Like for all glued solid wood floors, the subfloor requires high-quality workmanship. The surface must be dry and protected from humidity seeping in from below. The construction must be stable and not give way under the weight. The surface must have a good adhesion to the chosen glue, and minimum tenacity of 2.0 MPa to resist movement caused by changing humidity in the block. Remove dust and dirt from the subfloor to improve adhesion. The following is a description for a couple of subfloor types commonly used in new production and existing buildings.
For concrete surfaces, it is always important to control the humidity of the concrete, especially in new production; core sample should have relative humidity below 85%. If floor heating will be installed in the concrete slab, a humidity barrier must be installed first. If the humidity is on the upper end of the acceptable ratio it is important to select a hard floor glue intended for high humidity.
In these days additives are often used in concrete. Some of these additives can react with certain glues. If the floor will be glued directly on the concrete slab it may be necessary to first treat the surface with a primer. Consult with the supplier for the glue which primer is the most suitable.
The surface should also be smooth with a minimum tensile strength of 2.0 MPa. If screed will be applied on the floor, a screed/primer meeting the requirements should be selected. For direct gluing on concrete, the surface may need to be sanded smooth with an ABS sander, and the tenacity controlled.
Particle board on a joist frame
A subfloor constructed with wooden joists and a tongue-and-groove particle board fastened with screws is a good support for a block flooring. The construction is stable and the surface provides a good attachment for the glue. Risk for incoming humidity can easily be prevented.Humidity ratio of the particle board may not exceed 12%. It is not necessary to prime the surface before the glue is applied.
Assembly on existing floor
Begin with assessing whether the floor surface needs a strengthening construction or whether the surface is very uneven needing leveling. Control also the humidity ratio of the construction and investigate whether there is a risk for humidity seeping in. If the construction is stable, even and dry, a plywood or particle board can be fastened with screws on the floor as a base for the glue. A floor particle board has the advantage of making the construction more rigid. Old floor boards must be sanded before the assembly.
Alternatively, add a floating floor of tongue-and-groove particle boards that are glued together at the edges . If there is a risk for humidity add first a layer of age-proof plastic with overlapping taped edges.
Lay out blocks on the floor to cover a small area. For example, 20 blocks lengthwise and 12 blocks widthwise. Adjust the joint gaps until satisfied with the esthetics. The gaps should be between 0.5 and 1.5 mm wide. By adjusting the joint gap you will ensure to end up with an even number of blocks from wall to wall. Measure for example the total length of 20 blocks and total width of 12 blocks, and record these values.
Cover an area with loose blocks and adjust the joint gap to the desired appearance. Measure for example a length of 20 blocks including the joints and 12 rows including the joints, and use these values as your guiding lines.
Next, draw a grid on the floor; in this example, each square contains 20×12 blocks including the joints. Select a zero-point located a few block rows (about 3 to 4 rows) away from each wall in a corner. Next, draw a line starting from the zero-point to be used to lay out the blocks. This is the start line. Then, draw another line starting from the zero-point, perpendicular to the start line. Now, draw the lines that should end the installation so that you end up with a square with right angles drawn slightly away from the walls. Last, draw the grid with squares for 20×12 blocks each. If the room has an irregular layout you can simply extend the grid where necessary. The space between the square and the wall is installed last because walls are not always straight and parallel.
1. Establish a zero-point and draw a start line a few block rows away from the wall.
2. Draw a rectangle with right angles starting from the start line
3. Draw a grid inside the rectangle where one square is equal to an even number of blocks + joints. In this example, 20×12 blocks per square.
4 Extend the grid for nooks, if necessary.
Spread out glue with a notched trowel along the start line for 3 to 4 rows of blocks in one or two squares. Use a cross laser for guide. Spread out only the amount of glue that you can work on during its handling time – the time it takes for the glue surface to cure. The handling time varies based on the glue type, temperature and humidity ratio but is usually about 20 minutes. Each block should be pressed down rmly so that the glue gets contact with the entire block surface. The block can then be slightly adjusted sideways; make sure that no glue is pressed up between the blocks. Remove excess glue with a putty knife after each gluing step. Continue adding blocks and ensure that they align along the drawn guiding lines. When the entire grid surface is completed continue the installation along the walls. A block can be easily cut with a straight edged knife (such as a wood splitting knife) and a hammer, if necessary; lay the knife blade where you want to cut the block and use the hammer to tap the back of the knife blade. Select blocks without knots for cutting.
Spread an appropriate amount of glue and lay out 3 to 4 rows at a time. Use a cross laser as a guide for the installation. Scrape o any excess glue after each gluing step. The block should be pressed firmly in to the glue.
Let the glue to cure. The cure time required depends on the type of glue used, temperature and ambient humidity, and usually takes about 24 hours. The surface should be sanded with an orbital sander, not a band sander. We recommend Bona Flexisand with Powerdrive or similar. Begin with a 40-grain sandpaper, and continue then with 60-grain and 80-grain sandpapers. Change sandpaper often for even sanding result. Use a corner sander with similar type of sandpaper for corners and edges. If the floor requires filling of the joints, do it now (see below), or move to the final sanding using 100-grain or 120-grain sandpaper. If the floor will be treated with pigmented oil, maximum of 100-grain, or even 80-grain, sandpaper is recommended to allow the pigment to penetrate deeper in the wood.
Filling the joints
A block floor can have open joints but if a more uniform appearance and an easy-to-clean floor surface is desired the joints can be filled with a filler. A filler is a sealant that is pressed into the joints using a spackle. Some fillers are premixes in different colors, and there are also do-it-yourself adhesives that are mixed with grinder dust. We usually recommend Bona Mix & Fill, which is an example of the latter. The advantage of mixing it yourself is the possibility to create customized shades of colors by adding pigment to the grinder dust.
The floor joints need to be filled several times because the filler sinks and cracks when it dries up. During the first round of filling it is important to get the filler down as deep as possible in the joints so that it does not fall out when the floor is sanded. The filler material goes deeper if you draw the spackle along the joint rather that across it. Move the spackle crosswise over all joint directions. Let the filler cure for a few hours, and repeat the filling procedure to fill up the joints again. Repeat this step until you see no more cracks. Let then the joint filler to cure at least for 12 hours before the surface is sanded clear off all excess filler material.
Adding pigmentation and surface pigmenting
A block floor can be finished with either oil or hard wax oil, or a combination of the two. Oil provides a deep protection because the end grain absorbs the oil very effectively. It creates a beautiful and natural surface and retains the feeling of wood in the floor. Hard wax oil does not penetrate as deep as oil but creates a protective surface against wear and tear, dirt and water. By first finishing with oil and then adding hard wax oil will provide the best of the two worlds. The key factor for finishing with oil is to saturate the floor completely. Add more oil until no more is absorbed into the wood.
If color is wished, a pigmented oil can be used for the floor. Pigmented hard wax oils are also available but because this pigmentation does not penetrate deep there is a risk for the color to be worn away in heavily trafficked environments. A combination of a pigmented oil followed by a clear hard wax oil works best in the majority of the environments.
End grain absorbs large amounts of oil and pigment A rough floor absorbs more than a finely sanded floor. To determine the correct color shade, colors should be tested on a test surface that has been sanded exactly like the sanded floor. Test different mixtures of pigmented and unpigmented oil, saturate a test piece with the oil mix and let dry a few hours before assessing the result.
Once the oil has been applied, wipe of excess oil and possible excess pigment. Polish the surface with a rondell tool to re-expose the wooden texture. In case the surface needs post-finishing with hard wax oil, it is important to use a product that is compatible with the oil.
An example of pigmented Centerpiece Basic Spruce