PressurePros 248 Report post Posted April 29, 2007 For those of you that have done the undersides of decks, what is an accurate formula for computing s/f? I use 2.5 as a multiple of top surface. For example on a 1000 s/f deck underside would be calculated at 2500 s/f for a total of 3500 s/f. Close? Share this post Link to post Share on other sites
0 seymore 90 Report post Posted April 29, 2007 2x8 joist...1.75"+7.75"+7.75"/12=1.44 Sq"s FT X 12' joist X20 Ea. = total sqft of joist. that's my answer and i'm sticking to it! :cool: Share this post Link to post Share on other sites
0 MMI Enterprises 289 Report post Posted April 29, 2007 Wow..big difference between you two....Does that 2.5 factor include the PITA factor for them being raised decks? You by chance got that powerpoint for me Ken? Share this post Link to post Share on other sites
0 seymore 90 Report post Posted April 29, 2007 Wow..big difference between you two.... I am figuring the surface area x 2 + sqft of joist cause .5 doesn't add up to the total joist sqft. Share this post Link to post Share on other sites
0 MMI Enterprises 289 Report post Posted April 29, 2007 Shane, + .5 seems fair based on your actual measure method but do you add a Pita factor at some point. Almost have to assume that is what Ken is already including... bueler, bueler..where is Ken?. probly staining or some such.. Share this post Link to post Share on other sites
0 Beth n Rod 1,279 Report post Posted April 29, 2007 Joists are joists, underside of floor is underside of floor...two separate measurements. Beth Share this post Link to post Share on other sites
0 seymore 90 Report post Posted April 29, 2007 Joists are joists, underside of floor is underside of floor...two separate measurements. Beth Exactly and it's easy to lose your BUTT on all that joist surface area! Share this post Link to post Share on other sites
0 Beth n Rod 1,279 Report post Posted April 30, 2007 I just add 65% to the sqft to account for the joists. ie. 200sqft top measure + 65%(130) = 330sqft Rod!~ Share this post Link to post Share on other sites
0 Jarrod 22 Report post Posted April 30, 2007 I would tripple the floor price and hope they say "no." I will never do undersides. Share this post Link to post Share on other sites
0 PressurePros 248 Report post Posted April 30, 2007 I just add 65% to the sqft to account for the joists.ie. 200sqft top measure + 65%(130) = 330sqft Rod!~ Hey Rod, I think ou are cutting yourself way short. If you lay every joist butt to butt that has to be at least 1.5 times the s/f of the top surface. Then add in the exposed underside between the joists and that is where I get my number. MM, I don't want to factor in difficulty I just want to estimate stain correctly for a customer. Lets do a 20'x20' (400 s.f) deck to make life simple. Joists are what 16" OC? So a on a 20 ft span, thats 15 joists. If the joists are 2x10's.. lets call it 1.75 s/f per LF of joist. Times the 20 foot span you would get 35 s/f per joist times 15 joists. Thats 535 s/f in joists. The remainder of the underside is the same s/f as the top minus maybe 20 s/f where the joists mets the floor boards. So now we have 915 s/f under there. Now add few 6"x6"x8' posts and you get another 90-100 s/f. That's 1015 s/f of wood to stain. Top 400 s/f of floor Underside 1015 s/f (or about 250% of top) Total: 1415 s/f counting rails Share this post Link to post Share on other sites
0 PressurePros 248 Report post Posted April 30, 2007 Joists are joists, underside of floor is underside of floor...two separate measurements. Beth Beth, the problem is, I only have pictures and a total s/f measurement with which to work. Share this post Link to post Share on other sites
0 Greg R 82 Report post Posted May 1, 2007 I would tripple the floor price and hope they say "no." I will never do undersides. :) I'm with you Jarrod..... undersides are a PITA. We've have half a dozen or so in the last 11 years. Spraying them makes life easier but its not always possible. Ready Seal makes them far easier though. Share this post Link to post Share on other sites
0 PressurePros 248 Report post Posted May 2, 2007 Same boat Greg. I talk everyone out of it. This for a customer that is ordering product (stain) from me and insists on doing it. Share this post Link to post Share on other sites
0 Beth n Rod 1,279 Report post Posted May 2, 2007 Yeah, I know, but given the fact that the extra cost in itself is enough that most people don't really want to have it done once they hear the price. In all, we have done literally 8 decks on the underside and not all of them were sealed as well. Fact is, most decks underneath are not really that hard to clean and pretty easy to seal so I don't feel we are taking that much of a loss since we are there to do the rest of the deck anyway. It's just an add-on that most do not worry about. Rod!~ Share this post Link to post Share on other sites
0 MMI Enterprises 289 Report post Posted May 2, 2007 Well then hey folks, what do you sell them on?... Am thinking brighten and RS, or full on strip, sand, prime, solid stain on bottom with semi on top. The PITA part must be if there is old coatings under there they don't like to come off near as easy as up above in the weather and sunlight. Share this post Link to post Share on other sites
0 plainpainter 217 Report post Posted May 2, 2007 Ken - I have a degree in Mathematics and physics - although not needed for this calculation - but I did a quick calculation of 2x10's spaced 16" center to center - this doesn't take into account the outside 'skirts' area which may be the two outer joists - just what's 'underneath and out of view. And as well it doesn't take into account the bottom 'butts' of the joists since that area just replaces the area that the joist is covering up the bottom side of the decking boards. Anyways that being said this is area is 1.25 times the 'floor' area. So add 1 to that figure for 2.25 to get the total area. I like 2.5 - gives you extra fudge factor. Share this post Link to post Share on other sites
0 MMI Enterprises 289 Report post Posted May 2, 2007 So far I'm calling the inside joist factor as 1.56.. Please tell me if this example is wrong-- 1k deck (20 deep/50 long): 50/12=600"/16" center=37.5 joist .. Now subtract 1 joist worth of the left and right outer joist (counts as outer trim face) = 36.5 joist 2x10's- 1.75+9.75?+9.75=21.25"/12=1.77' per lnft. x 20' deep joisting=35.416' x 36.5 joist= 1292.684 under/inside of front trim = 9.75/12=.8125 lnft.x 50' deck length=40.625' +another 40.625' for inner face at house=81.25 + joisting=1373.934 How much more is missing in the way of joist seperator material I do not know...but woudn't it be a few more of the 1.77lnft.x 50' = 88.5' x 3? = 265.5? +1292.684=1558.184' in joisting top deck- 1k underneith- 1k joisting- 1559' =3.56k not counting posts, railings, steps, landings, trim face Share this post Link to post Share on other sites
0 MMI Enterprises 289 Report post Posted May 8, 2007 For 2x8 the factor is closer to 1 : ..to correctly figure a 2x8 10' long you have to first convert the underlying inches ( 1.75 x 7.75) into decimal.... .14583 and .6458 . So then take 10' x .6458 x 2 sides =12.916' + 1.4583 exposed edge x 10' = 14.3743' per joist. A 200 sq. ft. deck- 20' x 10' deck having about 14 joist would be 201.24 sq. ft. of joist which is almost same as top or bottom sq. ft. Share this post Link to post Share on other sites
For those of you that have done the undersides of decks, what is an accurate formula for computing s/f?
I use 2.5 as a multiple of top surface. For example on a 1000 s/f deck underside would be calculated at 2500 s/f for a total of 3500 s/f. Close?
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