A worm hole in European oak? Don't panic

Dear wood lovers,

The phone call comes about once a week. "Mr Van de Voort, there is a hole in my oak beam. Is that woodworm?" I can hear the panic in the very first sentence.

My answer is almost always the same: stay calm. A worm hole in European oak is often just part of the deal. Most of the time it is a bit of the tree's history, not the start of a disaster.

Where does a hole like that come from?

Usually long before the tree became a plank. Oaks have had a hard time in recent years with dry, hot summers. Weak or freshly felled oaks in particular attract ambrosia beetles, such as Platypus cylindrus and Xyleborus monographus.

Their holes are small, about 1.5 to 2 millimetres. Around the hole you sometimes see a dark rim. That is the fungus the beetle brings with it.

These beetles need fresh, wet wood. Once the wood is dry enough, that is the end of it for them.

What does the drying kiln do?

In the drying kiln the wood stays warm for a long time, right into the core. Most beetles that live in fresh wood do not survive that. Air-dried wood simply becomes too dry for them.

So the hole in your beam is usually an old hole, made by a beetle that left long ago.

Is it bad for the strength?

Not noticeably, with these beetles. Forestry researchers say the same. It is about looks.

That is why I do sort them out of the higher grades. If you buy a clean plank, you want a clean plank. That is a choice for the eye, not a verdict on safety.

A confession

In the old days we had to make holes ourselves. With a needle drill, for the customer, to imitate worm holes. You read that right. Now I get photos of exactly such a hole, with the question whether the house will stay standing.

When should you be careful?

There are also beetles that can live in dry wood: the common furniture beetle and the powderpost beetle. Their holes are also about 1.5 to 2 millimetres. But they choose the sapwood, the light wood just under the bark. In European oak that sapwood is only a few centimetres thick. They leave the heartwood alone.

The furniture beetle also needs moisture. Below 12 percent moisture content, its larva does not get far. Watch for these signs of an active infestation:

• Fresh, light bore dust under the holes

• New holes with sharp, clean edges (old holes are dark and dirty)

• Soft or crumbly wood, sometimes sapwood that has turned to powder

• Deep tunnels in a beam that has to carry something

Not sure? Do a test. Put a sheet of white paper under the wood, clean everything, and watch for a few weeks to see whether new dust falls. Pay most attention from May through September, when the beetles come out. One bit of dust means nothing, because old dust can drop out of a tunnel when the wood vibrates or is moved.

Does dust keep coming, or will the beam soon have to carry a roof? Then have it assessed. For a table with two old holes you do not need to.

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So, dear friends, just leave that hole alone. Oak has stood in the forest. A hole or two shows it is real wood.

Maarten van de Voort – Timber merchant, Tilburg

PS. Are you looking at a beam or table with holes and not sure what it is? Send me a photo or call: +31 (6) 22546945.