MECHANICAL FAILURE

If a building is able to rack or twist, placing individual members under excessive loads, then mechanical failure is likely to occur. This can happen if:

i) the frame is inadequately braced

ii) foundations have moved

iii) a cill beam has deteriorated bracing.

The resulting forces of compression, tension and torsion, will eventually lead to mechanical damage when the timbers can no longer absorb them through deflection or crush. Failure may occur in the form of shear, joint failure; plate or post fracture; rafter spread or ultimate collapse.

More often than not such events are initiated by alterations or ‘improvements’ which have involved severing structural members to accommodate inserted floors, communicating doors, additional windows or relocated stairways. Internally, the removal of partition walls or, the installation of services in trenches cut through floor joists, may have severely reduced the structural frame, while beam ends encased in masonry walls can decay, allowing the beam to descend along with the floor joists.

REPAIRS

Before timber repairs are put in hand, a drawn and measured survey of the building should be prepared, showing:

i) the timber type, condition and dimensions

ii) surface finish or treatment

iii) position, nature of construction and assembly marks

iv) details of mouldings or decorative features

v) the exact location of any damage or distortion.

Since timber frames and roofs are designed to stand alone as a unit, it is our opinion that the repair should be aimed at restoring that unity. Carpentry solutions may therefore be preferred to alternative approaches – although this will often entail a degree of replacement. Under these circumstances, ‘green’ or unseasoned timber may be used when the dimensions required exceed 4 – 5 inches. (Dry stock is difficult to obtain and prohibitively costly for most applications.)

B&w illustration of notched lap joint

Notched lap joint designed to resist withdrawal, Coggeshall Abbey Barn, 1120-1147 (Hewett)

When considering the replacement of such elements as shaped door heads, deep mouldings, large boards and panels or wide braces, dry stable material is essential. This is because subsequent shrinkage will cause distortion and possible damage to adjacent fabric.

An indication of conversion methods used in producing original members may be obtained by examination of the cross-section. Heart/sapwood orientation is important when considering the weathering of exposed timbers, whilst conversion methods dictate to a large degree, the surface finish, and at times, the final form.

For the individual trying to minimise the visual impact of a repair, the material which is to be permanently bonded to the original must be closely matched in type, moisture content, surface, sectional grain pattern and colour. This can easily be sourced from reclaimed material.

The glues used in bonding should be boil and waterproof (commonly two-part epoxy resins). Whilst this may also serve as a grout for bedding or to fill fissures up to 10mm, suppliers should be consulted regarding suitability for filling larger voids.

Where metal work is to be embedded in timber, as in flitch-plate repairs to fracture, stainless steel should be used for both plate and fixing pins or bolts. They should be set in a grout of resin and the entry point masked by the insertion of a timber fillet. Surface applied plates, straps or bands, may be of mild steel but adequate protection must be provided by the application of rust inhibiting coatings or paint, particularly, on contact surfaces.