You may treasure your bespoke front door or your lovingly restored original. Sadly, few others will treat it as gently as you do. London’s front doors have a hard life. They are likely to get pushed open and slammed shut by visitors using their toe-caps, heels, buggy-wheels, bicycles, skate-boards or the sharp corners of boxes and suit-cases. The resulting chips and scratches to the paint-finish are not just unsightly; they can expose the timber to moisture penetration. Moreover, to the familiar natural hazards presented by wind and rain, this summer we now have to add the harmful effect of prolonged exposure to strong sunlight. Your front door needs all the help it can get and paint is its first line of defence.
Wood does not benefit from frequent changes in temperature and humidity. Like most materials it expands when heated and contracts on cooling. Grain direction is important; lengthwise, timbers expand far less than steel when heated but across the grain movement due to temperature changes can be up to ten times greater than the linear movement. The most significant movements in timber result from changes in moisture content but these changes are frequently temperature-related. Warmer air increases the rate of exchange of moisture within the timber. Cells at the surface of a piece of wood will exchange moisture with the air very quickly but core cells will lag significantly – setting up stresses. Absorbing air-borne moisture causes timber to swell and ultimately to split along the grain or to warp.
The paint applied to London’s front doors is therefore not purely decorative; it serves to protect the timber from moisture penetration. The colour pigments in your chosen paint either absorb or reflect the light falling on the door’s surface. A white-painted door reflects most of the light falling on it. A red door absorbs every colour except red – which is then reflected back into your eyes. A black door absorbs all of the light and, since light is energy, that energy is converted into heat and emitted by the door at the infrared level of the spectrum. In simple terms, the surface of a black door exposed to sunlight becomes hotter to the touch than the surface of a white-painted door and as a result is more prone to expansion. If this movement exceeds the elasticity of the paint, cracks will develop and, consequently, movement due to moisture exchanges will accelerate.
If you simply have to have a dark colour then Accoya timber is essential, hardwood simply won’t cut the mustard. Accoya is the most stable timber that we know of and will help keep thermal expansion to a minimum but do bear in mind that with dark colours, you are pushing your door to its limits.
Paint it black, Mick? I don’t think so!