Chapter 21 Stairs.
The landscape also presents situations where a slope needs to be navigated with stairs even though no wall is planned. Such instances require the construction of freestanding stairs. Because the side slopes are so gradual, freestanding stairs are constructed without the benefit of retaining or cheek walls. Stairs in this situation are typically constructed with riser/tread dimensions that match the slope rather than matching the ideal mathematical relationships for steps. This chapter also outlines the methods of planning and installing various types of materials used for freestanding stairs.
By reading and practicing the techniques described in this chapter, the reader should be able to successfully complete the following activities:
* Identify appropriate stair style for a project.
* Install stairs that abut a retaining wall.
* Install stairs that interlock with a retaining wall.
* Install freestanding stairs.
RELATED INFORMATION IN OTHER CHAPTERS
Information provided in this chapter is supplemented by instructions provided elsewhere in this text. Before undertaking activities described in this chapter, read the related information in the following chapters:
* Construction math, Chapter 4
* Safety in the workplace, Chapter 6
* Basic construction techniques, Chapter 7
PLANNING THE PROJECT
The range of construction difficulty for stairs can be quite wide depending on the materials used and slope on which the stairs are being constructed. Prior to beginning the project, develop a plan or materials desired and the number of risers and treads.
Local Codes Governing Stairs
Before constructing stairs as part of a landscape, check with local building officials to verify stair requirements. Many communities have regulations on stair tread and riser dimensions, railing requirements, stair widths, and other stair-related attributes. Access up/down grades may also have to be accompanied by a ramp as well as stairs. Check with building officials for rules or requirements that might supersede techniques described here.
Types of Stairs
Stairs can be constructed as part of wall systems (Figure 21-1), or be independent of walls as freestanding stairs traveling up a gradual slope (Figure 21-2).
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In certain cases, stairs may be replaced by gradual sloping ramps. The type of construction selected depends on the steepness of the slope, the distance covered, and the mobility of the user. Whenever possible, select a ramped walkway over stairs because of the ease of construction and the diversity of audiences that ramps will accommodate. Building codes and access regulations may also require installation of ramps rather than stairs. When grades begin to exceed 5% longitudinal (over 5 feet of fall in 100 feet end to end), consider stairs as part of the project. If the slope to be traversed does not require a wall, incorporate freestanding steps to assist the client over the slope.
Stairs constucted as part of a wall require that cheek walls be built on either side of the area where stairs are desired. Cheek walls are short walls that run into the hillside the stairs traverse. Cheek walls are perpendicular to the retaining wall, should be the same height as the retaining wall, and should extend into the hillside at least as far as the stairs will extend. Timing of cheek wall construction depends on whether stairs will be interlocked with the cheek wall or built independently and butt into cheek walls.
Material Selection for Stairs Built as Part of a Wall
A preferred choice for stair materials is to use the same materials as for wall construction. This provides continuity in design and dimension from the wall into the stairs. In some cases, the same material may not be possible or desirable. One popular choice for stairs is cast-in-place concrete. This material, when properly formed, can be set at any riser and tread dimensions and can be poured to fit snugly with any cheek walls. Forming for stairs is covered in Chapter 23. Freestanding steps can be constructed of any of the wall materials just identified, as well as from cast-in-place or precast concrete.
Material Selection for Freestanding Stairs
Freestanding stairs can be built out of almost any material. To organize the presentation of choices and installation, materials are grouped into four categories as follows:
* Concrete stairs. Concrete can be formed with any riser height and tread depth and can be built without the benefit of retaining walls alongside (Figure 21-3) See Chapter 23 for forming and paring concrete stairs.
* Tie/timber framing with treads of concrete, brick, or loose material. Stairs along a hillside can be framed on the front and two sides with ties, timbers, or heavy-dimensioned lumber with the tread portion surfaced with a variety of materials. The front portion for each higher step rests on the sides of the lower step. For informal stairs, a tie or timber can be placed across the front of each step with the tread behind left untreated or covered with loose fill (Figure 21-4).
* Stacked slab materials. For informal stairs, large slab-like materials can be placed level in the ground and serve as both a riser and a tread. Each subsequent step rests on the back edge of the lower step. Materials that might be used for this type of step include precast concrete, large, flat stones, tree sections, or other materials with thickness and large dimensions (Figure 21-5).
* Stone or segmental precast concrete wall units. Both of these wall materials can be interlocked or butted against cheek walls to create stairs. In some precast units, the caps are necessary to cover hollow voids in the blocks.
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Riser/Tread Calculations for Freestanding Stairs
Unlike stairs that are constructed as part of a deck or wall, freestanding steps are adjusted to match the slope that is being covered. If standard riser/tread dimensions are to be obtained, it will either be a fortunate coincidence that the slope matched the mathematical formula for riser/tread, or landings and/or cheek walls will have to be added to accommodate the mismatch length. To determine the number of risers and treads in a set of freestanding stairs, use the following abbreviated formula:
* Measure the length and height of the slope.
* Convert the measurements to inches.
* Divide the height (in inches) by the thickness (in inches) of the material selected for stairs. The answer will indicate how many risers are needed.
* Divide the length of the slope (in inches) by the number of risers. The answer will indicate the depth of each tread. If the tread depth is less than 12 inches, the tread depth will not accommodate a normal step pattern, creating potential safety concerns. Consider routing the stairs diagonally across the slope or extending the stairs beyond the top or bottom of the slope to provide more length for calculating tread depth.
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When constructing the stairs, use the tread depth measurement to determine the placement of the front of each subsequent stair.
Building Freestanding Stairs on Irregular Slopes
When placing freestanding stairs on irregular slopes, adjusting the tread depth may be required to avoid significant excavation. If the slope is steep, shorten the tread depth to no less than 12 inches. The length cut from these treads can then be added to treads on the flatter portion of a slope. It is best to group at least three steps with short treads together. Avoid alternating a short tread depth with a long tread depth if possible. If the adjustment of treads is not acceptable, cutting and filling to create a more even slope is an alternative.
INTERLOCKING STAIR INSTALLATION
Interlocking stairs should be constructed as the wall is being erected, with the first course of the stairs installed with the first course of the wall and continuing for each subsequent course. The following steps outline how to install interlocking stairs for ties/timbers (Figure 21-6), segmental units (Figure 21-7), and dry-laid stone (Figure 21-8). These instructions are prepared for stair treads that are approximately 12 inches in depth. For deeper stair treads, additional stair materials are required. If special precast or one-piece stone treads are available, use those materials for stair construction.
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1. Widen the granular base trench at the stair location 24 inches in back of the entire width of the stair opening. This trench should be the same elevation as the base trench for the wall. Since the first course of the wall will be buried for stability purposes, this level will serve only as a landing and not as a riser (Figures 21-6A, 21-7A, 21-8A).
2. Route any tile around the back of the widened trench.
3. Fill this widened trench with base material and compact.
4. Place the first course of the wall up to the opening for the stairs, keeping the wall front aligned. Place the first tread behind the first wall course. This tread may be composed of multiple ties, wall units, or stones, or may be a single-piece tread. The tread should be approximately 24 inches wider than the stair opening (12 inches on each side), providing enough extra material to allow the tread to extend behind the wall on both sides of the stair opening (Figures 21-6B, 21-7B, 21-8B).
5. Construct the second course for the wall and stop short of the stair opening on each side of the stair.
6. Excavate a base trench behind the first tread for the second tread. This trench should be 18 inches wide by the length of the tread and 6 inches deep. Fill the trench with base material and compact. If a tile is present, fill over the tile without disturbing its level. Smooth the base material so that it is flush with the top of the previous tread.
7. Place the second tread across the opening for the stairs. The second tread should be 24 inches wider than the stair opening. The second tread should also overlap the back of the first tread enough to create the desired tread dimension for the first step. If using tie/timber treads, pin the second tread to the first (Figures 21-6C, 21-7C, 21-8C).
8. Build the cheek wall between the front wall and the tread. Trim wall material as required to fit between the front wall and tread. If building the wall with wood or stone, remember that the cheek wall must interlock with the wall face at the corner and that alternate courses of the cheek wall will run to the face of the wall.
9. Backfill and compact behind the wall and tread.
10. Repeat steps 5 through 9 for each subsequent course of the wall until stairs are complete (Figure 21-9).
11. If used, place tread coverings for all treads.
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BUTT STAIR INSTALLATION
While butt stairs are easier to construct, their stability is not as good as that of interlocking stairs. Because butt stairs are not interlocked into the cheek wall, they may move up or down at a different rate than the walls next to them, causing unevenness or irregularity. Construct butt stairs using the following steps for ties/timbers (Figure 21-10), segmental units (Figure 21-11), and dry-laid stone (Figure 21-12). The instructions are prepared for stair treads that are approximately 12 inches in depth. For deeper stair treads, additional stair materials are required. Precast or one-piece stone treads will simplify tread installation.
1. With butt stair construction, walls and cheek walls can be built prior to the stair installation or concurrently as the walls are assembled. If installing the walls first, leave an opening for stairs when constructing the wall and verify that the walls on each side of the opening are level and aligned. Adjusting the width of the opening to match tread material dimensions is also desirable, although use of a step-back batter on cheek walls requires cutting tread materials to fit.
2. Widen the granular base trench at the stair location 24 inches in back of the entire width of the stair opening. This trench should be at the same elevation as the base trench for the wall. Since the first course of the wall will be buried for stability purposes, this level will serve only as a landing and not as a riser (Figures 21-10A, 21-11A, 21-12A).
3. Route any tile around the back of the trench where the treads will be placed.
4. Fill this widened trench with base materials and compact.
5. Place tread materials between the cheek walls flush with the first wall course. Level treads side to side and front to back. A slight (1/4 inch or less) fall toward the front is desirable if it does not disrupt the riser installation. Cut and fit partial tread materials along the sides if required (Figures 21-10B, 21-11B, 21-12B).
6. If building the walls and stairs concurrently, continue construction of the second course of the wall and cheek wall before installing the second tread. With wood and stone wall materials, the wall and cheek wall must interlock at the corners. Excavate a base trench behind the first tread. This trench should be 18 inches wide by the length of the tread and 6 inches deep. Fill the trench with base material and compact. If a tile is present, fill over the tile without disturbing its level. Smooth the base material so that it is flush with the top of the previous tread.
7. Place tread materials for the second tread across the opening for the stairs. The second tread should overlap the back of the first tread enough to create the desired tread dimension on the first step. If a wall cap is used for the tread, the overlap should equal the wall unit width minus the cap-width (Figures 21-10C, 21-11C, 21-12C).
8. Backfill and compact behind the tread.
9. Repeat steps 6 and 8 for each subsequent course until the wall has been completed (Figure 21-13).
10. If used, place cap stones or tread coverings.
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FREESTANDING STAIR CONSTRUCTION
Site preparation is similar for all of the following types of stairs if they are to be placed on an even slope.
Mark the location for the stairs along the slope. Cut and remove any vegetative cover and excess soil in the stair right-of-way. Add 4 inches of granular base. Except for concrete stairs, begin placement of stair material with the base step.
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Concrete Stair Installation
Concrete stairs can be formed and poured similar to stairs that are built as part of a wall. After excavating the area for the stairs, fill with 4 inches of granular base material. Construct the forms and pour the stairs according to the instructions given in Chapter 23, Concrete Paving, for independent forming of stairs.
Wood-Framed Stair Installation
Stairs that have a wood framework on the front and both sides should be installed beginning with the base step. Add a small amount of granular material in the area excavated for the first stair. Cut the front piece the width of the stair. Cut two side pieces, each 1 foot longer than the depth of the tread. Place the front and two side pieces on the base material (Figure 21-14). Connect the side pieces to the front using galvanized lag screws or spikes. Level all three pieces by adjusting the base material. Fill the voids along the outside edge of the stairs with soil. Fill the tread area between the three pieces with material being used for the tread.
Cut the next set of step boards. Place the front board on the two side pieces for the step below. Position the front board so the distance to the front of the first riser creates the desired tread depth. Anchor the front board for the upper step to the side pieces for the lower step by drilling a 3/8 inch diameter pilot hole through the front board. Drive 12 inch spikes through the pilot holes into the side pieces. Connect the side board to the front pieces and repeat the filling procedure. Repeat these steps until the top of the stairs has been reached (Figure 21-15). The top stair may require an additional board opposite the front board to hold the tread surfacing material in place.
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Wood Riser Stair Installation
Install a small amount of granular base material in the area excavated for the first stair. Cut the first riser piece the width of the stair. Place the riser piece on the base material and level. Fill the void behind the riser with the material selected for the tread. Fill the area along the sides of the tread with soil. Cut a second riser piece. Place the second riser piece on the tread material for the lower stair at the correct distance from the front of the first riser to create the desired tread depth. Repeat the filling operation and continue with the remaining steps.
Stacked Slab Material Installation
Place a small amount of granular base material in the area excavated for the first slab. Place the first slab on the base and level. Fill around the edges of the slab. Place the second slab with an overlap on the first slab that provides the correct tread depth. Fill around the edges of the second slab. Continue this process with the remaining slabs.
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|Title Annotation:||Section 5 Landscape Retaining Walls and Stairs|
|Publication:||Landscape Construction, 2nd ed.|
|Date:||Jan 1, 2005|
|Previous Article:||Chapter 20 Gabion retaining walls.|
|Next Article:||Section 5 Summary.|