Timber-framed Buildings

Timber frame

Timber-framed buildings can withstand a degree of flooding but will structurally fail when submerged for long periods of time. They will also fail if they suffer continuous cycles of wetting and drying, or where force from debris impacts the infill panels.

Timber is sensitive to moisture and has the potential to be attacked by decay fungi, wood boring insects and provide a surface for mould to breed. It is important to note that mould is associated with health problems, whereas decay fungi can result in structural defects and health problems.

Structural timber when saturated (that is, when the moisture content stays above 27–28 per cent for a long period of time) is vulnerable to decay caused by fungal growth and insect attack. Like other living things, the organisms that cause this decay need food (wood and water) to grow. In most building environments, the missing element is water. So, as long as structural timber is kept dry, it can last for centuries.

A one-off event, such as a flood, should not cause serious damage if the timber is allowed to dry appropriately afterwards. Problems occur when moisture is trapped in the timber and cannot escape. This can happen in wall cavities, in sill plates under floors, behind panelling and under impermeable finishes such as oil-based gloss paint.

Timber-framed buildings have several points of vulnerability. Their plinths are generally low and not historically fixed to the frame itself. When floodwater rises above the plinth, it is likely to lead to structural problems. Timber-framed buildings may also have infill panels of varying material types, with several junctions. Newer timber replacements will not necessarily be of the same strength as the older parts.

Older hardwood timber can be more resilient than its modern equivalent. The grain is tighter, which makes it harder for water to pass through. Slow-growing ring porous hardwoods, such as oak and ash, may be structurally weaker because their annual growth rings are predominantly earlywood, with large, thin-walled pores. The opposite effect is seen in fast-growing ring porous hardwoods.

Infill panels

The type of infill panels will affect how the building withstands floodwater and how it will recover. Timber-framed buildings have gone through various iterations of panels, from wattle and daub, to brick or cement infills, to more modern options such as wood fibre. Understanding how the panels work with the frame will help to identify where structural failure is likely to occur, and where water might enter.

From a structural perspective, cement and brick infill panels are unlikely to be properly bonded to the inside of the timber frame. In some cases, nails might be present and left proud in line with the mortar joint. These harder infills will provide some racking resistance (ability to withstand horizontal load), whether bonded or not.

Wattle and daub panels have a low racking resistance, and the laths will likely split or snap under high pressure. Currently, there is no evidence pertaining to the racking resistance of wood fibre panels.

Cement and brick panels can be dried effectively in situ and returned to their pre-incident state. Wood fibre panels would likely need to be replaced after a flood event because the integrity of the material cannot be guaranteed.

Installing removable panels could facilitate drying and cleaning. Not all panels need to be removable, just those at risk of floodwater. However, removable panels are only an option if they are not providing lateral stability (or raking resistance) to the building.