Replies

1. 
   Gene_Davis | Sep 26, 2006 07:52pm | #1

   Sorry to be flippant, but it is not a rule of thumb.  It is just math, and I think that nowadays, the kind you need is taught in about 8th grade.

   Find the length of the hypotenuse of your 6 rise 12 run triangle, divide the hypotenuse by the run (12), and there is your factor.  Multiply your plan area by that.

   Here, let's try it without a calculator.  6 squared is 36, 12 squared is 144, add them together and get 180.  The square root of 180 is something between 13 and 14, but let's be conservative and use 14.

   14 over 12 is one plus 2/12, or 1 and one sixth.  Let's round it up to one and one fifth, because everybody knows that one fifth is 0.2.

   Your factor is 1.2, and is nicely on the high side so you won't run out of material.

1. 
   PlaneWood | Sep 27, 2006 03:43am | #5

   Well, I figured it out as about - 1 / (cosine (arctan( rise / run)).

   That comes out to a factor of about 1.12.  Add 10% and you get about 1.2.

   PlaneWood by Mike_in_Katy (maker of fine sawdust!)PlaneWood

2. 
   Stilletto | Sep 26, 2006 11:47pm | #2

   Gene explanation is confusing,  but he is alot smarter than me. 

   Measure the height of the roof from drip edge to the peak,  then measure the width of your roof and divide that number by 2. 

   Example height 10',  length 20'.   Multiply 10' x 10' and thats your sq. ft.  

   Add 10-15%  for scrap on hips and valleys. 

   Thats how this hack measures.   :) 

   And so castles made of sand melt into the sea eventually

    

1. 
   MikeSmith | Sep 27, 2006 02:02am | #3

   you mean measure the rafter length , not the height , right ?

   also .... he's talking about a hip  roof.. assuming it's a regular hip and not a bastard hip

   then gene's factor is correct

    1.2

    what the poster didn't give us is the length & width of the building

   you can estimate the whole roof from the ground  if you have a visual pitch guide  ( the shingle mfr's used to give them away )

   hold the transparent guide up and measure the pitch... measure the base width of the roof... measure the base length of the roof

   find the pitch factor for that pitch ( like Gene did... or look it up in a table )

    and solve for the two planes  ( hip end  & common roof ) multiply by two and you have your area.. since it is a hip  you have more waste so  add your 10-15%  and then determine wether you need a special cap for the hips  ( architectural shingles )

   and a diffferent cap if you are using a ridge vent

    Mike Smith Rhode Island : Design / Build / Repair / Restore

1. 
   Stilletto | Sep 27, 2006 02:35am | #4

   Rafter length including overhang.  I couldn't think of the right wording for it. 

   I just take the hip and turn it into a square.  10x10 in the example and tack on the waste.   If you use your drops from the first side of the hip on the other side waste is considerably less. 

   I usually shingle left to right,  the drops off the left side get nailed onto the right side then cut again.  A few sq savings is a good thing. 

   I agree Gene's right,  just confusing.  I was just tryin to simplify it for the OP.  Cut the hip in half and make it a square.  Easier figuring than factors.  At least for me. 

   I haven't used a visual pitch guide yet,  I like to get on the roof and measure,  check for nailing and other things on new construction.  On remodels it gives me an idea how much sheeting is rotted without climbing into the attics. 

    And so castles made of sand melt into the sea eventually