Four flights, continuous geometric.
Project status: Survey stage.
Parallel flights with 180º winders and quarter space landings at each end.
The project is to enclose the steel frame staircase, to give the impression of a full timber flight.
This will include manufacture of the cut strings, including drums for the winders. Solid timber treads, ply risers for painting and the fake wall strings, that flow into the skirting. Many of the surrounding properties from the Edwardian period. These flights are to reflect the style used during that period.
The steel frame had already been installed with an allowance made for the balustrading to be set over the steel frame. This then required the timber components to be designed around the existing steel frame and balustrading.
This means that the steel frame dictates the position of the stringer faces, the size of the stairwell that is left between the flights and the flow of the handrail.
The better way to do this is to design the handrail flow first, once you have this you can determine the stringer faces and work from there to get the layout off the steel staircase allowing tolerance to maintain a constant rise and going of the finished timber cladding.
The stringers will be continuous geometric cut strings, with drums through the winder area and plaster bead detail to the bottom.
The bottom detail for the stringers is to allow the plaster soffit to finish level with the bottom of the stringer, for this project it was decided to maintain a 3mm – 1/8″ shadow gap between plaster and stringer bead detail.
The bottom rebate behind the plaster bead will vary between the straight and drum sections.
The straight sections will have 2 layers of 12mm 1/2″ plasterboard with a skim coat, therefore the rebate will be approx 28mm 1 1/8″ to allow for this make up.
In the 3D curved sections, there will be a rebate depth of 20mm 3/4″ to allow for EML as the base to bond to with up to approxamatly 12mm 1/2″ of bonding and a finish coat.
N.B. the top of the rebate may need chamfering and not stay at 90º the faces of the stringers, to allow for the angle that the EML. may have to come into the rebate.
On the straight sections plaster stop bead may be applied to the plasterboard edges to assist in creating the shadow gap, through the curved sections this is normally scratched in by hand.
Dependant on contractor, the 2D curved sections of straight stringer may also be built up with EML others will score the back of the plasterboard and use this as kerfs to allow the plasterboard to bend around the 2 dimensional curves.
Stringer riser connection.
With the stairs being cut string in design, the risers are to be mitred into the stringers.
Due to the fact we had two different contractors working on these stairs. A flooring contractor for the treads and risers, and a staircase specialist for the stringers. Also due to the fact that we are fitting on site and many of the joints that are easier to create in a workshop will have to be fitted in situ.
It was decided to assist in this that the arris to the riser stringer joint would be increase closer to a pencil round, thus giving a larger margin for adjustment. this is also assisted with the fact that the stringers and risers are both for painting.
This adjustment also allows the machining of the return mitres into the risers from the stringer faces to have slight flat rather than a pointed end, help remove the chance of breakout during the manufacture of the stringers.
Tread and scotia.
The treads and scotia are to have a traditional look to them.
Due to the fact that the steel subframe to the stairs had already manufacture and fitted, it did not allow enough room for the treads to be of a standard thickness for that period, therefore we went with a 20mm tread. This had a ply under tread that could be drilled and screwed to the steel sub frame and the solid Oak tread cover could be adhered to that under tread. this means that a good fixing between the steel and timber cover could be achieved.
It is worth giving consideration to this, as steel will expand in heat and timber is likely to shrink as it dries, although timber is dried before fitting it is still very likely to shrink in its first year of being fitted. Ply will not move as mush as natural timber and creates a good barrier between the steel and timber.
Having the ply under the tread cover also gives room for any adjustment in the leveling of the steel stairs, the finished product must have a consistend level, rise and going. leveling the ply under tread means the glue faces between ply and underside of tread should be in full contact, thus getting a full strength bond between them.
Survey of steel work.
While taking some initial dimensions and checking the levels of these stairs, it became apparent that the stairs had not been installed correctly. not only could we not find any level surface
The issues we dicovered during the survey stage consisted of:
- No level tread surface.
- Rises not perpedicular to walls or aligned with each other.
- Inconsistent rises and goings through each flight.
- Steel to high to allow for proposed coverings.
- Flights made to incorrect ffl.
For the cladding to work these issues will have to be corrected in the layer between steel and finished coverings.
The scan data collected using a microscribe arm with the surfaces to repesent the metal stairs being built from the scan data.
During the survey stage a number of extra stages were included into the scan, This was for my peace of mind while producing the drawings.
I transfered the ffl onto the bottom riser of each flight. I then ran the maths to calculate the correct rises for each flight. I then marked the walls with the correct finished tread levels, with a laser level I projected these heights over thier respective tread positions. I could then measure from the laser line to the steel tread in each corner of the tread and get a measurement for each corner.
This gave me the information I needed to confirm against the digital scan.
With the digital scan completed I could build the surfaces to represent the rises and goings. Then check the dimensions against those taken on site.
Once the surfaces for the rises and goings have been added to the scan data I extract a 2D plan of the stairs, this will show how far out the substructure is and you can start calculating the space required for correcting the issues.
Here you can see the plans in black with the grid set over showing lines at 90º to the centre line of the stairs. the treads should be parralel to the grid lines. The steel riser faces should be set the distance from finished riser face material to steel riser face either side of the grid lines, so the finished faces are in line.
The side elevation showed the same discrepencies from level for each tread, as the plans do for perpendicular to the walls.
The next stage is to adjust the plans and elevations to get constant rises and goings while allowing enough space to be able to correct the out of square and level issues.
Packing the treads and risers.
With the steel subframe requiring additional work to get correct levels to work to, amendments had to be made to allow for packing between the finished tread and riser material. The packing had to be adjusted from back to front and left to right, as no surface was constant in any direction.
To keep this as thin as possible we allowed for 9mm ply sub treads and risers, to adhere the face finish treads and risers to. Here you can see the way we allowed for this as opposed to the proposed lay out mentioned above.(Tread and scotia makeup)
In this image you can also see a proposed mounting bracket for the balustrade, by fixing this at this stage the 16mm 5/8″ Ø metal spindles can be dropped into these sockets after the treads have been fitted, this also means the soffits can be plastered in before the oak tread covers go on, prevent the chance of plaster coming into contact with the Oak and burning it.
Stages to be completed.
Drawing of the components.
Production of the components.