Wednesday, 28 November 2012

Timber Repairs - Rot Engineering

The repair of timbers weakened by fungal decay or insect attack (wood-boring insects such as woodworm beetles) is one of the important parts of a successful structural refurbishment exercise.
In some projects, repairs can be made so that chemical treatments can be minimised or avoided.

However, where structural damage has occurred timber repairs are essential.  Before commencing timber repairs and remedial actions to remove the rot and some types of beetle infestation, it is most important to eliminate the ingress of moisture and dampness. This is the primary method of control - together with promoting rapid drying of the structure. There are options for chemical treatment of course.

Procedures for timber repairs

There are various procedures for timber repairs. British standard BSS268 gives general guidelines. However specific information cannot be obtained in each case from the standard because of variation in construction details. An individual case will require detailed instructions from a structural engineer.
Before any cutting out of timbers is carried out the beams must be propped. Propping itself can be complex.  Anything other than simple single level propping requires to be designed. An engineer or competent scaffold contractor can usually help with propping design.

Epoxy resin repairs

These are a very useful way of repairing timbers. The technology has improved over the last 20 years.  Various methods exist for these repairs, increasing the scope for “keyhole surgery” procedures.

Basic splice repairs

This can be done bolting side by side or in line. The fixing method must result in a secure connection that can support the load and will not move in the future.

Dwangs (noggins) help prevent twisting of the repair. Roof joists should always be dwanged to prevent rotation. These should be located at the mid point of the bolt pairs. Too many splices in a line can produce undesirable bending moments along the splice line. To overcome this the fifth in each line of splices should run back to a bearing wall.

To splice timbers in thickness in excess of 150 mm it will normally be necessary to use steel plates, or some other system than a splice  An engineer’s detail will normally be required.

Beam replacement

Sometimes it is necessary to replace the entire beam. This can be done using a non-organic material like steel or concrete. Concrete can be used to replace Timber lintols over windows and doors. It is important to make sure that the bearing ends have secure seating. Some types of lintols have to be bricked up so that they can stand the designed loading.

Bearing beams may be replaced in steel. An engineer’s detail will design the steel beam. The beam must be securely mounted. If you are in any doubt at all about a repair get advice from a qualified structural engineer.

Steel plate or shoe connections may also be used. These can be made up to a specific design by a blacksmith, or commercially available plates can be bought from some specialist suppliers. 

For more information about expoxy resin repairs please click here.