between the components within this assembly, rather than at its boundary. An entry exists only where the condition between two elements governs how the assembly behaves — two parts touching is not on its own a junction worth naming.
Balustrade Post to Stair Landing
Where a guarding post is fixed into the edge of a landing, delivering everything the guarding resists into a small platform.
Structural transfer · Movement · Buildability
Beam Bearing on a Corbel
Where a beam lands on masonry projecting from a wall face, a seating held in place only by what bears on the back of it.
Structural transfer · Movement · Buildability
Floor Joist to Head Binder
Where joists land on the member laid over the top plates of a framed wall, and a line built as separate panels is asked to behave as one support.
Structural transfer · Movement · Fire continuity · Buildability
Joist End to Masonry Bearing
Where a floor joist is supported by a masonry wall, and where structure meets a wetting risk.
Structural transfer · Moisture · Thermal continuity · Buildability
Stair Carriage to Stair Tread
Where an intermediate member beneath a flight meets the underside of each step, so that the step is picked up between its ends rather than only at them.
Structural transfer · Acoustic continuity · Movement · Buildability
Stair Landing to Stair Stringer
Where a flight's edge member lands on the platform at its end, delivering everything the flight carries into a small structure of its own.
Structural transfer · Movement · Acoustic continuity · Buildability
Stair Landing to Trimmer
Where the platform of a stair bears on the member framing the floor opening the stair passes through.
Structural transfer · Movement · Fire continuity · Buildability
Stair Stringer to Stair Tread
Where the end of each tread is received by the edge member of a flight, which is the stair's primary load path.
Structural transfer · Movement · Acoustic continuity · Buildability
Trimmer to Trimming Joist
Where a trimmer delivers the load of interrupted members into the joists alongside an opening.
Structural transfer · Movement · Fire continuity · Buildability
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Acoustic
- Footfall and airborne sound travel by different routes, and both are properties of the complete construction together with the walls around it. What a floor achieves is judged by a qualified acoustic professional, and this entry states no performance.
- Movement
- Timber changes dimension with moisture and the floor deflects under use. Both appear as gaps, squeaks and springiness, and both are influenced by how the deck was fixed and how the members were restrained rather than by the covering above.
- Fire
- A floor between storeys usually has a role in how the building resists fire, and that role belongs to a tested construction including its ceiling. Penetrations, recessed fittings and later openings all bear on it, and the relevant authority governs.
- Buildability
- Structure, services and finishes are installed by different trades in sequence, and each arrives to find the depth already partly used. The order in which they turn up is the practical reason services end up cut through members.
- Maintenance and access
- Once the ceiling is closed, everything in the depth is reached by opening something. Planning a route or an access panel for the parts likely to need attention avoids the later choice between living with a problem and taking a ceiling down.
- Interfaces
- The floor edge is at once a bearing, a restraint to the wall, an air path and a sound path. It is drawn by the structural engineer and finished by a decorator, and it usually shows the consequences of that gap.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What joist type is in this floor, and does the service routing plan match what that member allows?
- Where do openings fall, and how is load intended to be carried around them?
- Is stiffness being considered alongside strength, given how the room above will actually be used?
- How is the ceiling fixed, and is that arrangement doing an acoustic job as well as a visual one?
- What occupies the space below the floor, and does that change what the floor is being asked to do?
- Are access panels or planned routes provided for the services most likely to need attention?
- How is the perimeter sealed where the floor meets the surrounding walls?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Drilling a joist is treated as a plumbing task, when where a hole may go is decided by how the member carries load.
- A springy floor is assumed to be an inadequate one, but stiffness and strength are different properties and a floor can satisfy one without the other.
- Adding insulation between joists is thought to solve noise, when the paths that matter run through the structure and around the edges.
- The ceiling is treated as decoration, when in most floors it is part of what the construction is relied on to do.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What type of joist is used here, and what does its manufacturer permit by way of openings for services?
- Has deflection and floor stiffness been considered for the intended use of the room above?
- How is this floor restraining the walls it bears on, and would altering it affect them?
- What role does this floor have in the building's fire strategy, and what does that mean for penetrations?
- Is the ceiling arrangement part of an acoustic design, and has the flanking construction been considered?
- How will the services in the depth be reached once the ceiling below has been closed?
What this page does not do
- Spans, member sizes and the position of any opening cut in a joist are structural matters for a qualified engineer.
- Removing or notching members to make room for services can affect both the floor and the walls that it restrains.
- Acoustic and fire behaviour are properties of tested constructions including their ceilings and junctions, and none is stated here.
- Existing floors often contain unrecorded alterations, and what is found on opening one may differ from whatever was drawn.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Commonly built alongside
Systems routinely present in the same building without a junction recorded between them and this one. Where two systems do meet, the junction is set out above instead.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Intermediate Floor · Structure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Floor Structure and Build-Up Systems
Horizontal decks read together with the stack placed on them, told apart by what spans, what is infill and what is only a topping over both.
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