Sometime last year I was approached by a client who wanted a very unusual staircase for a new house he was having built. It had to consist of a single beam with the treads straddling the beam. To make matters interesting, the beam had to have the shape of a compound bow when seen in plan view. To make matters even more interesting the beam also had to have concave sides when seen in section, wide at the top, narrow at the bottom.
No structural plans were available and the architect had only provided a conceptual plan view and elevation with no details or dimensions. I don’t think his computer could cope with all the compound curves! The architect felt it was impossible to build the staircase in timber. Now if you want my attention, tell me it is impossible to build something in timber! I immediately accepted the challenge and told the prospective client in a diplomatic way that the question of possibility depended only on the size of his wallet! This however, did not seem to be an issue.
And so with my big mouth I quoted a price, accepted a deposit and tied myself to a delivery date. The agreement even included a penalty clause! Thinking back, I must have sounded very confident to the client. Or something. No drawings, a hefty deposit, me telling him to trust me with the design of form and structure, construction and installation, all in the face of a respected architect that said it was not possible. In my favor was the fact that I had built a few pieces o custom furniture for the client before. He knew that I could work wood.
Gluing the laminates.
The time came where I had to put my money where my mouth is. I laid out the plan view shape as I saw it on some hardboard, then mocked it up in the house to get a feel for where I was going. The client approved of the shape. I took the hardboard pattern back to the shop and built a laminating form out of 19mm (3/4”) plywood. The total span or length of the beam was to be around 4,5m (15’). Using my very scientific thumb suck approach, I figured a beam of 250mm (10”) x around 140mm (5, 5”) would do the trick That meant 15 laminations of 9mm(3/8”)exterior Pine plywood. I decided to use resorcinol glue for the job. It is widely used in industry for structural bonding i.e. lam beams, plywood etc and is very good for laminating stressed beams. Also I’ve had much experience with it, building masts when I was boat builder.
The completed beam prior to removal
It took a while to get all the laminates done. With my luck, the job happened in winter and resorcinol glues needs at least 20 degrees Celsius to cure properly. I struggled to maintain temperature in my shop and had to wait for warmer days. Not having enough clamps, I used a combination of temporary screws and all the clamps I had to get enough pressure on the glue joints. Resorcinol like a lot of pressure and ideally glue lines no thicker than 0.25mm.
Free from the jig!
The day came where I could finally take the beam off the jig and clean up the glue lines. I had a lot of time whilst gluing up to think about the best way to get the curved sides done and finally opted for a technique we used in boatbuilding. For all practical purposes the laminated staircase beam now became a boat. Shaped ribs were screwed and glued to each side of the beam at 100mm (4”) intervals. The ribs were then connected by beveled stringers top and bottom. I had to incorporate the landings for the staircase treads into this framework system and the curved sides made this quite complicated. There was also the heavily pondered question of maintaining horizontal levels on the landings of what is essentially a warped spiral.
. Ribs and landings fitted
Much head scratching and measuring and calculating was done to get the rise and horizontals and angles and bevels and curves and twists to work together. At one stage I started doubting the whole lot and thought it would never work. I closed the shop door and took my bike for ride to clear my head! The next day I went straight to the site to stare at the hole I had to fill and rechecked all my dimensions. At the start of the job I had made an angled steel bracket for attaching the foot of the beam to the floor. After pulling a tape measure in various directions I marked X on the floor and fastened the bracket to the concrete floor. This was now screeded over by the builders and just the vertical plate remained, angled in two dimensions. It had better work out!
. A close up shot of ribs and stringers
. Construction of the landings
Back in the shop, I started cladding the “boat’s” frame with 3mm (1/8”) plywood. I cut the ply into strips 150 mm wide and glued them onto the ribs diagonally so they could take the curve. The strips were temporarily held in place with staples and wooden pads. When the glue had cured the pads and staples were removed. Another layer of 3mm plywood followed, this time running 90 degrees to the first layer. The final layer was 3mm Poplar veneer, again in the opposite direction. I used a boatbuilding epoxy for gluing the layers. Epoxy has good gap filling properties, requires no pressure, and dries clear.
. The first layer
. The final poplar veneer layer
I was getting somewhere! Now followed much shaping to get the landings flat, followed by even more sanding to make everything sweet. A final job was the veneering of the top and bottom surfaces which was relatively straight forward. It remained to build a cradle for transport to the jobsite. The whole contraption was manhandled out of the shop and onto a trailer and at the end of the day it was lying in the house, ready to be hoisted into place.
. Cutting the slot for the bottom bracket
. Ready to leave the shop
. Will it fit?
The day of final judgment arrived. Armed with block and tackle, common sense, lots of rope and lengths of wood, the games began. Nervous moments, some not so delicate removal of material (think of a chainsaw here), sweaty T-shirt, no time to take photographs, temporary supports, the beam was in place! I slept well that night. The next day’s concern was if the beam would be rigid enough. The proof of the pudding is always in the eating! Bolts at the bottom, steel brackets and some more cabinetmaking with the chainsaw at the top and the temporary supports could be removed. My back up plan was a hidden steel bracket at the middle of the span. This could tie the beam to the adjacent wall if need be. I walked up and down like a cat and it felt good. Then I walked up and down like a human being and it still felt good. Then I stood in the middle and jumped up and down and there was just a little flex. All was well, no need for that central bracket. Phew!!
. The impossible staircase installed
The treads were made from Poplar, 50mm(2”) thick, tapering to 20mm(3/4”)at the ends to lighten the look and give the feel of wings. Positioning the treads took a while. I opted to do it by eye rather than measurement which would have been too complicated. An old boat building saying goes: “If it looks right, it is.” I attached the treads to the landings with coach screws which were plugged over. A bit of trim here and there was still required. Final sanding and varnishing and that was it, the job was done!
. Looking up the staircase
Off course, in retrospect, I would have done things a little differently but that’s how it goes with custom projects. The staircase was delivered on time, it works well and the client is extremely happy. I just wish I could have seen the architect’s face!
-- Div @ the bottom end of Africa. "A woodworker's sharpest tool should be his mind."