How much Finish to finish?

Well, as you can see the guys have been hard at work applying finish to the Roof Rafter assemblies and Floor Joists. They gave them two coats and it took 5 gallons. What have they been learning?

The finish really sets off the look of the rafters and doing it now before they are installed really is a labor savings. We will be finishing the underside of the roof boards before they are installed. Likewise the lattice trusses will be finished before they are assembled also. 

This step in the process presented a teachable moment for the guys.

We discussed finish coverage per gallon and project cost estimating. Along in the process I have been sharing with them the costs of materials going into this project. They understand and appreciate that this finish sells for $25 per gallon. In reading the label they found out the coverage listed as a range of 250 to 350 sq. ft. per gallon depending on surface texture. The question I put to them was could they "apply" what they have been taught in school to "learn" from their "actual experience" what the real sq. ft. coverage per gallon is for this project material? Then taking it a step further calculate with a degree of certainty the number of gallons that will be required for the entire bridge? My early estimate called for a total of 16 gallons. That is not considering the change in the roof covering. Will that be close?

They were up for it!  Easy answers, "a lot" and wild guesses "30 gallons" filled the air. As I pulled out my note pad, pencil and calculator you could see their body language change. They knew I meant business and the "serious fun" was about to begin. In the picture they are sitting on the 3" x 6" x 10' long floor joists. We started with these. They were like Deer in the headlights but we plowed forward.

The number of square feet of surface to be finished for one Floor Joist is.... Make your best guess.
  5  -  10 -  15 - 20 - 25 - 30 - 35 - 40 - 45 - 50 - 55 - 60 - 65 - 70 - 75 - 80 - 85 - 90 - 95 - 100 Sq. Ft.

The common mistakes were made right off the bat. Calculating volume, not changing the units so they were all the same and dividing by 12 not 144. If you guessed 15 sq. ft. of surface area per Floor Joist you are right! You have it made! You have a "gift" and for you coming up with the total number of square feet of surface area for the 6 Roof Truss Assemblies and 10 Floor Joists in the picture will be a snap. For the rest of us mere mortals some pencil and calculator work will be needed.

If you want to "play" along here are the numbers will need to know, or what's your guess?
400 - 425 - 450 - 475 - 500 - 525 - 550 - 575 - 600 - 625 - 650 - 675 - 700 - Sq. Ft. area total.

10 - Floor Joists - 3" x 6" - 10'
12 - Primary Rafters - 2" x 6" - 9'
12 - Secondary Rafters - 2" x 6" - 7'
12 - Collar Ties - 1" x 6" x 4'
6 - Cross Ties - 2" x 8" x 11'-6" (just had to do it :-)

If you calculated 579 total Sq. Ft of area we agree.  Note: Building this calculation into the spreadsheet would be a great application for the business students.

So applying two coats on rough sawn material they are getting about 116 Sq. Ft coverage per gallon.

Puzzler - In the original estimate I called for 16 gallons of finish for the entire bridge. Could you figure out what the actual number of gallons needed will be?

Tech Vocab - Perimeter - Area - Coverage