How To Use A Scientific Calculator For Rafter Calculations

This 6 minute lesson will show you how to calculate a Common Rafter Rise, Pitch and Length Using A Scientific Calculator.

[@SkilsawDoctor]

Excellent, excellent video! Thanks!

[@michaeldownes516]

You are welcome!

[@marcmcgettigan3580]

very good thanks Michael

[@michaeldownes516]

your welcome

[@connorramm1038]

Is there a calculation that can work out the angle of my roof which includes the size of timber using I know how to get my angle by using rise and run but when use a 6x2 for example I’d have a 100mm height above plate which will leave my rafter 100mm higher than my finish point which would result in an angle change, Is there a formula that can over come this?

[@michaeldownes516]

The angle of the rafter will not change.....if you check out this video on made on marking a common rafter on a site square you will see that the right angled triangle that give the length and plumb cuts does not change even if you move that right angled triangle from the 2/3 rds line to the back of the rafter. Some people go to the the trouble of marking the 2/3rds line on the side of rafter, measure along that 2/3rds line to bm. But if you hook you tape at top end of rafter and measure rafter length along top edge of rafter it will bring you above the BM....the mistake one could easily make at this point is to draw a line square in to get BM....but this would shorten the rafter length along 2/3rds line....so instead, make sure you measure in to BM along plumb (heel line) rather that square in (i.e....you should never see a square line on side of rafter when marking BM)...............you will this heel line marked 6 minutes into the attached video link below. . Check out this video where you see i used the top edge of common rafter to mark rafter length and continued to mark BM as outlined above.... kzbin.info/www/bejne/on2Wm2Bopstnpas hope this gives a clearer understanding

[@Sherlockly]

Thank you for this video, How would you find out the length for the hip/valley using calculator , I understand how to work out the rafter length using cos and the pitch

[@michaeldownes516]

Glad you find the video usefull....To find the length of the Valley/Hip line note the following....the common and typical Hip/Valley have the same Rise, but the run of the Hip/Valley is longer...so the rise and run are 90 degrees to each other and the diagonal of those 2 measurements is the rafter line (Common /Hip/Valley).....if you multiply the run of the common by the square root of 2 (which is 1.4142) you will get the run of the Hip/Valley. Now that you have the Rise and Run of the H/V you just need to find the the pitch of the H/V...so use this formula, ....... .Rise divided by the H/V run...hit = button. ...then hit 2ndF (AKA...Shift or Inv)button, ....hit Tan button, ...hit= button to get the Pitch of the H/V. Now that you have the pitch of the H/V use formula .....H/V run divided by Cos H/V Pitch to get the length of H/V.... Use the following example to check if you are using formula correctly... A rise of 1, with a run of 1.7321, will give a pitch° of 30°, and rafter line length of 2.

[@Sherlockly]

@@michaeldownes516 thanks for this detailed reply, so the run for the common rafter is 1.7321 multiply that by the square root of 2 (1.4142) and this will give me the run of hip/valley being 2.449

[@michaeldownes516]

@@Sherlockly ...yep...thats it...if the common run is 1.732...the Hip run will be as you state, 2.449. (This is for a regular Hip/Valley...hip/valley is 45 ° on Plan View)

[@Sherlockly]

@@michaeldownes516 thank you it’s a great help, what are your thoughts on the reckoner book using that for roofs ?

[@michaeldownes516]

​@@Sherlockly ...reckoner is fine for standard hip roof (Hip 45° on plan)..but its best to know how to use the calculator/trigonometry /Pythagoras for the roof situations that are not standard.....Always be able to get around a right angled triangle your self for the non typical roof scenario. And this ability will be helpful in other areas of Carpentry Joinery/ Construction other than roofing. So when you recognise a right angled triangle on a roof surface surface or with in its structure that you need to calculate, you will be able to figure it out, but the person who is totally dependant on the reckoner is lost. Also, the reckoner can only tell you the line lengths of all members (it sees the roof members as tread lines basically), so you will need to have a good understanding of the roofing geometry involved and have a understanding of how to determine what your deductions are when you factor in various roof members thickness for standard and non standard roofs.