Making a handrail for your back steps requires a few basic tools assuming the rail is to be made of wood. A few basic handrail guidelines must be followed. According to the International Residential Building Code, open sided stairs shall have handrails not less the thirty-four (34) inches measured vertically from the tread nosing and not more than thirty-six (36) inches in height total to the top of the rail. Our local codes require two handrails for any stairs over thirty-six inches wide. It seems to make people feel much safer with two hand rails. There are many fancy pre-formed handrails available in your lumber yards today. Already shaped and sanded, they do eliminate some of the grunge work of building your own hand rails.
To make an inexpensive handrail you will need some four inch by four inch, pressure treated posts and some two inch by four inch by at least as long as the stairs material for the top rail and side rails. Four inch lag bolts or threaded rod with nuts and washers, a good drill, ratchet and socket for the lags, a circular saw with a sharp blade, tape measure, four foot carpenters level and and a pencil. A chalk line comes in very handy when it is time to cut the tops of the posts to their finished height. The overall length of the stairs will of course determine how many posts you will need. An eight foot set of stairs would have three posts. One at each end and one at a four foot center. Keep your support posts at four foot centers for maximum strength of the handrail. Cut your four by fours in lengths long enough to reach from the bottom of the stringer to at least thirty four inches above the stringer in a plumb line. For a thirty-five (35 1/2) and half inch height above the stringer the post would be forty-seven and a half inches long (47 1/2).
After cutting all the posts, lay them out on a pair of saw horses. Take one post and hold it in position on the side of the stair stringer. (a little help here would be great). Holding the post flush with the bottom of the stringer, plumb the post and carefully mark the stringer slope on the four by four post. If at all possible, place your vertical posts between the treads so the post cuts can be sloped and not flat. Measure from the pencil mark up vertically making sure that when you install the two by four top handrail, the overall height above the tread nosings is between thirty four and thirty six inches from the nosing of the tread. Once you are sure you have marked your sample correctly, return it to the saw horses. You want to cut and remove one half of the posts width for the full height of the stringer. The notch you cut will have a sloped top. Back to the stairs and test fit the post. If when the post is plumbed and the sloped cut is sitting directly on top of the stringer your vertical measurements are correct, go ahead and cut all the rest of the posts the same way. I like to use three lag bolts per post to prevent twisting although many contractors use only two. If you are through bolting the posts, two rods should be sufficient.
Now install all your posts. (get your helper back for this part). Once all the posts are installed and lag bolts tightened, using your chalk line you want to strike a chalk line from the top to the bottom post on the slope of the finished rail. Measure up from the nearest tread thirty four inches and make a mark on the top and bottom posts. Connect the two marks with the chalk line and snap it. Doing it this way will assure the top handrail is in a flat condition and does not go up and down at each post due to slight differences in the stair tread heights. Overall it all will be the same. Now carefully cut the posts off at the sloped chalk line marks. If possible use one piece of two by four for the handrail. Making mid-point joints is tough to do on top of a four inch wide post but it is possible. If you do, remember to bevel cut the adjoining ends at a forty five degree angle with the top angle sitting on top of the bottom one you will have a great joint and when sanded less chance of a splinter.
Install your top rail leaving two inches overhanging the first post. Screw the rail in place and with your helper holding the other end of the rail piece, they can bow the two by four back and forth to make it center over the post despite any bowing in the lumber itself. Screw the railing in place as you go and it will stay there. Cut off the excess rail leaving the same two inch overhang as the bottom of the rail. Now you want to add another two by four under the top railing on the outside of the posts and directly under the top rail. this will help keep the rail straight over time.
A variation here if money allows, is to use a two by six inch top rail. It does provide more surface area to grab on to. You can now add a bottom side rail, balusters, decorative lattice or what ever you wish and can afford, to your new posts and handrails.