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For years now in online forums like JLC  Online and FaceBook groups I have been arguing about The Hidden Dangers of Square Foot Estimating saying things like…

EstimatingMarkupIcon“There is no such thing as a good, fair, or accurate Square Foot Price for an estimate. At the very best, a price based on square footage (the footprint) is nothing more than a WAG or SWAG ( Wild Assed Guess or Sophisticated Wild Assed Guess). Square Foot Estimates are inaccurate, undependable, and dangerous to use.”

My focus has mostly been in regard to the contractors that look for or think there is something like a Price per Squre Foot for interior finishing for house based on the house’s footprint (SF of living space) that they can use to bid a project with.  That kind of thinking makes my head explode.

A 36′ x 28′ single story home with an 8:12 pitch gable roof has 1008 Square Feet of Living Space and  if it has a gable roof with 1′-11″ of overhang the surface area of the roof is 1365.84 Square Feet which is 35% more Square Feet than there is living space. There is no corelation between living space and the square footage of roofing to be installed.

But I would also get something like this argument I once got from a builder that doesn’t didn’t seem to understand the distinction between that kind of Square Foot Estimating and using the number of Square Feet of roof surface as a Unit Cost:

I disagree that subcontractors shouldn’t use square foot estimating. It depends on the tasks. Example, say you are a roofer, and you charge $50/sq to install, labor only. You are basically charging by the square ft. You’ve roofed a 100 houses at 8/12 pitch, your job log and data tells you the deviation factor from one 8/12 roof to another is miniscule. As a basic price, you charge $50, and adjust to the circumstances of the job. A 75 sq straight run roof, all gable, no hips or valleys, you may reduce to $45, whereas a 40 sq job with several hips or valleys you might charge $55. But your basic price of a 5/12 pitched roof is the same, $50, that’s your point of reference that’s proven profitable, you don’t reinvent the wheel on each job trying to estimate the time needed to roof a house. This is a simple example that can be expanded and applied to just about any task.

The accuracy of this method is function of how much actual accurate data you have on past jobs.

As I said the roofer described in that quote about is really performing a Unit Cost Estimate using the Square Foot as the particular Unit of Measure but there is still a problem with figuring an estimate based on just that. The roofer that uses Square Footage as a Unit Cost still has a a couple of problems to consider. The shape affects the cost too. Sal Alfano who has written about this subject before recently wrote in Pro Remodeler Magazine: Shape Affects Cost | Pro Remodeler  (July 25, 2019).

This isn’t the only time that Sal Alfano has written about the problem with SF measures for estimating. All the way back in 1999 he wrote this article for the Journal of Light Construction that I have cited before in other post and online discussions.

Unit-Pricing Pitfalls A building's shape affects square-foot costs

Unit Pricing Pitfalls
By Sal Alfano
Journal of Light Construction July 1999

A building’s shape affects square-foot costs

But there is still another problem with using a square foot cost for an assembly or system like a roof.  While your price for the Square Footage of area to be roofed can include the cost for the roofing underlayment such as felt (does anyone still use roofing felt on roofs?) or the more modern roofing underlayments. But this SF roof price doesn’t included how the edges and intersections of the roofing planes will be treated.

  • Roof Edge or Soffit Ventilation is often measured by the LF (linear foot) although sometimes buy the piece (Ea. or Each) as in the case of those horribly ineffective circular soffit vents.
  • The Metal Drip Edge along the bottom edge of the roof is measured by the LF.
  • The The Metal Drip Edge or we call Drip Edge Flashing along the rake edges of the roof which is a different material from the bottom edge is measured by the LF unit.
  • Ice and Water Shield Underlayment is applied along the bottom edge (the bottom 3′ of the roof) and is measured by the LF.
  • Ridge Ventilation is also measured and installed by the LF unit.
  • If the roof butts up to another vertical wall or has a chimney penetration the Step Flashing there is typically measured and installed by the LF unit although some roofing estimators culculate it by the piece (Ea.).
  • The flashing  for plumbing penetration is measured and installed by the piece (Ea.).

While it sure is possible to make sure your SF price for a roofing project has enough padding in it to account for the cost of all those other items that may not be so wise when the roof is really cutup with lots of intersections and penetrations and not accounting for these LF items also means if you do get the project with your padded price you will have to go back and get these measurements again to place your materials order so why not just get a more accurate price that saves you the time.

In short the edges matter and sometimes they can matter a lot.

In the case of tile installations we found over the years that the installation of the tile that had to be measured and cut to fit took 3 to 4 times as long as laying the field tile so we had our labor cost for laying the field tile by the SF and a LF cost for all the edges which was 3 to 4 times the rate of the field tile installation depending upon the tile we were cutting.

And even if you look at this illustration from Sal Alfanos article and each tile is 1′ x 1’… how often do you encounter a room that is exactly 5′ x 5′. It will probably be more like 4′-11″ x 4′-9-2/2″ so you are cutting the tile to fit along the perimeter.

In sort blanket trust that your square foot price for installion of a building system assembly may very well come back to haunt you and become a profit leak.

I can also recall years ago meeting with a contractor to talk about problems he felt he was having with his employees slacking on some projects. I saw in one specific incidence where he used a comprehensive SF price for building wood decks which ignored that the size and shape of the deck dramaticaly affected the linear footage of railing the deck would require and also didn’t account for differnent railing designs being more time intesive per LF than others. It turned out it employees weren’t slacking, he was instead a terribly weak estimator.

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J. Jerrald Hayes
Primus Inter Pares at Paradigm Projects, Ltd.
I am an architectural woodworker and general contractor turned IT, Business and Project Management consultant, software developer wannabe senior division triathlete and ski racer, Yankee fan and founder of, 360 Difference, and now too.
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