Almost all intermediate floors in domestic-scale buildings have timber joists with a timber based sheet material deck. Suspended timber ground floors account for about 80% of ground floors in Scotland and for about 15% in England and Wales. Both types of floors have a long-established and successful record in use. Timber floors are easy to install, light in weight and have the advantage of utilising a material which is environmentally friendly, both in terms of a renewable resource and using little energy in its production or use.
This WI Sheet gives guidance specifically intended to minimize problems of movement and noise in timber floors in masonry or timber frame buildings. It does not cover the design information readily available in other documents, for example thermal insulation and ventilation. The guidance relates principally to dwellings although much is applicable to floors in other building types, eg timber compartment floors in flats. The sheet can be used as a checklist for designers and site supervisors to ensure that timber floors give continuing satisfactory performance.
As in all types of building, careful specification, good site practice, including correct materials storage, and supervision are essential to ensure satisfactory long-term performance. If these are neglected, problems of movement and noise in service can develop. The production of squeaks and creaks results from the movement of two adjacent surfaces and in timber floors this is often the movement of the timber against timber, or timber against the nail which should be holding it in place. The amount of movement necessary to produce a significant noise may be very small (less than 0.5 mm) and considerably less than the degree of movement normally necessary to produce complaints of the movement itself. Whilst noise problems are few in number, their impact on building occupants is disproportionately large. Noise and movement problems do not generally result in significant structural defects but the disruption to occupants and the cost to builders of remedial action can be high.
The design and construction of timber floors is included in the TRADA Technology book "Timber Frame Construction" which covers both ground and intermediate floors.
The guidance in this WIS is derived from a TRADA research project which was part-funded by the UK Government.
Joists which are not sized correctly for the given span may result in excessive deflection and, in extreme cases, could be structurally inadequate. Timber joists for floors may be calculated in accordance with BS 5268 Part 2 “Code of practice for permissible stress design, materials and workmanship”. For dwellings, designers normally use pre-prepared span tables to size joists. Span tables are now included in the TRADA Technology publication "Span tables for solid timber members in floors, ceilings and roofs (excluding trussed rafter roofs) for dwellings". Designers may set a more stringent deflection limit for ground floors than that used for the span tables, since there is no plasterboard ceiling to provide additional stiffness.
All joists should be regularized or should be to ALS or CLS sizes to ensure that floor decks and ceilings are level. If the joists are not of regular depth either the deck or the ceiling may span over more than one joist space resulting in localized deflection and/or noise caused by movement on nails.
All joists should have a minimum bearing length of 35 mm.
Joists built into a masonry wall (Figure 1) should bear on to solid masonry or a solid packer (eg slate) and not on to friable or compressible material (eg mortar) in order to avoid downward movement. The pockets at the sides and top of the joists should be tightly pugged with mortar to minimize rotational movement.
Joists should fit tightly into joist hangers (Figure 2). It is especially important that the hanger width is specified to suit the joist width in order to restrict rotational movement. If joists need packing in the joist hanger, a solid material such as slate should be used to avoid downward movement. All nail holes in the hanger should be filled with the manufacturer’s recommended nails to minimize movement within the hangers.
Vertical or lateral movement of the joists in a platform floor in a timber frame wall is restricted (Figure 3).
Joints in joists may allow excessive deflection if they are not specified and made correctly. Joints should only occur over support walls unless they are specially designed.
Joists which overlap on support walls should be securely nailed together and should not project more than 100 mm beyond the support wall. This is to minimize upward movement of the joist against the deck as the joists in each span deflect.
Joists which are butted end to end over a support wall should be joined with a plywood or proprietary metal plate on both sides of the joists.
Metal straps which are required to provide lateral restraint to masonry walls should be supported by and nailed to noggings fixed between the joists. If they are loose, foot loading on the floor can cause the straps to move against the deck, causing a vibrational noise.
Moisture content and joist shrinkage
The moisture content of joists is the most important aspect of avoiding noise and movement in floors. Joists which are installed too wet (ie at a moisture content higher than that which they will reach in service) will dry out and shrink.
Intermediate floor joists in a building are likely to attain a moisture content of about 10-12% whilst those at ground floor are likely to be about 14%. Ideally the moisture content at the time of installation should be similar to these values. This is not always possible and a practical upper moisture content limit of 20% is recommended. However, even at this moisture content level, some shrinkage will occur.
Installing joists at too high a moisture content has a number of possible consequences:
200 deep joists installed at 30% moisture content may dry down to 10% in service. On average, the joist will shrink by 8 - 10 mm but individual members may shrink as much as 15 mm. Even allowing for joists cut oversize to compensate for the 30% moisture content, they will be undersized for the given span and deflection of the floor and “bounce” may result.
Shrinkage across the joist width will cause loosening of solid strutting which could result in noise through movement on nails. This is one of the most common causes of noise in timber floors.
Internal non-load bearing walls are normally fixed at the ceiling and floor level. Any lowering of the floor level through joist shrinkage will result in straining of these fixings. This may result in noise from movement on nails due to downward deflection of the floor through foot loading.
A joist drying down from say 30% moisture content may warp or twist and this could spring the floor deck or ceiling causing noise from movement on nails. If “dry” timber is used, any warp or twist will be evident before installation and affected pieces can be rejected.
An amendment to the stress grading standard, BS 4978, published in December 1994, requires structural softwood, less than 100 mm thick, for use in buildings to be graded at a moisture content of 20% or less. The timber will be marked DRY, or if kiln dried may be marked KD. The amendment is due to be implemented after March 1995 and therefore in future, structural timber should be delivered to site at a moisture content of 20% or less.
Even with relatively “dry” timber, different moisture contents in different joists can result in differential shrinkage which may cause joists to drop away from or lift the deck, or vice versa to the ceiling. Again this may result in noise from movement on nails.
Trimmers and trimming joists sometimes come from a different source from the other joists (they may be wider or of a different Strength Class). It is important that their moisture content is similar to the other joists.
Shrinkage in double member trimmers or trimming joists may result in loosening of their fixings and they will then not act together. This may result in noise from movement on the loose fixings, noise from timber to timber movement, or differential deflection causing nail popping at the deck or ceiling.
If timber joists bear on to a beam of different material (eg steel) an allowance should be made for differential movement to prevent distortion or movement of deckings or ceilings. As a guide, the top of the beam should be 12 mm below the joist top and the bottom of the beam and 2 mm above the joist bottom.
The use of modern structural composite materials (manufactured from laminated veneers or strands of timber) for joists will avoid shrinkage problems since these materials are supplied at a low moisture content.
Excessive notching or drilling can in extreme cases result in structural failure. In other cases it is likely to result in increased floor deflection and increased “bounce”. Notching and drilling should not exceed the limits shown in Figure 4 unless the notches and/or drillings have been specially calculated.
Trimmers and trimming joists should not be notched unless this has been calculated.
If water pipes are in direct contact with timber surfaces they may produce noises either due to normal deflection movement of the timber, or due to thermal expansion movement of the pipes. Pipes should either be cushioned with a fibrous insulating material or stand clear of timber surfaces. Notches should be deep enough to accommodate pipes so that the deck does not bear on the pipe and result in noise.
Figure 4: Notching and drilling - Link to PDF
