Essentially, oak beams are maintenance-free and it is not necessary to treat them. Over a very long period of time natural tannins in the timber react with the surrounding air and it changes gradually from a light honey colour to a more silvered hue. However, there are various methods of controlling this, depending on your requirements.
Since oak requires very little care we tend to leave any treatments other than fuming to the discretion of our clients. Usually we recommend that you leave external green oak frames to weather naturally. Left untreated the oak will weather attractively to a silver colour in around a year. If required, the original, lighter colour can be retained if the wood is treated or sealed before it is exposed to the elements. Remember though, that if you do this, you are signing up to a lifetime of repeat coats as the weather erodes the treatment off! We also recommended you check the colour on an off-cut of the same timber if you do intend to treat it so you can get exactly what you want.
Areas of water forming on the surface of the beams during construction are often unavoidable when building outside and will affect the colour and leave water marks. Internal oak features can often be sheltered from the rain, particularly if the timber has been fumed dark as this increases the staining. If staining occurs it can be cleaned with oxalic acid after the structure is weather tight and the staining largely removed.
Splits and cracks are always to be expected in an oak frame or set of components. As oak dries out, it shrinks across the grain, which is caused by the the fibres in the cellular structure thinning and pulling closer together. Some of them stick one one side and pull away from another causing shakes (splits) along the grain, which is completely natural and does not effect the frame strength in most situations. The shrinkage has a strengthening effect on the frame with and can also tighten joints between components.
Buildings made of green or semi-seasoned oak will do most of their drying out during their first few years in a central heated environment. This drying period will see the moisture content of oak change from around 60% to around 16%. This is a huge change, and so it's not surprising that the oak shrinks a little and that cracks occur. However, it is generally agreed that it is these very cracks and splits which add to the character and charm of the finished product.
For more information about caring for green oak, take a look at our Green Oak Maintenance Guide.