There are many different kinds of stairs (interior and exterior), each serving the same purposethe movement of personnel and products from one floor to another. All stairs have two main parts, called treads and stringers. The underside of a simple stairway, consisting only of stringers and treads, is shown in figure 6-14, view A. Treads of the type shown are called plank treads. This simple type of stairway is called a cleat stairway because of the cleats attached to the stringers to support the treads.
A more finished type of stairway has the treads mounted on two or more sawtooth-edged stringers, and includes risers (fig. 6-14, view B). The stringers shown are cut from solid pieces of dimensional lumber (usually 2 by 12s) and are called cutout, or sawed, stringers.
The first step in stairway layout is to determine the unit rise and unit run (fig. 6-14, view B). The unit rise is calculated on the basis of the total rise of the stairway, and the fact that the customary unit rise for stairs is 7 inches.
The total rise is the vertical distance between the lower finish-floor level and the upper finish-floor level. This may be shown in the elevations. However, since the actual vertical distance as constructed may vary slightly from that shown in the plans, the distance should be measured.
At the time stairs are laid out, only the subflooring is installed. If both the lower and the upper floors are to be covered with finish flooring of the same thickness, the measured vertical distance from the lower subfloor surface to the upper subfloor surface will be the same as the eventual distance between the finish floor surfaces. The distance is, therefore, equal to the total rise of the stairway. But if you are measuring up from a finish floor, such as a concrete basement floor, then you must add to the measured distance the thickness of the upper finish flooring to get the total rise of the stairway. If the upper and lower finish floors will be of different thickness, then you must add the difference in thickness to the measured distance between subfloor surfaces to get the rise of the stairway. To measure the vertical distance, use a straight piece of lumber plumbed in the stair opening with a spirit level.
Lets assume that the total rise measures 8 feet 11 inches, as shown in figure 6-15. Knowing this, you can determine the unit rise as follows. First, reduce the total rise to inches-in this case it comes to 107 inches. Next, divide the total rise in inches by the average unit rise, which is 7 inches. The result, disregarding any fraction, is the number of risers the stairway will havein this case, 107/7 or 15. Now, divide the total rise in inches by the number of risers-in this case, 107/15, or nearly 7 1/8 inches. This is the unit rise, as shown in figure 6-15.
Figure 6-15.Unit rise and run.
The unit run is calculated on the basis of the unit rise and a general architects rule that the sum of the unit run and unit rise should be 17 1/2 inches. Then, by this rule, the unit run is 17 1/2 inches minus 7 1/8 inches or 10 3/8 inches.
You can now calculate the total run of the stairway. The total run is the unit run multiplied by the total number of treads in the stairway. However, the total number of treads depends upon the manner in which the upper end of the stairway will be anchored to the header.
In figure 6-16, three methods of anchoring the upper end of a stairway are shown. In view A, there is a complete tread at the top of the stairway. This means the number of complete treads is the same as the number of risers. For the stairway shown in figure 6-15, there are 15 risers and 15 complete treads. Therefore, the total run of the stairway is equal to the unit run times 15, or 12 feet 11 5/8 inches.
In view B, only part of a tread is at the top of the stairway. If this method were used for the stairway shown in figure 6-15, the number of complete treads would be one less than the number of risers, or 14, The total run of the stairway would be the product of 14 multiplied by 10 3/8, plus the run of the partial tread at the top. Where this run is 7 inches, for example, the total run equals 152 1/4 inches, or 12 feet 8 1/4 inches.
In view C, there is no tread at all at the top of the stairway. The upper finish flooring serves as the top tread. In this case, the total number of complete treads is again 14, but since there is no additional partial tread, the total run of the stairway is 14 times 10 3/8 inches, or 145 1/4 inches, or 12 feet 1 1/4 inches.
When you have calculated the total run of the stairway, drop a plumb bob from the header to the floor below and measure off the total run from the plumb bob. This locates the anchoring point for the lower end of the stairway.
As mentioned earlier, cutout stringers for main stairways are usually made from 2 by 12 stock Before cutting the stringer, you will first need to solve for the length of stock you need.
Assume that you are to use the method of upper-end anchorage shown in view A of figure 6-16 to lay out a stringer for the stairway shown in figure 6-15. This stairway has a total rise of 8 feet 11 inches and a total run of 12 feet 11 5/8 inches. The stringer must be long enough to form the hypotenuse of a triangle with sides of those two lengths. For an approximate length estimate, call the sides 9 and 13 feet long. Then, the length of the hypotenuse will equal the square root of 92 plus 132. This is the square root of 250, about 15.8 feet or 15 feet 9 1/2 inches.
Figure 6.16.Method for anchoring upper end of a stairway.
Extreme accuracy is required in laying out the stringers. Be sure to use a sharp pencil or awl and make the lines meet on the edge of the stringer material.
Figure 6-17 shows the layout at the lower end of the stringer. Set the framing square to the unit run on the tongue and the unit rise on the blade, and draw the line AB. This line represents the bottom tread. Then, draw AD perpendicular to AB. Its length should be equal to the unit rise. This line represents the bottom riser in the stairway. You may have noticed that the thickness of a tread in the stairway has been ignored. This thickness is now about to be accounted for by making an allowance in the height of this first riser. This process is called "dropping the stringer."
As you can see in figure 6-14, view B, the unit rise is measured from the top of one tread to the top of the next for all risers except the bottom one. For the bottom riser, unit rise is measured from the finished floor surface to the surface of the first tread. If AD were cut to the unit rise, the actual rise of the first step would be the sum of the unit rise plus the thickness of a tread. Therefore, the length of AD is shortened by the thickness of a tread, as shown in figure 6-17, by the thickness of a tread less the thickness of the finish flooring. The first is done if the stringer rests on a finish floor, such as a concrete basement floor. The second is done where the stringer rests on subflooring.
Figure 6-17.Layout of lower end of cutout stringer.
When you have shortened AD to AE, draw EF parallel to AB. This line represents the bottom horizontal anchor edge of the stringer. Then, proceed to lay off the remaining risers and treads to the unit rise and unit run until you have laid off 15 risers and 15 treads. Figure 6-18 shows the layout at the upper end of the stringer. The line AB represents the top, the 15th tread. BC, drawn perpendicular to AB, represents the upper vertical anchor edge of the stringer. This edge butts against the stairwell header.
Figure 6-18.Layout of upper end of cutout stringer.
In a given run of stairs, be sure to make all the risers the same height and treads the same width. An unequal riser, especially one that is too high, is dangerous.
We have been dealing with a common straight-flight stairwaymeaning one which follows the same direction throughout. When floor space is not extensive enough to permit construction of a straight-flight stairway, a change stairway is installed-meaning one which changes direction one or more times. The most common types of these are a 90° change and a 180° change. These are usually platform stairways, successive straight-flight lengths, connecting platforms at which the direction changes 90° or doubles back 180°. Such a stairway is laid out simply as a succession of straight-flight stairways.
The stairs in a structure are broadly divided into principal stairs and service stairs. Service stairs are porch, basement, and attic stairs. Some of these maybe simple cleat stairways; others may be open-riser stairways. An open-riser stairway has treads anchored on cutout stringers or stair-block stringers, but no risers. The lower ends of the stringers on porch, basement, and other stairs anchored on concrete are fastened with a kickplate (shown in fig. 6-19).
Figure 6-19.Kickplate for anchoring stairs to concrete.
When dealing with stairs, it is vitally important to remember the allowable head room. Head room is defined as the minimum vertical clearance required from any tread on the stairway to any part of the ceiling structure above the stairway. In most areas, the local building codes specify a height of 6 feet 8 inches for main stairs, and 6 feet 4 inches for basement stairs.
A principal stairway usually has a finished appearance. Rough cutout stringers are concealed by finish stringers (see fig. 6-20). Treads and risers are often rabbet-jointed as in figure 6-21.
Figure 6-20.Finish stringer.
Figure 6-21.Rabbet-and-groove-jointed treads and risers.
Vertical members that support a stairway handrail are called balusters. Figure 6-22 shows a method of joining balusters to treads. Here, dowels, shaped on the lower ends of the balusters, are glued into holes bored in the treads.
Figure 6-22.Joining a baluster to the tread.
Stringers should be toenailed to stairwell double headers with 10d nails, three to each side of the stringer. Those which face against trimmer joists should each be nailed to the joists with at least three 16d nails. At the bottom, a stringer should be toenailed with 10d nails, four to each side, driven into the subflooring and, if possible, into a joist below.
Treads and risers should be nailed to stringers with 6d, 8d, or 10d finish nails, depending on the thickness of the stock,
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015