Building Confidence

Volume 15 Issue 5
ISSN 1923-7162

Welcome to Dave's Shop Talk's Home Improvement Newsletter of questions from our members on their construction projects, a Tip of the Month and a home remodeling article, both from our website at

Tip of the Month

An easy way to figure out the length of rail between two stiles. Place the two stiles together, edge to edge. Put your tape measure on the right side edge at the measurement wanted, say 13 15/32 and read off on the opposite edge what the measurement will be.

And a Bonus Tip:

When using a router always try the setup on a scrap piece of identical material to be sure it is correct before trying it on the actual piece.

Ask Dave!

I had an interesting email from my Grandson, Dylan, who is working in the oil patch in Northern BC, Canada. Dylan is a 4th year Carpenter Apprentice and is doing very well, I am very proud of him. He asked this question:

What is the formula for switching back and forth between degrees of slope and percent of slope? 2 percent of slope is what in degrees? (It's 1.15 degrees.)

I answered. Yes, 1.146 degrees. We have a slope Conversion calculator on our website. Do you still have access to our site?

Dylan: I don't have access currently, how do you do the conversion long hand? I am building a large retaining wall out of lock blocks and there is a drainage pipe running through.. without any dimensions on the pipe.. I took the start and finish elevation of the pipe and got the rise with the 2% slope I got 1.15 degrees and used trigonometry to get my run of the pipe through my wall, Which worked. I made a triangle with a 3 meter section x the specified 2% and got a smaller unit triangle and used inverse tan to find the degrees and ratio'd it to my pipe length... but there has to be an easier way?

I wrote this article that Dan, my brother the Math Whiz, checked for me and I thought some of our readers may appreciate it, as well:

Slope Conversion

There are two ways to measure a slope and each method of measurement can be converted to the other. The two ways are: ratio of rise to run or degree of angle.

Measurement One

Most carpenters are familiar with the ratio of the rise and run for stairs and rafters. The rise is vertical and the run is horizontal. A typical slope of the roof is 5:12, so the rise is 5" and the run is 12". Another way to look at this is the ratio of a slope when the rise is one compared to another value. 5:12, or 5 in 12, becomes 1:2.4 or 1 foot in 2.4 feet. Road builders refer to the gradient of a slope as a percentage. It is still a ratio but the rise is figured in 100 units of run. For example, a 10% gradient is 10 units of rise in 100 units of run, where the units are the same. 5 in 12 becomes 41.67% (percent which means per 100) or 41.67 feet rise in 100 feet or 41.67 inches in 100 inches or 41.67 metres in 100 metres. No matter what unit you use you must measure both the rise and run in the same unit.

Example: To calculate the slope of a roof (referred to as a pitch) of 5:12 to a percent of slope: Divide 5 by 12 = .4167, then multiply by 100 = 41.67%.

As a ratio to a rise of one: 5:12 becomes 1:2.4. (12 divided by 5 = 2.4). To convert 1:2.4 to a percentage, simply divide 1 by 2.4 = .4167, then multiply by 100 = 41.67% - the same as 5:12.

Measurement Two

The second way to measure a slope is by the angle of the slope (in degrees). If you know the angle of the slope you can convert that angle to a rise to run ratio.

This is done by using the tangent, which simply means the rise divided by the run at any given angle. The tangent of a known angle can be looked up in a table or by using a scientific calculator (a calculator that has a tangent button and usually lots of others). You can also find the angle of the slope if you know the rise to run ratio by using the inverse tangent on your calculator (the tangent in reverse, usually looks like this: tan-1) or by looking up the angle in a table of tangents where the angle corresponds to your known slope ratio. That's all a tangent is. It's just the ratio (fraction) of rise over run for any angle of slope.

One important note about angles in calculators. There are three ways to specify an angle (and you only need to know one of them unless you're an engineer): degrees, radians and gradians. You guessed it: degrees is the one to use. You can skip the other two and I won't bother you with their definitions!

The reason the above note is important is because you MUST set your calculator to degrees (instead of rads or grads) otherwise the results of your calculations will NOT be useful to you. There are many types and models of calculators. You want one that can calculate in degrees and has the tangent key (tan) and inverse tangent key (tan-1).

Using your Scientific Calculator to get the rise to run for any known angle of slope:

  1. Set calculator to Deg. (Not Rad or Grad.)
  2. Tan 22.62 degrees = .4167. (On some calculators you need to enter the angle first, then press the tan key. On others you press the tan key first, then enter the angle.)
  3. To convert this to a rise to run ratio multiply the answer by 12. So, in our example .4167 x 12 = 5. The rise to run is 5:12 (5 inches of rise for every 12 inches of run. A typical and useful slope for roofs.)

Using your Scientific Calculator to get the degrees of a slope from a known ratio of rise and run:

  1. Set calculator to Deg. (Not Rad or Grad.)
  2. Divide rise by run. (Example: 5:12 or 1:2.4) 5/12 = .4167
  3. Click the inverse tan button (tan-1). In our example, you will get 22.62 degrees as the result.
  4. Or on some calculators you need to click the inverse tan button first, then enter .4167 to get 22.62 degrees as the result.

Hope this helps. If all else fails check out our Conversion Calculator at and set the Convert field to Slope.

Feature Article of the Month

(taken from our website:

Remodeling 19: How to Replace a Sliding Patio Door

In conversing with a member of our site, recently, I was shocked to hear that he was quoted $900 for labor to remove an existing patio door and replace it with a new unit. I sent him these instructions, hopefully they will help you too.

Here are the steps to remove and replace a sliding patio door unit with the same size unit:

Drawing of the parts of a sliding patio door.

1. Remove the window liner of the patio door and casing from the inside.

2. Removing the existing patio door: If the house is stucco, by using either a circular saw or an angle grinder with a masonry blade, remove the stucco about 1 1/2" from all around the patio door unit. These patio door units have a flange on them that is designed to hold the patio door units against the sheathing on the wall. You have to remove the stucco, siding and trim for the first 1 1/2" around the patio door unit, right down to the sheathing, to expose this flange. Get a nail claw and remove the nails driven through the flange into the sheathing. I remove the sliding patio door to make the frame lighter. Lift up and out on the bottom when the patio door is in the full open position. The new patio doors have stop blocks in the frame header which prevents upward movement unless the patio door is opened enough to slide by these blocks. Older patio door units do not have these. Remove any screws through the frame into the cripple, especially around the latch. Remove the patio door frame unit.

3. Replace the new patio door unit - Set the new patio door unit in position on the sub-floor and shim up one side to level the patio door unit. Shim up under the patio door unit in various places to keep the threshold level. A helper is usefull here to keep the patio door unit from falling out. I usually work by myself so use nails as a third hand, by nailing the nails above the upper flange, enough to allow shiming (if required), and bend them over the flange to prevent the top from tipping out. When the bottom threshold is straight and level nail the bottom flange about 6" apart. Aluminum flanges have a thin groove to nail through, vinyl flanges have elongated holes to nail through. Use roofing nails to nail the flanges, especially on vinyl, center the nail in the slot.

4. Plumb the side of the patio door unit by installing the patio door back in and close the patio door to get an even margin. This should require very little movement and should be perfect from the patio door manufacturer. It stands to reason that if the bottom is level the sides should be plumb as long as the patio door unit is square. If it is not, the patio door may need to be adjusted if the side frames look plumb with a level. The patio door is levelled using a screw driver on the sides on the bottom, through a hole. By screwing this screw in or out will lift or drop this side of the patio door. Keep trying until the patio door slides against the jamb evenly. That's why it is important to level the patio door base well before continuing, so that the patio door will roll along the bottom nicely and straight rather than going up and down and rubbing on the track. When happy with the patio door unit being plumb, nail up the side flanges on both sides.

5. Remove the nails above the top flange used to temporarily hold the patio door unit at the top. Do not nail any nails in the top flange at all. The top is held against the sheathing with the trim or siding and allows the header to move down without breaking the patio door unit as the wood shrinks. This is mainly for a new house where the lumber still has a percentage of water in it. Still good practice, though.

6. Open and close the patio door and adjust if necessary.

7. Depending on the type of siding or stucco, you have to replace or install a 1x4 trim on the outside of the patio door. Pack out above the flange to match the thickness of siding or stucco, usually a ripping of 5/8" plywood works nicely ripped about 1 3/8" wide. The trim will cover the flange and provide shelter from the elements. Caulk the frame to the trim and trim to the siding or stucco, unless it is bevel siding. Use an exterior latex with silicon caulking that is paintable, for example Alex Plus by Dap.

8. Insulate between the patio door unit and the cripple (jack stud), threshold and header loosely with fibreglass insulation or the spray foam. Finish the inside liner as was the original and install the casing. Caulk up the casing with the same as used outside.

Enjoy your new patio door.

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(Ask Dave) (About Dave)

Your source for building tips, woodworking & furniture plans, house plans and building advice directly from Dave...

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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