Roof timber treatment: when and how to protect the wood?

Roof timber treatment is one of the most important interventions for the longevity of a house, and one of the least understood. A timber roof structure can last for centuries if it stays dry and sound. But damp, insects and fungi can weaken it silently for years, until the strength of the roof is compromised. Here is how to recognise the warning signs, which treatments exist and when you need to go beyond treatment.

The enemies of roof timbers

Wood-boring insects

These are insects whose larvae feed on wood. The most common in Belgium are:

• The house longhorn beetle, which attacks softwoods. Its larvae dig wide tunnels beneath the surface, sometimes for several years, before emerging through oval holes. The damage can be extensive without being visible from outside.
• The common furniture beetle, or woodworm, which mainly attacks old timber and leaves small round holes of one to two millimetres, with fine bore dust.
• The deathwatch beetle, which prefers hardwoods such as oak, especially when damp.
• The powderpost beetle, which attacks the sapwood of certain hardwoods.

Wood-destroying fungi

They develop when timber stays damp. Dry rot is the most feared: it can spread through masonry and quickly destroy the timbers of a building. Other fungi cause brown or white rot. Their presence is betrayed by a mushroom smell, white strands or timber that cracks into small cubes.

Damp

Damp is behind most problems. An unrepaired roof leak, chronic condensation in a poorly ventilated loft, a rafter foot sitting in a leaking cornice: damp timber becomes a target for fungi and some insects.

Signs that should alert you

• small round or oval holes in the surface of the wood;
• fine dust at the foot of the beams or on the loft floor;
• timber that sounds hollow or gives way under the tip of a screwdriver;
• blackened timber, damp stains or mould;
• a smell of mushrooms or a cellar;
• white strands or a cotton-wool-like mass;
• visible distortion of the ridge line or roof slopes.

If in any doubt, have a survey carried out. The earlier an attack is detected, the simpler the treatment.

The survey: an essential step

Before any roof timber treatment, a professional must identify the problem precisely: which insect or fungus, how widespread, how deep, and above all what the source of damp is. They probe the wood, measure its moisture content and inspect the bearings and hidden areas. This survey determines the treatment method and shows whether any members need strengthening or replacing.

Roof timber treatment methods

Preventive treatment

It protects sound timber against future attack and is applied by spraying or brushing on all accessible faces. It is good practice during a roof renovation, when the structure is fully exposed, and for new timbers fitted as replacements.

Curative treatment by spraying

For a surface attack, the timbers are brushed or cut back to remove the damaged parts, then sprayed with an insecticide and fungicide that penetrates the surface.

Curative treatment by injection

For large sections or deep attacks, particularly by house longhorn beetle, holes are drilled in the timber and fitted with injectors. The product is injected under pressure to reach the core of the members. This is the most effective method for heavy beams.

Treating dry rot

This is specialised and more extensive: removing the source of damp, cutting out affected timber well beyond the visible area, treating contaminated masonry and drying everything out. It must be entrusted to a specialist.

Treat, strengthen or replace?

Treatment kills insects and fungi, but it does not restore the strength the wood has lost. When tunnels have weakened a load-bearing member, it must be strengthened, by sistering or with steel plates, or even replaced. The survey determines how much sound timber remains in each member. Our roof carpentry service carries out this strengthening.

How much does roof timber treatment cost?

The price depends on the area of timber to be treated, access, the method and the extent of strengthening. Preventive spray treatment remains affordable, especially when carried out during a roof renovation. Curative treatment by injection, with cutting back and strengthening, is a larger budget. An accurate quote can only be given after a survey. For a home more than ten years old, reduced VAT at 6% in principle applies.

Preventing attacks

• Keep the roof watertight and repair any leak quickly.
• Ensure good ventilation of the loft and the roof covering.
• Insulate with a correctly installed vapour barrier to prevent condensation.
• Maintain cornices and box gutters.
• Inspect the loft once a year, torch in hand.

Roof timber treatment during a renovation

The best time for roof timber treatment is when the covering is being renewed anyway. The structure is then fully exposed, every member can be reached and weak pieces can be replaced or strengthened on the spot. The extra cost is small compared with the peace of mind it gives for decades to come, which is why we include this check in every roof renovation quote.

Frequently asked questions

Is timber treatment dangerous for the occupants?

Today’s products are much less harmful than older ones, but you must allow an airing period before using a treated loft again. The professional will tell you which precautions to take.

The holes are old: is the attack still active?

Not necessarily. Fresh, pale dust and holes with sharp edges indicate recent activity. Only a survey can confirm it.

Does a new roof structure need treating?

Structural timber is often supplied pre-treated. Additionally treating the cuts and joints made on site remains good practice.

In summary

Effective roof timber treatment starts with an accurate survey and removing the damp. Preventive during a renovation, curative in the event of an attack, it sometimes needs to be combined with strengthening. Our teams work in Brussels, Walloon Brabant, Hainaut and Flanders: request a free visit.