"I wouldn't put a milk bottle out on a night like this!" said a gentleman we once knew, looking askance out of his window at the weather. Perhaps not, but his oak frame had to weather every storm that came its way... as do they all.
Before too long, the frame is up, pegged and ready for the next stage of the development. We strongly recommend the frame be made secure against the elements as soon as possible. To this end, arrangements should be made for the relevant trades to set to work immediately on such things as roofing, cladding, glazing, fittings and plastering.
Water marks are often left on the surface of the timbers where they have been exposed to rain or moisture during or after installation. We advise that the frame is cleaned once the roof is watertight and walls fitted or wrapped. Sand-blasting, heavy sanding or treating with oxalic acid are the most effective ways to remove marks on the surface of the oak.
Hover over the image above to see the difference cleaning makes...
Press on/off the image above to see the difference cleaning makes...
Surface marks may also result from handling and working in the workshop, during delivery and on site. They appear as gun-metal marks at points of contact which will disappear completely when sand-blasted or sanded. They are often left by the action of metal tools on the timber during fabrication and post-build fitting - a reaction between the iron of the tools and the tannic acid in the oak in the presence of moisture. The greener or wetter the oak is, the greater this reaction. Similar marks have been known to occur when green oak comes into contact with wet plaster.
Blasting can remove deeper milling lines and creates a more grain textured surface than cleaning with oxalic acid or sanding. The green oak is blasted with a non-silicone material whilst adjacent surfaces are shielded. Blasting and sanding can produce a considerable amount of dust and debris so it is advisable to mask off any doorways and fittings such as electrical outlets, lights and vents.
Where plaster is to be applied after cleaning, the oak should be protected with non-marking masking tape and/or polythene sheeting. There are many local sand blasting companies with experience of green oak around the country. Alternatively, we are happy to arrange for a completed frame to be sand blasted through our regular contractor.
Oxalic acid is used to remove tool, tannin and water marks on timbers, as well as to remove silvering on exterior surfaces. It is sold crystalline and is diluted in either water or methylated spirits at typically around 110g/litre (4 tbsp/pint). It may be applied with a brush, cloth, sponge or spray and left for 10 to 20 minutes before washing with clean water or meths and wiped dry. Nearby surfaces should be protected from any run-off. Gloves, face and eye protection to meet supplier's recommendation is vital.
As the green oak frame matures, it will start to lose much of the moisture from its timbers. A newly erected green oak frame will comprise timbers of around 60-80% moisture content, which will dry over time internally to around 20-30% depending on ventilation and heating.
This drying leads to a certain amount of shrinkage across the timbers' width, which may cause surface cracks to appear and some internal joints to open up at the shoulders. These are perfectly normal and do not affect the strength of the frame. Indeed, the increase in strength of the oak as it dries more than compensates for any cracks in its surface. It adds to a frame's individuality and character.
Oak is an excellent exterior wood and there are many open structures that are known to be many centuries old.
When untreated green oak is exposed to the weather its surface gradually becomes 'silvered'.
As it dries and matures, it will blend into its surroundings to give a naturally aged and venerable appearance.
Where exposed timbers are subject to prevailing weather, periodic treatment with a clear wood preservative can be applied when the surface is dry enough to absorb the treatment.
However, due to the way surface cracks in oak widen as a frame matures, the moisture content of the exposed oak will often reflect that of the air around it.
Oxalic acid crystals are available at:
AG Woodcare
A summary of approaches to installing glazing in green oak frames, with either direct or externally mounted units or frames.
Step into our oak frame gallery to explore the green oak building projects we've brought to life over time. From small-scale porches to large three-storey extensions, there's bound to be something that catches your eye!
This is a brief overview of some aspects of green oak frame cleaning and
finishing, rather than a comprehensive guide. Whilst some tasks may be
undertaken as part of a self-build or DIY project by those with sufficient
competence, this should also be accompanied by appropriate professional guidance.
