Timber floors are simply a series of timber joists or beams suspended across and supported by load bearing walls sized and spaced to carry all the dead and imposed loads. The ends of these timber joists are built into, or suspended by, the external walls of the property.

Primary function of timber floor is to:

• Provide a level surface with sufficient strength to support the imposed loads of people and furniture plus the dead loads of flooring and ceiling
• Reduce heat loss from lower floor
• Provide required degree of sound insulation
• Provide required degree of fire resistance

Trimmer and Trimming Joists are required at stairwell openings. Proprietary timber to timber connectors (Jiffy Hanger) are used to connect trimmer to trimming joist.

Note: depending on size of opening, bearing loads etc., the opening may require specialist design by an engineer.

Joists are spaced at 400m, 450mm or 600mm centres to accommodate floor sheeting and plasterboards.

Allowable spans and spacing between joists:

Size of Roof Joist (mm)	Maximum clear span in metres for Joist Spacing of:
	400mm	450mm	600mm
50x97	1.98m	1.87m	1.54m
50x122	2.60m	2.50m	2.19m
50x147	3.13m	3.01m	2.69m
50x170	3.61m	3.47m	3.08m
50x195	4.13m	3.97m	3.50m
50x220	4.64m	4.47m	3.91m
75x147	3.56m	3.43m	3.13m
75x170	4.15m	3.96m	3.61m
75x195	4.68m	4.52m	4.13m
75x220	5.11m	4.97m	4.64m

The joists in the loft space should already have been insulated. However, this may have taken place years ago or insufficient insulation may have been used. This insulation may now not meet current standards. Now is the opportune time to improve this insulation before floor is sealed. Both rigid sheet and insulation roll can be used.

If restricting noise transfer is a priority, select acoustic insulation. Insulation can be placed between floor joists after plasterboard for ceiling is attached. Make sure insulation is tightly packed between joists to provide maximum benefit.

Increase the insulation (and soundproofing) properties of a floor by fitting a good quality acoustic underlay. It is simply laid under most floor finishes to improve the reduction of impact sound and it can replace conventional carpet underlay. It is manufactured from various different materials such as vulcanised rubber, sheepswool, agglomerated cork and recycled tyre rubber.

Structural Wood Panels (e.g. Plywood, Chipboard, OSB) commonly form the subfloor of a Timber Floor System.

These panels are typically manufactured in 1.2m x 2.4m sheets.

The panels are fixed to the floor joists with 3mm gaps left between the sheets to allow for expansion and to prevent buckling.

The maximum moisture content at time of fixing should not exceed 16%.

Plywood should be conditioned to the expected in-service moisture content prior to fixing.

Laying and fixing:

• Plywood panels should be laid with the face grain at right angles to the direction of the floor joists.
• All end joists should occur over joists and be staggered transversely across the floor.
• All longitudinal joints should be kept in line.
• Where boards are square edged (i.e. not tongued and grooved), the edges should be supported by noggins.
• Typically fix nails at 150mm centres to perimeter and 300mm centres to intermediate supports.

Only flooring grade chipboards are suitable for use as floor decking. These are C4 (moisture resistant), C5 and Type II/III.

Board thickness: 18mm at 450mm centres (max) or 22mm at 600mm centres (max).

The maximum moisture content at time of fixing should not exceed 15%.

Plywood should be conditioned to the expected in-service moisture content prior to fixing. Steps should be taken to ensure that chipboard is not exposed to wet weather.

Laying and fixing:

• Square edged boards should be laid with the long edges parallel with and butt-jointed over the joists.
• End joints should be staggered and supported by noggins.
• Tongued and grooved boards are normally laid at right angles to the joists with the short edges butt jointed over the joists.
• Board edges around the perimeter of the floor are continuously supported by joists or noggins.
• Typically fix nails at 300mm centres to perimeter and 300mm centres to intermediate supports.
• All tongued and grooved joints should be glued to prevent creaking.

OSB is a structural panel board manufactured from softwood flakes bonded with phenolic resin adhesives, chemical bonders and wares.

A voluntary colour coding system exists for OSB. Two colours are used in each case.

The first colour defines the panel as either intended for general use or load bearing applications.

The second colour identifies the panel as being suitable for use in either dry or humid conditions.

Colours used as follows:

First Colour	White	General Purpose
	Yellow	Load bearing
Second Colour	Blue	Dry Conditions
	Green	Humid Conditions

Load bearing decking for dry conditions is also known as OSB 2 and load bearing decking for humid conditions is also known as OSB 3.

Steps should be taken to ensure that chipboard is not exposed to wet weather.

Board thickness: 15mm at 450mm centres (max) or 18mm at 600mm centres (max).

Laying and fixing:

• All boards must be laid with long edges at right angles to the floor joists.
• Cross joints should be staggered.
• All cut edges which are not supported on joists and all edged boards should be supported on noggins.
• All board edges must be supported at the perimeter of the floor.
• Typically fix nails at 150mm centres to perimeter and 150mm centres to intermediate supports.