Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
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The construction of the deck stairs as shown in the above Floor Plan is a very complicated build, which incorporates the fundamentals of stair building combined with that of roof construction. If this is your first time in making stairs, refer to the How to Build Stairs articles on the web site. Also, details are given in How to Build a House 2: The Foundation.
I will compare the support of the miter of the steps with a hip rafter in a roof and the supports for the steps between the handrails with common rafters. Notice that the steps are 7' in both directions to produce a 45 degree angle with the deck line.
The height of the deck off the ground is 67", in our example; you can vary yours accordingly. The main thing I would like to accomplish here is to clarify the principle of building steps with mitered returns so you can apply your own measurements to suit your individual needs. I'll use this example of a set of return deck stairs throughout this article, but the chance of your actual project being the same is very unlikely, so I'll explain how the measurements are arrived at as we go.
You can see the perimeter measurements are 20'-4" x 7'-0"; your footing will be inside of that. The top of the deck is the reference for these elevations, so measure 68 1/2" down to the top of the concrete, which is the total rise plus 1 1/2" for the thickness of the 2x8. Concrete should be a minimum of 18" below the soil, depending on the frost depth in your area. The concrete should be 12" wide with a pressure treated 2x8 bolted to the top of it, flush with the outside edge of the footing. The stringer bottoms and supports will be nailed into this 2x8 sill plate. When the concrete is smoothed off on the top, insert 1/2" x 6" anchor bolts, leaving the threads up about 2" to bolt the 2x8 down. Place them no more than 6' apart. The footings in the middle are to support posts for the stringers, the two pads are for posts under the short stringers. Don't just rely on nails holding up the stringers; they should all be supported with posts—2x4 posts are good. Any wood laid down onto concrete should be protected with a 30 pound roofing felt or sill gasket under it or the wood pressure treated.
The rise and run ratio is 7.44/10.5 or 7 7/16"/10 1/2" on the common stringers. For the 45 degree stringers the ratio is 7.44/14.85 or 7 7/16"/14 7/8". Lay these out as shown in the article Stairs 2: How to Cut a Stair Stringer.
Here is a detail of the 45 stringer:
Lay this stringer out with 8 rises of 7.44" and 8 runs of 14.85". After laying it out and cutting it out, remember to cut it off the bottom the thickness of the tread. Measure back from the first riser 11.98" and cut off the end plumb and on a 45. This is a compound angle of 45 and square with the top. You need two stringers like this, so copy one from the other. It's important to be very accurate with this.
Now cut the 7 common stringers; 3 will go in between the ones against the 45 stringers and 2 will be shortened by 1 1/2" at the top end to go up against the common stringers, against the 45 stringer. These are parallel to the deck as shown on the Stringer Layout drawing. These are laid out as normal, 8 rises at 7.44 and 8 runs at 10.5". Cut the bottom off by the tread thickness, 1 1/2" if you're using standard 2x6's for treads. (See How to Cut a Stair Stringer for more info on this.) Lay one out and use it as a pattern to copy the others. You need to make 12 jack stringers, so use the pattern for these, as well. Notice the plan for their different lengths, just subtract the number of runs at 10.5 each to make up the set.
Here is a detail of the Jack Stringers:
Notice how these stringers need to be extended by 1 1/2", then cut off on a 45 degree to fit the 45 degree angle of the 45 stringer. The rest of the stringer is the same as the common stringers. You need 4 sets of 3 with 2 of the sets cut off on the opposite 45 degree (mirror-image to the other 2 sets). The bottoms of these stringers are all alike since they all rest on the concrete pad, just cut them to different lengths.
When ready to assemble start with the common stringers and nail them into the deck starting at the inside edge of the 6'-4" measurement and nail into the 2x8 sill on the concrete. They come down from the deck surface 1 riser of 7.44 plus 1 1/2" tread or 8.94".
Before installing the treads and risers, place posts under the stringers next to the deck and in the middle of the span. This applies to all the stringers including the jack stringer, although a center post is not necessary for the two shorter ones.
The red lines next to this text show the backs of the riser, the same as the 10 1/2" run. Put the risers on first, then the treads over them to allow for a bit of overhang. The treads are mitered on the center of the 45 stringer, as are the risers. Two 2x6's will work well for each tread.
The treads should be 1 1/2" thick material for the spacing of the stringers. The stringers should all be made from pressure treated 2x10, except for the two 45 stringers which should be 2x12 pressure treated.
Dave(Ask Dave) (About Dave)
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