Rain droplets can remove the finish and destroy the wood. What solutions do we have?

I saw the wooden door in the picture recently and thought it was the perfect illustration of a phenomenon that is often overlooked but can have very unpleasant consequences: raindrops bouncing off. It's the cause that led to the complete destruction of the door's finish at the bottom and the appearance of mould on the wood. Otherwise the door looks very good, even if it is not protected against rain and sun. Water from bouncing drops can destroy the base of windows, doors, support posts or cladding boards. Therefore, wooden houses should be built on a concrete or masonry foundation of at least 30 cm. In Germany, due to the rebound of raindrops, it is mandatory that the wooden elements of a house are no lower than 50 cm from the ground.

Water, the big problem for wood and protective materials

I was told at one point by a manufacturer of timber frame houses that wood's greatest enemy is water, not fire. Fire is an accident, whereas water can get to the wood through a lot of ways and destroy it. Water is just as dangerous to the materials that protect wood on the outside. The smallest crack in the varnish film allows water to reach the wood, and it's only a step from there to mould.

Nor is the protection given by oils any safer if not regularly maintained. Repeated splashing washes away oil applied to the base of the door, window or fence, creating a gateway for water to enter. The areas become covered with mold over time and if not refinished in time, the wood will begin to rot.

The water is harmless as long as it does not remain for a longer period, it does not puddle. Staying longer creates conditions for micro-organisms to penetrate the wood and destroy it. The phenomenon is explained at length here. In order to avoid it, technical solutions must be found so that the water drains away and the surface becomes smooth. These are the two fundamental conditions for the wood and the protective coating to last as long as possible.

What are the solutions to prevent mould on finished wood

No varnish, paint or oil can protect wood in the long term without maintenance. In areas where we know for sure that raindrop rebound will occur, it's best not to rely heavily on the finish. Fitting a metal guard on the bottom of the door over varnished or oiled wood can help.

There are other solutions to make doors, windows, veneers and other wooden elements of the house as protected as possible. Here are some of them:

• protecting them with large eaves and awnings,
• covering areas in contact with the ground with water-resistant materials (metal, plastic),
• covering (blocking) areas of fibre ends to prevent absorption,
• fitting window glazing,
• the use of profiled planks for horizontal cladding to allow drainage (glazing),
• fitting ventilated facades so that air can circulate and shake the wood,
• technical solutions to promote water drainage, avoid contact with the ground and ensure air circulation.

The vertical surfaces, rounded or triangular profiles of the wooden elements allow better water drainage. Vertical wood cladding is preferable to horizontal cladding. However, if horizontal cladding is desired, the recommendation is that the elements should be fitted sufficiently overlapping so that the falling drops do not reach the lower element. The specialist recommendation is that the overlap should be 12% of the plank width and min.10 mm.

Direct contact with the ground can create big problems. Support posts should not be fixed directly to the ground, but with fixing screws that provide a comfortable distance from the ground while being safe and strong. A slightly slanted section will ensure better water drainage. If there is a possibility that the drops will ricochet down to the base of the post, it is recommended to clad this area with metal. This cladding should not allow water to accumulate.

Why wood preservative materials yield differently

The different outdoor resistance of the finish, even if the same materials and technology were used, is a common phenomenon. Wooden window frames or shutters will be more quickly in need of rebuilding if they are placed on the side of the house more exposed to rain and wind. Windows under the eaves or awning will be more resilient than those standing directly in the path of rain and wind. The more the finished wood is protected from water and wind, the longer it will last.

Although not as dangerous as water, wind can also cause problems. Together with dust in the atmosphere, it has an abrasive effect on the protective layer, removing about 15 µ/year, according to experts. If maintenance is not done on time, the protective layer will be completely removed. Wind acts differently on the sides of the house, with some parts being more sheltered than others. This will cause the finish to fail harder on the sheltered parts and faster on the others.

Information on the choice of species and construction solution suitable for outdoor use and on suitable protective materials can be found at here and here.

I hope you find the information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.