NEWSLETTER from DaveOsborne.com  
Volume 4 Issue 5  “Building Confidence”  May 2006 
Welcome to those new members to our site as well as to this newsletter. We are a bit behind in getting these newsletters out, please bear with us.
Thank you to several readers for the help regarding the water filter for Mexico. I'll keep you updated.
Here are some of the questions and my answers for the month. The first email uses words that I define later in this newsletter for those of us who are a little fuzzy when it comes to Geometry (mathematics of shapes).
Dave, It seems to me that there was a mistake in the discussion regarding the trapezoid. The question indicated that the two sloped sides were 7.75, which is not the height. Your answer used 7.75 as the height, which I don't think is correct. By my calculations, using the Pythagorean Theorem would indicate that the height is 7.59. Just wanted to let you know what my thoughts on this question. Sincerely, Owen
Hi Owen,
Thanks for the correction to the trapezoid problem. I'll resend the calculation to Brandy, with your note.
Dave
Hi Brandy,
I used your question in our last newsletter and thanks to a reader's response I found out that I made a mistake in calculating the area of the shape you sent me.
I took the sides (7.75) as heights, which they were not.
Here is my corrected area:
The formula for the area of a trapezoid is:
Area = Height X (top + bottom) / 2
Let's look at a drawing:
This is a trapezoid, a four sided shape that has two of its sides parallel to each other. Take the average length of the top and bottom. In other words add the two lengths, in this case: 8.75 plus 11.875 = 20.625, and then divide by 2 = 10.312. The average length of the top and bottom is 10.312. Then multiply this average side by the height to give the area.
To find the height (H) here we need to use Pythagoras' Theorem, which is that each of the right angle (90 degree angle) sides of a triangle when squared (multiplied by themselves) and summed are equal to the hypoteneuse (the long side of a 90 degree triangle) squared. Here's the formula: a² + b² = c², where c is the long side and a and b are the two shorter lengths and the little 2 means the length is multiplied by itself. Another way of writing this formula is:
(a X a) + (b X b) = (c X c)
In our case we know the lengths of one of the smaller sides and the hypoteneuse and we need to figure out the length of the other smaller side (the height). So, first we just plug in the numbers we do know so we have:
(1.5625 X 1.5625) + (b X b) = 7.75 X 7.75
The first step of simplifying this is to do the multiplying so we have:
2.44140625 + (b X b) = 60.0625
Then subtract 2.44140625 from both sides of the equation to get:
b X b = 57.62109375
The way to simplify this now is to take the square root of both sides:
b = 7.59 (rounded off to two decimal places, which is as precise as we need)
This is the height (H) in our little diagram, so the area of the entire trapezoid is:
Area of the trapezoid = Height X (top + bottom) / 2
area = 7.59 X (8.75 + 11.875) / 2
area = 78.27 square inches
Dave
Hi Dave, Refinishing the basement and need two additional air ducts in the bedroom and bathroom from the existing central air conditioning plenum. Need suggestions and/or diagrams on the proper assembly. I want to use rigid ducting and am looking at possibly 4 or 6" diameter with the total length of both locations of 8 linear feet.
Hi,
Use a 5" or 6" diameter duct. You need to buy an adapter from the 5" duct (for instance) that will fit a standard 4x10 or 3x12 floor register. The adapter or boot is secured to framing in the wall or a square hole cut in the floor. The other end which goes into the plenum is fitted through a proper sized hole in the plenum, notched about 1" x 1" to form tabs. Each alternate tab is bent into the inside of the plenum and its alternate is bent onto the outside. Space out 3 screws around the duct on the outside and screw these tabs into the plenum. Wrap proper foil adhesive tape (not duct tape) around the outside of the 5" duct and the plenum to seal off any air leaks. Also tape the joints with this tape as well.
Dave
Dave: I am interested in taking down an interior wall that separates my kitchen and dining room and the wall runs parallel with the floor joists. Are there any special precautions that I should keep in mind while undertaking this project? Thanks, Dan
Hi Dan,
Just check to be sure the wall is not a bearing wall, doubtful, but... A bearing wall may support a floor, ceiling or roof. Check out in the attic to be sure.
Removing a wall leaves a space in the floor and ceiling which has to be filled to match the existing covering. Removing the top and double plate if in an attic may leave a hole for loose insulation to pour out of. Maybe check out the attic and see if you have blown in insulation first so you can remove the insulation over the wall area.
Other than that I can't think of anything else.
Dave
Hello Dave, I live in Kaneohe, Hawaii and I am in the process of adding a deck above a deck at an elevation of 20 feet above ground and a span of 28 feet from inside of the column to inside of the column on the opposite side. I plan on four columns. The deck will be 26 feet wide by 12 feet. My question and your expertise help needed is, how and where can I find the beam and size to span the 28 feet inside to 30 feet outside. I know it will be a paralam, but I need the size. Please help and this is a GREAT site. Thanks and ALOHA from Hawaii. Norm
Hi Norm,
These beams usually come in 1 3/4" thicknesses. With a 28' span you would probably need a double layer making the beam 3 1/2" thick. I'm guessing for a 28' span you would need a 14" or 16" deep beam. These are engineered beams found at truss plants. They will design the beam to fit you span, load, etc.
Dave
Dave what's the standard size board for a stringer?
The standard board used for a stringer is either 2x10 or 2x12. The requiring factor is that after the stringer is cut it should have 3 1/2" left below the notches for strength.
Dave
Hi Dave, I'm confused about whether or not to tape the joints on (1/4" wonderboard, a cement backer board for tile) The manufacturer says not to, but people in the trade say to tape them with fiberglass drywall tape. Any insight on this would be greatly appreciated. Bob
Hi Bob,
Actually James Hardie recommends the use of a 2" fiberglass tape embedded in thinset mortar to tape and mud the joints.
Dave
Hi Dave, I really appreciate your knowledgeable help. My plans have changed again. Now I am building a chalet style home. It is 28'x30 with 8' 2x6 walls and a 12/12 open pitched roof. The end wall in the living area will be 28' wide by about 22' high to the inside peak. Much of the wall will be glass (trapezoid windows). My question is, What is the best way to frame it for wind sheer and stability? There's not much strength in glass. Thanks, Rich
Hi Rich,
I'm glad you are framing the walls with 2x6. You probably are not having the floor reach the gable end wall. Rather have a loft and full ceiling. I would suggest putting in a header over the windows, although not required to support a floor or roof. It would act more like a strongback to give rigidity to the wall. A 2x10 double beam with a double 2x6 plate on top of the beam and a single 2x6 plate below it. The beam and plates would be tied to the gable end truss with plywood gussets, the same thickness as the plywood or OSB sheathing. This would help stiffen the wall up. With large windows they will use heavier glass, as well. You would have a beam and plates over the top of each row of windows.
Dave
Hi Dave, Do you have any instructions/tips for installing a ridge vent on an existing attic roof? Thanks, Jim
Hi Jim,
Attic ventilation is a system which requires air coming in and air going out. The air comes in at the eaves or soffit of the roof and comes out at roof vents, ridge vents or gable end vents. You need the soffit vents in order for the roof vents to work.
To put in a ridge vent in an existing asphalt roof is not that difficult. It requires removing the existing shingle cap at the ridge. These can be used over the new ridge vent if they can be saved. According to the vent purchased, an opening is cut out from the sheathing on each side of the ridge. If the roof is quite old and rafters were used the roof would have a continuous ridge board with the rafters nailed to this on each side. With trusses there is no ridge board. Set the depth of your circular saw to the thickness of the sheathing. Don't cut the rafters, ridge board or the trusses, just the sheathing. A small strip, about 3/4" to 1" is removed on each side of the ridge. The instructions with the particular vent should give this exactly. After the caps are removed, start with removing the shingles by using a utility knife. Snap a line the required distance from each side of the ridge line. Cut the sheathing out with a circular saw keeping clear of the asphalt shingles. Start this cut about 6" back from the edge of the roof, leaving the shingles intact for the first 6" and about 6" away from any chimneys, etc. After removing the strip of sheathing and shingles nail the vent into position. Keep the vent flush with the gable end of the roof, for looks, even though the vent cap starts about 6" from the edge. Nail the vent into position using 1 1/4" roofing nails. Some vents need end caps at the roof edge and some require covering them with either the old caps, if still in good condition, or left as is.
Another solution to ridge vents is adding roof vents. These are individual vents installed near the top of a roof, usually on the back side of the roof. The amount of ventilation used is dependent on the size of the attic ceiling. "The unobstructed vent area shall be not less than 1/300 of the insulated ceiling area for a slope greater than 4/12. For slopes less than 4/12 the vent area shall be 1/500 of the insulated ceiling area."
The code mentions unobstructed vent area. Make sure the soffit vents are not covered with the attic insulation which renders them useless. Styrofoam baffles should be installed at the insulation stage to prevent the insulation from being blown over the soffit vents. Check this out first.
Dave
Hi Dave. We just became members today to your site. It looks great. We printed out the plans for the 10x12 gambrel shed but didn't realize that they are 4 ft. walls. We really want 7 ft. walls instead of 4 ft. How difficult would it be to change the height? If you could email us back and give us some help, that would be awesome! Thanks! Louie and Liz
Hi Louie and Liz and welcome to our website,
If you want 7' walls that's no problem. Just remember, though, that the lower part of the gambrel roof forms a sort of wall in itself. 7' with the gambrel on top may be too high. But in any case , the height of the walls doesn't affect the trusses.
Let me know what you plan on doing, if you need any further help.
Dave
Well, I hope your renos are going well. During the summer I will be out and about, as all of us will, I would hope, so bear with me if I don't get back to you right away with answers to your questions.
Enjoy the summer with your families!
Dave
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