Despite the fact that hard and softwoods have been utilized as flooring mediums for several centuries now, pre-finished engineered flooring is a relatively recent development, and was invented by Kahrs, who in 1941 were granted a patent for the engineered wood floor construction that is now the standard for non-solid wood floor manufacturing all over the world.
Engineered wood is now the most common type of wood flooring used globally, with only the USA having a larger solid wood market than engineered. Even then, engineered wood is quickly catching up in market share.
The construction of all engineered flooring nowadays is essentially composed of the following component layers;
(1) On the surface 4-6 layers of UV hardened acrylic or natural oil are applied onto (2) the hardwood decoractive wear-layer (properly known as the “lamella”), which is typically a sawn piece of hardwood timber. This wear-layer is glued onto (3) a core-board, or substrate, which is commonly composed of either multi-layer plywood, softwood lathes, or HDF (high density fibreboard). The board is then backed with (4) a thin stabilizing layer of pine or spruce, although on Multi-layer boards this backing is generally not necessary.
A ply, or multi-layer board, is composed of multiple layers of thin wood veneer, each laid at 90 degrees to each other. Whilst used by some manufacturers in 14-15mm boards, it is more often found in 21mm thick boards where the top wear layer is 6 or 7mm thick. This construction provides the best dimensional stability and in the case of the 21mm thick boards, is what provides the strength and density that enables these floors to be nailed to joists.
Soft wood Cores
Soft wood cores are used exclusively in 14 to 15mm thick engineered floors that have 4mm wear layers. The core material is composed of a series of softwood laths which are arranged at 90 degrees to the top layers of the board; the crossed grain direction providing the dimensional stability. The board is finished at the back with a thin veneer of softwood.
A variation on the softwood and multi-layer boards is where on the rear-side of the board a final layer of the same timber type and thickness that is used for the wear-layer is applied. Because of the structural symmetry (hence the term “balanced board”) in cross-section, this is thought by many to be the most stable of engineered floors.
HDF, or High Density Fibreboard, is generally used in boards with thinner veneer style wearlayers; it’s high compressive strength being able to provide additional support. You will often find the more exotic wood species available in this construction, as the scope for using a thinner wear-layer (and hence less material) can make what would otherwise be a costly floor in a solid exotic hardwood, comparatively inexpensive.
It should be noted that ALL wood floors are susceptible to the effects of moisture, but that those with an HDF core are particularly hygroscopic and must never be exposed to large amounts of water or very high humidity - the expansion caused from absorbing water combined with the density of the fibreboard, will cause it to lose its form.
Environmentally speaking, although HDF is notable for being made of waste material from the timber industry, and as such does not involve the cutting down of trees, it is not VOC free and is in that respect not environmentally friendly.
Engineered flooring is available in several thicknesses; 12mm, 14-15mm and 21mm. 12mm generally has an HDF (high density fibreboard) core, or substrate, and usually comes in a Click system. The 14 - 15mm and 21mm thicknesses have either softwood or plywood cores. All 3 thicknesses can be either floated or glued, but only the 21mm is load bearing and therefore can also be secret-nailed direct to joists or timber sub-floors.