Sprung Wood Dance Floor

I’ve been involved in Traditional American Dancing for over 30 years and have thoroughly enjoyed dancing on historic “sprung” wood floors that actually have some “give” when dancing. I recently got the opportunity to build one of these floors using techniques that haven’t been used in 50 years. The floor came out great with a heavy learning curve.

As a founding member of the local Hackerspace, Xerocraft, I build and fabricate a lot of things, but a complete wood floor is one of the biggest jobs I have taken on. In this Instructable I will go through the steps, materials and measurements that I used.

Modern wood products are not what they used to be so I had to work around a lot of material issues. I used conventional Tongue and Groove (T&G) Oak flooring on a ¾” Tongue and Groove plywood subfloor supported on a flexible lattice of “sleepers” to provide the floor’s “spring”

Step 1: Design for the Space.

Our floor was being laid in on a newly poured concrete slab on compacted fill above the existing grade. The extension of the house was built on the “downhill” side of the house and room had been built with a 20” retaining wall on the lowest side to allow for a level floor. I had the general contractor leave me 3 inches of depth from the concrete slab to all exterior thresholds to accommodate our floor.

We chose the two longest sides to give us our first corner for control with a true 90 degree angle. Wood floors require a moderately dry sub-floor so we had to wait for the water of hydration to absorb into the concrete as it set. We checked the slab with a moisture meter and didn’t start laying material until we had the moisture below 8%. In the desert environment this took several months with average daytime humidity levels below 15%.

The house this floor was installed in has a newly installed Air Conditioning system that will keep the room at a uniform humidity level. This will minimize the expansion and contraction of the wood floor. Since all wood floors will expand and contract with major changes in humidity, it is best to design for maximum flexibility before installation. In the desert southwest, it would not be advisable to install a conventional hardwood floor in a building with evaporative cooling and expect the floor to not experience buckling or severe gapping. Many of the older homes in Central Tucson were built prior to 1940 using the old floor installation techniques of hardwood tongue and groove over diagonal 1x10” subfloor on pier and beam supports over a basement.

We used a self-leveling laser that projected a level fan to determine how level the original slab was. This was necessary since the highest point on the slab is normally a final control elevation. In our case, there was a small hump in the middle where a fireplace had been that was 1 ½” higher than the newly installed base elevation. We choose to modify our elevation plan to work around the hump rather than try to grind the hump down. This was done by aligning the lattice work in such a manner as to not have any bottom sleepers project across the hump. All sleepers were on an 18” grid and the lump was less than 18” wide.

Step 2: Develop the Elevation Plan.

I built a 2’ x 2’ mockup of our proposed floor construction layers for use at the thresholds and then used it to determine the available space for adjustment. The system I built uses three layers of sleepers. The middle layer has to be uniform with equal thickness in all springs.

For the initial planning, an elevation map of the existing concrete base floor was created using a laser level. I first chalked in contour lines at the highest and lowest points of the concrete floor, I then drew in 1/4” elevation contours. This told me where the floor needed to be thicker and thinner to make the final elevation. Concrete floors are seldom perfectly level. In our case we had a rise in the center of 1 ¾” from the base level across the entire west end of the room. I marked the elevations on the concrete itself on an 18” grid and then drew in the contours to define my sublevel map. The grid defines my final-floor, elevation control points. The problem was an old hearth from a fireplace that had been incorporated into the old half of the concrete floor and the contractor had matched to the existing levels to create an appearance of a smooth floor flowing out into his new expansion from the old room.

I couldn’t find any good measurements for a sprung wood
support so I was forced to build a large mockup of a piece of floor that I could test for loading. The wood that would have been used in 1950 was FAS Spruce. The rating “First And Second” (FAS) that was used then meant no knots and clear wood. Today’s FAS is actually considered Builder’s Grade and has a lot of knots.

The wood being used needed to be flexible and relatively knot-free. I was able to locate a better grade of Spruce that is sold for the Truss Building industry and can be purchased without knots in lengths of up to twenty feet. This was a 2x4” Spruce out of Canada that is Machine Stress Rated (MSR) for use in the Truss building industry. This lumber is rated for 2100 psi bending (Fb) strength. It is the closest you can come to the old quality of wood without buying “Old Growth Timber”. This is referred to as MSR 2100

Step 3: Materials Preparation

The list of materials was
generated and our sizes were calculated for sleeper thicknesses. The sleeper material was MSR Spruce purchased in nominal 2”x4” lumber. This material was used by cutting the material lengthwise on a cabinet saw to achieve our necessary list of thicknesses. The first cut was a two inch (2”) deep cut lengthwise down the middle of the 1 ½” thick edge. After the removal of the saw kerf, the resulting cut left two pieces with a thickness of a little over 5/8” (closer to 11/16”) that will be the standard sprung sleeper thickness throughout. The second cut was lengthwise across the 1 ½” thickness to remove the two sprung sleeper we just created at a width of exactly 2”

The remaining piece of wood is 1 ½” by something less than 1 ½” due to the kerf width of the saw blade. This wood is then cut lengthwise into various thicknesses of top and bottom sleepers as determined by the thickness layout to provide a final, level floor. I had some sleepers as thick as ¾” and the thinnest sleepers were 1/4” to achieve a level finish floor. All of the top and bottom sleepers are a standard 1 ½” wide.

When setting up to cut the sleepers, be prepared to have a well-oiled saw and a new blade. I used the shop cabinet saw rather than an onsite construction table saw since this gives a more precise cut and is designed to go through a lot more material before maintenance service and cleaning. I was thoroughly pleased with the job of the Freud Diablo 80-tooth blade. Cutting this much material definitely created a lot of sawdust from the saw. Although the wood is relatively dry, it needs to be bound into straight bundles and moved onto the job site as quickly as possible for use.

Step 4: Installing the Wood Lattice Spring Structure

The new floor has five layers that contribute to the final elevation and the “springiness” of the finished floor. The first three are the Spring mechanism to allow the floor to “give”, followed by a sheet plywood nailing floor and then the actual hardwood planking of the final floor.

Layer 1: The Ramset Sleepers –These are the first of the sleepers to go down on the concrete and are set into place with powder activated “Ramset” nails to nail them deeply into the concrete. Their thickness can be adjusted since they are designed to maintain a clear space for the Sprung sleeper to flex into. Due to the fact that this was a new concrete sub-floor, we also used a polyethylene vapor barrier between the concrete and the wood lattice. The slab will continue to entrain it’s moisture over time and there will be a tendency for outside moisture to migrate into the soil within the foundation so we don’t want any future moisture being able to migrate up into the wood lattice work. In wetter environments, this could present a major problem without venting of the subsoil.

Layer 2: The Sprung Sleepers –These are the actual “springs” of the flooring system that will flex as the floor rises and falls under load. These are all 5/8ths of an inch thick by 2 inches wide. All of these sleepers are identical in thickness to give a uniform flex throughout the floor. These sleepers “float” without being anchored or pinned at any point. I used small alignment blocks fastened to the bottom “Ramset” sleeper to keep the Sprung Sleepers from migrating out of alignment.

When the three layers are completed, the load of the floor above is only supported on one or two of the three layers at any one point.

Where two Sprung Sleepers were matched for a longer run, sliding alignment blocks were mounted to the bottom of the Sprung Sleeper one Ramset sleeper back from the join to prevent the joint from sliding open. The sliding blocks are attached with an extremely strong adhesive and then stapled into place to hold the pieces secure until the adhesive has time to completely set.

For the outside ends of the Sprung Sleepers, the sliding block mounts were used to stabilize the floating, middle layer so the spring boards would not migrate out of alignment, yet still had the ability to move and flex as the floor above is put to use.

Step 5: Finishing the Lattice and Sub Flooring

Layer 3: The Top Sleepers –These are the sleepers that sit atop the Sprung sleepers and create the clear space for the Sprung sleepers to flex into on the top side. These are stapled into place with the sub-floor sheeting that goes on above them. The Top sleepers can also have an adjusted thickness to level the final floor.

Layer 4: The Sub Floor –This is a conventional, ¾” Tongue and Groove (T&G) plywood sheeting that is laid in an alternating pattern/ The length of the plywood runs at a right angle to the length of the Top Sleepers. In addition, the four foot “butt” joint of the plywood is nailed down the center of a Top Stringer. This layer is a uniform ¾” thick throughout. ¾” T&G plywood only has a tongue and groove down the two long sides and are square cut on the 4’ ends. Unlike conventional plywood, subfloor consists of 5 or 7 layers that have each layer plugged and filled to prevent void spaces inside the laminate. Regular plywood only finishes the top and bottom layers for visual effect and two overlapping voids inside the sheet can cause failure or massive squeaks when used as flooring.

Layer 5: The Hardwood Floor - This is the final floor that will be the dance surface. This was a mixture of Red Oak and White Oak Tongue and Groove (T&G) flooring with an unfinished ¾” thickness before sanding. We were also using mixed widths of 2 ¼” and 3 ¼” planks. This layer is also a uniform ¾” thick throughout. In order to minimize floor noise (squeaking) we laid in a layer of 80 pound asphalt felt paper before installing the hardwood flooring.

Step 6: Installing the Hardwood Flooring

Layer 5: The Hardwood Floor - This is the final floor that will be the dance surface. This was a mixture of Red Oak and White Oak Tongue and Groove (T&G) flooring with an unfinished ¾” thickness before sanding. We were also using mixed widths of 2 ¼” and 3 ¼” planks. This layer is also a uniform ¾” thick throughout. In order to minimize floor noise (squeaking) we laid in a layer of 80 pound asphalt felt paper before installing the hardwood flooring.

Both the ¾” plywood subfloor and the Oak flooring itself is installed in a manner that leaves a clean ¾” gap along all walls. This serves the purpose of allowing the floor the room to expand without buckling and it also provides a channel to a clear space below the tight floor structure where we could drop audio and multimedia cables for use with amplified musical instruments or professional recording sessions.

In our project, there were five door openings into the room. Each one required different wood fittings to match to the adjacent floor surface. This involved design and construction of custom pieces to provide lintel transitions. The kitchen had a 3” drop to linoleum, the bathroom had an 11” drop with one step, the laundry room was close with only a 3/8th” drop and the two exterior doorways had an elevated threshold above the floor. All door openings had the subfloor to concrete gap stiffened at the threshold to stop any flex at the threshold itself.

Step 7: Floor Finishing

Final finishing on the floor consisted of a dark wood filler to fill most of the visible blemishes and significantly noticeable holes in the highly figured oak planking. After the blemish filling, we troweled on a very thin layer of a “full-trowel” filler like ZAR, a commercial, latex-based floor filler that is oak colored, easy to sand and will take a stain. ZAR can be thinned with water and dries quickly. This was our final crack and joint sealer and was used for thin cracks, nail holes and pinholes. This dried to a color that matched the flooring very well and is only visible in close-up inspection.

Once the final planking had been installed, we troweled on the crack and joint filler and then started the floor finishing with a drum sander. Our first pass was with a 36 grit coarse cut sanding belt cutting the floor in a diagonal pattern to take out any slight elevation differences in adjacent hardwood pieces. At this time we were able to pick out any major errors in floor elevation due to faulty wood. This also took off the excess spot filler used on the pinholes and figuring. This is the point where we are getting ready to finalize the job and a critical eye is needed to identify any irregularities and fix them before they become a permanent blemish.

In addition to the drum sander, we used a heavy rotary sander as a floor “edger” to work up close to the walls and we used a small orbital sander to work into the corners that a 10” rotary sander wouldn’t reach. Notice that all sanding and finishing was done prior to any baseboards being installed. The second drum sanding was done with 80 grit paper and ran parallel to the hardwood boards to take out any cross-cut marks.

After sanding, the floor was screened with a 150 Grit abrasive screen on a large rotary floor buffer to take out final marks and prepare the floor for our penetrating sealer.

Step 8: Adjust Irregularities Before Finishing

Only one board was found to be too far out of spec and had to be removed and replaced. This is done by matching a piece and removing the bottom tang of both groove sides. The new piece was top-nailed in using the same technique as the last three rows against the South wall where the pneumatic floor-nailer did not have room to operate. As with the last three rows, the nail heads were driven below the floor and then filled above with the ZAR. It is always better to make replacement before the floor gets it’s final finishing.

Step 9: Floor Sealing

Once the floor sanding had been finished, we applied a penetrating oil sealer. The sanding used a rented drum sander and our first cut was with a coarse 36 grit and then with an 80 grit and finished up with a 150 grit screener. After sanding, we had to do an extensive cleanup of vacuuming with a canister Shop-vac following each run of the sander to keep the belts from clogging too badly. We then had to hand sand the corners and edges and then re-vacuum the entire room with a HEPA filter equipped vacuum to trap any of the fine dust that would normally pass through a filter bag.

Once the floor was prepared, we cleaned the floor ahead of our finishing area with mineral spirits and tack rags and then we started to apply our penetrating oil sealer. The product we used was a high-VOC solvent based sealer from Dura Seal. The nice thing about penetrating oil sealers is that you are dancing on the wood itself and not a plastic layer that will scuff and gum and cloud up. We used the natural finish without dyes or colorants so the color of the wood can come through. In addition, any touch-up needed in coming years can be applied directly and will blend in perfectly with the old finish.

We only sanded to a 80 grit and then screened with a 150 grit so the floor will have a bit of “tooth” to it after finishing but the wood will wear in and develop a smoother “rubbed oil” effect over time. As this picture shows, the finish really pulls up the color of the mixed woods and gives a great feel to the whole room.

Step 10: Baseboards

After the floor was finished, we had a fairly large amount
of hardwood available, so I made a baseboard structure that would adequately cover our ¾” wall gap, ride above the floor when the floor flexed downward, and could be easily removed to alter or change the wiring placed below the floor in the wall gap. I used pieces of highly figured, 3 ¼” floorboard as the back piece and ripped a piece of 2 ¼” for the front piece. Both pieces were bull-nosed on a shaper to give an Art Deco look. The front pieces were nailed into the back pieces with a pin-nailer and the back piece was mounted to the wall using narrow-head, Torx driven cabinet screws. There is an overlap of wide and narrow boards to give structural integrity to each wall run since we did not have any long pieces left at this point in the project.

Step 11: Project Specifications

The room was approximately 750 square feet with a 20’ width at the East end and a 24’ width at the West end. The total length of the room was 33 feet. The floor was finished in August of 2013 and has now had a year of use. The owner is hosting a number of musical jam sessions and dances of various sorts. The floor has a downward deflection of up to 1/4” and the deflection is spread over an 8’ circle around four dancers.

With the use of a well-planned elevation map and a good cut schedule on our wood I was able to complete the job with a level floor that is less than 1/8th” out of perfectly level across the entire floor. The “give” in the floor is such that a single person jumping rope on one spot can have movement of the floor six feet away and can be felt as movement up to 10’ away. This is a perfect level of spring for dancing.

List of materials that were used.

MSR Spruce

10 mil polyethylene vapor barrier

80 pound black felt paper

¾” Tongue and Groove plywood sub-flooring in 4x8 sheets

¾” x 2 ¼” or 3 ¼:” wide Tongue and Groove oak flooring

DuraSeal penetrating Oil Finish

ZAR latex floor filler

¼” crown staples for non-weakening attachment

Liquid Nails construction adhesive for sliding block stops

2” wide 20 /mil PVC waterproofing tape to seal polyethylene vapor barrier