Today we're learning how to figure and cut stair stringers for a basic set of stairs if this is
your first time building your own set of stairs I imagine you're pretty overwhelmed right now and
that's okay because by the end of this two-part series you're going to be able to figure the math
out and cut your own stringers and then learn how to install them using three
different methods to get it started you're going to need to know some basic terminology
the first term is stringer stringers are normally made up of dimensional lumber and they make up the
main structure of the staircase a typical stairway will have three to four stringers
the second term is stairwell header the stairwell header is important because it's the structural
member that supports the floor above and it's also where the stringers are attached
the next term is tread treads are the finished part of the stairs that you actually step on
and the last basic term is riser risers are the finished material that run vertically up
the rise of the stairs if we dig a little bit deeper there are four other important terms
that we need to know and they are total run total rise unit run and unit rise first is the total run
this is the stairs total horizontal length next is the total rise total rise is the total vertical
distance from one finished floor to another sometimes that finished floor can be concrete
like in a basement or a garage it can be in a home where it's hard wood or tile and it can even be
dirt or patio blocks for an outside project like a set of deck stairs to better understand unit run
and unit rise i've made this very simple visual aid unit run which is represented by this side of
the triangle is the horizontal distance from the face of one riser to the next adjacent one and for
our stairs that number is going to be 10 inches or 254 mil and that's a pretty standard dimension
for a rough tread the unit rise which is this side of the triangle is the vertical distance from the
top of one tread to the top of another and for our stairs here that number worked out to be 7
and 5 16 or 185 mil and i'll show you how we worked out that number here in a minute
the last side here is the stair slope which is determined by the unit run and unit rise it's
important to know that the sum of one rise and one tread should add up to between 17 and 18 inches
or 431 to 457 mil for example if we had a tread that was 12 inches and a rise that was 7 inches
that would add up to 19 inches which would not be ideal and in some cases would not pass local
building codes and speaking of building codes be sure to check your local codes before you build
any set of stairs because this video is really demonstrating the math and the basic techniques
to stair building okay with that out of the way let me show you how i solved for our unit rise the
first step to determine unit rise is to determine the total rise again the total rise is the total
vertical distance from one finished floor to another finished floor for example here this
plywood represents the subfloor on the inside of a house which is not the finished floor therefore we
need to take a piece of sample finish floor and hold it into its position before measuring the
total rise the other thing to think through before getting your total rise is the floor below for our
example the concrete floor here is the finished floor but let's say that it wasn't your finished
floor you would have to do the same thing as we did up top which is to take a piece of sample
finished flooring and lay it down to measure off of but since our concrete floor is the finished
floor the total rise from the top of the finished concrete to the top of the finished pine floor
measures 36.5 inches or 927 mil and before we move on i forgot to mention something pretty important
and that is you need to check how level the floor is further out away from the header to check this
extend the level beyond the header and measure down to see if the total rise is the same in that
area as it is in the area underneath the header if the measurement further away from the header is
different you either must make them the same which can be very difficult or go with the measurement
that's out further where the actual stringer will be setting so for our example here the measurement
is quite the same so i'm going to go ahead and go with the measurement that's right under the header
with the total rise now identified let's go ahead and move on to step two
which is the math behind solving for unit rise the ideal rise for a stair is seven inches or 177 mil
knowing that take the total rise which is 36.5 inches and divide that by seven which again is
the ideal rise to get 5.21 this number represents the total number of risers but of course you can't
have a partial riser so you have to round to the nearest whole number and in this case that would
be 5. now divide the total rise of 36.5 inches by 5 this time to get 7 and 5 16 or 185 mil
which is indeed the unit rise for our stairs so how many total treads and risers will we
need for our stairs well it all depends on how we decide to attach the stringer to the header
there are two main ways to do that the first is to have the tread at the top of the stairs
even with the finished floor doing it this way makes the number of risers and treads the same
the second way to attach the stringer and the way we're going to do it today is to drop the stringer
down and use the top of the finished floor as one of the treads doing it this way makes the number
of treads one less than the number of risers therefore for our stairs today we need to cut
four risers and four treads knowing that the stair header is acting as one of the risers and this
will make more sense as soon as we start to lay out our first stringer at this point if you didn't
already have your total run of your stairs you could take the total number of treads and multiply
that by the unit run so for the first method that would be 5 treads times 10 inches which is our
unit run for a total of 50 inches and for the second method four treads times 10 or 40 inches
for your total run now that the math is all done we know that our stairs are going to have a total
of five risers and four treads and we know that we need to cut four treads and four risers it's time
to lay out for our first stringer the first step is to find a really nice straight piece of lumber
in this case i'm using a 2 by 12. next grab a framing square a straight piece of wood and a few
clamps using the blade of the square line up the 10 which is our unit run with the edge of the wood
and clamp it in place then using the tongue of the square line up the 7 and 5 16 which is our rise
with the edge of the wood and clamp that in place i like to do a test mark to make sure
i'm set up in the correct position so i use the back of the lumber and draw both lines
once that's done i double check my marks with a tape measure to ensure that i have exactly what
i'm intending for some reason if you don't simply make some adjustments to the square and clamps
now slide the square down leaving enough room to fit one riser in before the end of the material
with the square held firmly into position mark out for one tread and one riser then slide the square
up carefully lining up the 10 mark with the 7 and 5 16 and make another set of lines
so far we have two treads and three risers one two and a third one which we haven't yet marked out
again we need to cut a total of four risers and four treads continue marking another two times
but once you've marked out for the last tread
stop at this point you'll need the turn 90 degrees to finish off the top of the
stringer to do that remove the square from the clamps and line up the blade with the tread line
making sure it's lined up perfectly draw the 90 degree line now it's time to move back down
to the bottom and draw the last riser line up the tongue with the tread making sure you're perfectly
in line with the pencil mark and in the corner here and then measure and mark out the riser
distance again in this case that's seven and five sixteenths then turn the square 90 degrees
and mark out where the stringer will contact the floor the next step before we actually cut the
stringer is that we need to drop the stringer the thickness of the tread now what this does
is it makes the unit rises equal throughout for example the tread thickness of our mock-up is
an inch and a half or 38 mil therefore we need to remove that amount from the bottom of the stringer
that way once the tread is added our rise still remains 7 and 5 16 on the first and last steps
however if you have a finished floor going down you need to drop the stringer the thickness of the
tread minus the thickness of the finished floor again this only applies if your stringer is going
to get attached to the sub floor first and then a finished floor is going to be added on top of that
when it comes to cutting the stringers take your time make sure that the saw is flat before making
each cut you also don't want to over cut the lines in the corners use a circular saw for the majority
of the cuts and finish the rest with a handsaw the last thing to point out is that it's best to leave
no less than three and a half inches or 89 mil of material left here if you go any
thinner than that you can weaken the stringer significantly well that does it for part one
and part two we're gonna go ahead and check the fit of this stringer we're gonna make some
adjustments and cut some more and then we're gonna learn how to install these using three different
methods thanks for joining me today thank you for watching i'll see everybody next week for part
two