Timber Joist Floors

Sizing · Span tables · Insulation · Standard and US spans

Timber joist floors are the backbone of Norwegian timber-frame construction. They carry loads from the floors above, distribute them to the walls below, and form the platform for floor finishes, insulation and acoustic separation. Good sizing and correct installation prevents creaking, deflection and noise problems that are extremely difficult and expensive to fix after the house is complete.

Floor construction types

Suspended joist floor over crawl space

Timber joists span over a crawl space (typically 50–80 cm clear height). Most common in older Norwegian houses built 1950–1990. Provides easy access for pipework and services, but requires good ventilation and drainage of the crawl space to prevent rot and moisture damage.

Platform frame floor

In modern timber-frame construction, the joist floor is mounted directly on the top plate of the first-storey walls and forms the inter-storey floor deck. The joists are structural elements and are sized according to span and imposed load.

Concrete deck

Precast hollow-core planks or cast-in-place reinforced concrete are used at ground level in modern new-builds and during refurbishment. Provide excellent sound and fire resistance. This guide focuses on timber joist floors.

Joist sizes and span

Standard joist spacing in Norway is 600 mm centres. The clear span (free distance between supports) determines the required joist size. Treat the table below as indicative only: it assumes a design imposed load of 2.0 kN/m² (habitable rooms, standard residential load) at 600 mm centres, but real allowable spans also depend on the timber grade, the full load (partitions, tiled floors, point loads) and — often decisively — deflection and floor-vibration/comfort limits, which frequently give shorter spans than strength alone. Confirm against a current span table for your grade, or with an engineer:

Inter-storey floors and sound requirements

Acoustic requirements apply between separate dwelling units (flats, terraced houses). These requirements do not apply inside a single-family detached house.

• Impact sound: L'nw ≤ 53 dB (lower is better)
• Airborne sound insulation: R'w ≥ 55 dB

Solutions for good sound performance

• Insulation between joists — 95–100 mm mineral wool reduces airborne sound transmission
• Resilient channels / spring bars — decouple the ceiling from the structural floor deck, reducing impact sound transmission significantly
• Floating floor — the subfloor is laid on acoustic isolation pads without rigid connection to the joists (breaks the impact-sound path)
• Mass screed — a heavy concrete screed (>80 kg/m²) on top of the joist floor reduces impact sound markedly

Insulation in joist floors

Joist floor over crawl space (exposed to unheated/outside conditions)

• Minimum 250 mm mineral wool between joists (U ≤ 0.18 W/m²K)
• Wind barrier on the underside of the joists — this layer must be vapour-open (a proprietary wind/breather membrane or a wind-barrier board), never a polyethylene/vapour-tight sheet. A vapour-tight layer on the cold underside traps moisture in the insulation and rots the floor; the vapour barrier belongs only on the warm (top) side
• Vapour barrier on the warm side (top): 0.15 mm PE sheet
• Ventilation gap between wind barrier and any ceiling battens below

EU/NO: U-value ≤ 0.18 W/(m²K) for floors exposed to outside or unheated conditions

Inter-storey floor over heated room

• 200–250 mm mineral wool in the joist cavity for sound attenuation (primarily acoustic, not thermal)
• A vapour barrier is not normally required in an internal floor deck between two heated spaces

Subfloor and floor covering

Subfloor

• OSB 3 — 18–22 mm. Standard subfloor for timber joist construction; good stiffness and cost-effective. OSB/3 (EN 300) is rated for load-bearing use in humid conditions, but it is not an exterior or permanently-wet board — keep it dry and protect it from rain during the build
• Plywood — 18 mm. Somewhat stiffer and more dimensionally stable than OSB; preferred where ceramic tiles are laid directly
• Lay panels with short-side joints staggered (brick-bond pattern) and tight long-side joints
• Leave a 5 mm expansion gap against walls

Floor finishing

• Hardwood, engineered timber, laminate or vinyl are laid directly on the subfloor (see the parquet guide)
• For ceramic tiles on a timber floor: the deck must be much stiffer than for timber finishes — design to a tighter deflection limit (e.g. around L/600), add a rigid plywood overlay or a tile backer / uncoupling board, and use a flexible (S1/S2) tile adhesive. On a bouncy or under-stiffened floor the tiles and grout will crack whatever adhesive is used

Step-by-step — joist floor installation

1. Mark joist positions — starting from one edge, measure out 600 mm centres and mark on the top plate and any intermediate bearing walls.
2. Fix joist hangers — nail or screw galvanised joist hangers to the top plate at each marked position.
3. Install joists — joists may be butt-joined over a central bearing wall with a minimum 90 mm bearing. Check all joists are level and at the correct height.
4. Blocking — install solid blocking (cut from the same joist size) at mid-span for spans over 3.0 m to prevent joists from rolling and to increase the stiffness of the system.
5. Vapour barrier — for crawl-space floors, lay 0.15 mm PE sheeting across the top of the joists with 200 mm overlaps at joints and adhesive-sealed laps. Turn up 150 mm against walls.
6. Insulation — lay mineral wool batts tightly against the joist sides. Cut batts slightly overwidth (about 10 mm extra) so they friction-fit and stay in contact with the joists.
7. Fix subfloor — screw down with ø4.5×70 mm screws, 150 mm along joist edges and 300 mm in the field. Stagger the panel joints.

Reference table

Common mistakes

• ✗Installing parquet or laminate without acclimatisation — boards swell or shrink and produce gaps or buckling
• ✗Skipping a vapour barrier below a floor over a crawl space — rising moisture attacks the floor structure from below
• ✗Too little expansion gap along the walls — timber flooring needs 8–12 mm clearance around the entire perimeter
• ✗Ignoring floor levelness before laying — height differences over 3 mm per 2 m cause creaking and board cracking
• ✗Notching or drilling joists outside the permitted zones — holes and notches in the wrong place seriously weaken a joist; keep them within the limits given in your joist/span guidance
• ✗No solid blocking or strutting between deep joists — without it the joists can twist and the floor feels bouncy; add blocking or herringbone strutting at mid-span
• ✗Cutting skirting boards too early — let the floor acclimatise and settle first, then fit the trim

Calculate your materials

Use the calculators to find the number of joists, subfloor panels and insulation needed: