This is a two part series on how to work out a Hip or Valley raking length for Estimating purposes.
Having done a bit of estimating for large residential builders now, I have come to learn that many estimators do not know how to work out a Hip or Valley length. The matter has recently come up again so in order to get the word out about how to correctly work it out I thought it best to put up a post and an excel spreadsheet demonstrating a method of correctly working them out.
But firstly, and it may seem obvious, why do estimators need to know the Hip and or Valley lengths? Could they not get their Fascia and Gutter supplier or Roofing supplier(Tile or Sheet Metal) to supply them with a quote with the measurements? Of course they could but how would you know they are quoting to you correctly, if you have more then one quote and both match for quantities your probably safe, but what if they differ? You could be building 500 homes a year and they could be supplying you with quotes saying you have an extra 2m of Hip capping on each job. If each lineal meter of hip capping was $15.00, just the roof tile supplier could be getting an extra $15,000 from you each and every year(Are you a builder that has been operating for more then 10yrs?). That is just the start, you also have to pay for your valley irons and valley trimming and some builders pay their carpenters for the lineal meterage of hips as an extra. Those who calculate these simple things the wrong way or rely on a quote for the measurements could be paying more then they need to. It is a fact that I more often then not see these things over allowed rather then correctly allowed, it is primarly the estimator not understainding how to do the calculation correctly to begin with and then adding a little extra “fat” and then also rounding up as well, just to be sure.
So lets get into it shall we. Both a hip or valley length is calculated the same way, this is so because they basically form a corner for a roof and the distance from fascia to apex/ridge does not change be it a Valley(generally an internal corner) or a Hip(generally an external corner).
If you have tried to work it out for yourself before and your thinking to your self the only angle you need to work with Is the roof pitch (in degrees) form the plans, you would be mistaken. This is the common mistake I come across over and over, so don’t feel bad about it you’re not alone. I have found a lot of estimators use the roof pitch factor based off just the main roof pitch to work out the length of their hips and valleys and this leads to over estimating as it is not the right way to work it out.
Consider the following to understand why the pitch of your hip or valley is different to your roof pitch; Your hips and valleys if measured in plan view are longer then your standard truss to the ridge line, this is because they are at 45 degrees to the normal trusses and the distance therefore is further. If your hip or valley was rising at the same pitch as the truss and as we worked out before they are indeed longer, the top of the hip or valley would be higher then the top of the truss. Obviously because your ridge line should be horizontal having your hip higher is going to be an issue.
There are three(3) triangles to consider when working out the length.
The first triangle would represent half of a normal truss in elevation (side) view.
The second triangle would represent the 45 degree angle to the hip or valley in plan view.
The third would represent the hip or valley itself in elevation.
We now have knowledge of the basic building blocks we require to write the formula to work out the raking length, in the next installment we will go through the math and I will present you with a excel spreadsheet which you can use yourself.