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Factor for figuring roof area

G_Willey's picture

Hi guys. Several months ago I read in this forum in one of the posts a chart of sorts with factors to figure the area of a roof from the square footage of the floor directly under it. There would obviously be a different factor for each different pitch. I have searched the archives and cannot find it. I believe it was in a post with a heading not necessarily related to this specific subject. If anyone would know where this is or maybe have a chart like this, I would appreciate the information.

Thank you,
Red dog

(post #169945, reply #2 of 31)


Thank you, I believe that is what I saw. Maybe it was you who posted it. Am I correct in thinking that the first chart is for gable roofs and that the numbers under slope are pitches such as 4/12, 5/12, 6/12, etc? And maybe the second one is for hip roofs? Am I also correct in thinking that I would need to figure in the size of the overhang?

Thanks again for your time,
Red dog

(post #169945, reply #9 of 31)

Thanks guys. That's exactly what I'm doing, trying to get the roof area estimate off of the print for my new house for a material take off. It's a pretty complicated hip roof with a 10/12 pitch and these charts sure make it easy. Your input is appreciated.

Red dog

(post #169945, reply #12 of 31)

That'll do it, Pete. Muchas gracias.

(post #169945, reply #21 of 31)

I understand there are different ways to do things. I don't see what could be much easier with a print in front you than to take the house square footage, add in the overhang square footage, and multiply by the slope factor. Not too complicated at all. Thanks for the discussion.

Red dog

(post #169945, reply #26 of 31)


Kiss it?

Yup, I'm using a chart to ease an estimating task. It's hard to believe there are people like me, isn't it? And I'm not telling anyone where I'm keeping my chart.

Red dog

(post #169945, reply #30 of 31)

Hi guys. Several months ago I read in this forum in one of the posts a chart of sorts with factors to figure the area of a roof from the square footage of the floor directly under it. There would obviously be a different factor for each different pitch. I have searched the archives and cannot find it. I believe it was in a post with a heading not necessarily related to this specific subject. If anyone would know where this is or maybe have a chart like this, I would appreciate the information.

Thank you,
Red dog

(post #169945, reply #1 of 31)

Hey Red Dog,

I have a conversion chart here that may be of some use. I do not do roofing work, except for very small projects and repairs, so someone else may have a better understanding of the charts. I do know there are several factors that add to the total (dormers, eaves, etc...)

Here goes:

Slope --------------- Area/Factor

4 --------------------- 1.054

5 --------------------- 1.083

6 --------------------- 1.118

7 --------------------- 1.157

8 --------------------- 1.202

9 --------------------- 1.250

10 -------------------- 1.302

11 -------------------- 1.356

12 -------------------- 1.414

Here's one for valleys and hips:

Slope --------------- Hip/Valley Factor

4 ------------------------- 1.452

5 ------------------------- 1.474

6 ------------------------- 1.500

7 ------------------------- 1.524

8 ------------------------- 1 564

9 ------------------------- 1.600

10 ------------------------ 1.642

11 ------------------------ 1.684

12 ------------------------ 1.732

These came out of a roofer's book, and I must assume they are correct. Again, a roofer may have better info.

Just helping out...

James DuHamel

(post #169945, reply #3 of 31)

Red Dog,

I have never posted these charts before, but maybe whoever did can tell you more about them.

Like I said before, I am not a roofer. Aside from small roof work, and repairs, I do not do roofing. I am not experienced at calculating roof area so that I can be exact in determining how many squares are needed. On small projects that I do, I have no problem at all.

As far as the charts go, the first set of numbers is the pitch (4/12, 5/12, etc...) The second set of numbers is the factor to multiply by to get the actual area of roof. A perfectly flat 4' x 4' roof would be 16 sq ft. But, if that 4' x 4' roof was on a 4/12 pitch, the actual area would increase by multiplying 16 sq ft x 1.054

Again, you really need the expert advise of an experienced roofer for more detail.

Sorry I could not be of any more help here...

James DuHamel

(post #169945, reply #4 of 31)

< Obsolete Link > Here is a thread that I beleive answers your question. I was going to answer the question directly, but decided it would be easier to just post the link. Once you get the concept of "slope factor", you don't need a table. It works on hips, and gables. Whatever method you use, don't forget to add to the house square footage the area of the eve/gable overhangs.

(post #169945, reply #5 of 31)

I remember this thread from before, and I am still confused as to why on earth anyone would even want to go through all of those mathmatical contortions factoring with tables and roof pitches etc.I have the same type of tables gathering dust somwhere that I NEVER use ,nor do I know anyone who would use that table.

Use of the table implies PRECISION,but that precision will not likely be ACCURATE.I don't even understand why anyone would even want to know the pitch when figuring the area since you only need 2 numbers(LxW)Our dumbest roofers can figure that and they don't even need a calculator let alone a table.LxWx2sides plus 5%for starter,cap,and scrap and away you go.Even if the house is L shaped or worse,or the ridge is offset it is all just a few LxW problems.

It is incredible that you guys have taken something so simple ,that even roofers who count on their fingers can handle,and needlessly turned it into an exercise in higher mathmatics.Maybe I better find a PHD to help me with my roof estimates.

Thanks for the laughs,Stephen

(post #169945, reply #6 of 31)


I am not quite sure that I follow you, but I assume that what your are insinuation is that one gets up on the roof and measures it, and multiplies the LxW x2?

If this is the case, let me ask you how your method would apply to these 3 hypothetical (but very plausible) situations:

1) House is not yet built - what is the roof area?

2) Roof is on top of a 4 story building and has a 18:12 slope.

3) Roof is a hip roof with 3 gables.

Also, the tables/calculations produce estimates, not precision.

(post #169945, reply #7 of 31)

Steve -

Using your suggestion, that you simply measure roof area - then how would you estimate roofing material and labor on a house that hasn't been built yet ?

Seems to me that these formulas are quite valuable in a situation like that.

(post #169945, reply #8 of 31)

Hey guys, Stephen is right on. As for figuring the roof from the plans, you do the same thing, except you use your scale to measure the roof hight and width from the elevations,you don't figure the roof area from the floor plan. If you simply take the floor plan area and multiply by the "pitch factor" (or whatever technical term you want to call it) you have ignored the overhangs at the gables and eaves.

Matt G- I answered #1, #2-count the shingles up and across(make sure you know if they are metric or english), and for #3, how in the world is that table going to tell you the square footage, incorporating the dormers and valleys? Like Stephen, I know this isn't rocket science-it's just the simplest geometry you can imagine.


(post #169945, reply #10 of 31)

Matt,Although JRSpretty well covered it,I will elaborate....

1)If the house is not yet built yet,surely you have plans?Surely you are capable of reading those plans and determining the width of the roof to within 6 inches or so.Surely you are capable of determining a fairly capable dimension for the length from ridge to gutter.So now you have the 2 dimensions you need...WxL

2)The roof is on top of a 4 story building and has an 18:12 slope.......Once again what does the slope have to do with the area of the roof. I still need only the dimensions described in situation 1).If this is an existing building You can absolutely depend that I would be making a trip to the roof to gather the requisite info EVEN if it was available to me from as built plans.In this case there is apt to be a whole lot of factors more important to me than the roof area for me to consider when pricing this roof...Height,wind,neighboring buildings,Osha,Mechanical systems,Insurance factors,site access,traffic patterns,is it a tear-off,what building department will I have to deal with..... and on and on and on.In this case ,the area of the roof is of comparitively small importance.

3)Roof is a hip roof with 3 gables.....So what. Big deal. I still measure it like situation 1) or 2).The fact that it is a hip roof has no important effect on my calculations. The hip roof will consume slightly more caps and scraps,but the actual area seems to be less so in actual practice the the same basic calculation works fine.( LxWx2sides plus 5%)If the roof was L shaped or U shaped instead of a simple square or rectangle I simply break it down into a few easy rectangles for the purposes of my estimates.

The presence of dormers has no real factor in the roof area calc..The same approximate area will be covered.Of course the presence of dormers will have a big effect in the price I charge,but that is another discussion.

So you see Matt,the tables and charts will produce very precise answers,but that doesn't mean their precision is indeed accurate or even contains any information usefull in real world application.

By the way,I put MY money where my mouth is with this method 100plus times a year,year after year after year.On a 30 sq roof I might end up with a bundle or a bundle and a half left over.I can't recall the last time I was SHORT,. I would not trust a table to produce those results.

Really,you guys are trying to turn something a semi-literate roofer can do with a pencil on a piece of scrap lumber,into a form of rocket science.

I guess thats why I have my happy satisfied customers,but you have CLIENTS.

Thanks for the laughs,guys....Stephen

(post #169945, reply #11 of 31)

Red Dog,

It was I that posted the chart with the factors. The factors remain the same for gable or hip roofs. There is no difference between the two, area wise that is.


No need to be a whiney prick about it. I am sure most of us don't regularly use the table but it's nice to keep around for refence in certain situations. Hell, I can eyeball a lot of roofs and know how much shingle I need without even breaking out the tape. So stop laughing and just let the guy have his info. You said you have them too.

Pete Draganic

(post #169945, reply #13 of 31)

no problemo!

(post #169945, reply #14 of 31)

Using those "simple" factors on a "complicated" roof is sure to produce flawed results.


(post #169945, reply #15 of 31)

Couple of things, Stephen -

1. I don't like the smart-assed attitude of your post. We're all here trying to learn, and I didn't find the insulting tone your posting all that helpful.

2. Yes, I am quite capable of using a scale on a set of blueprints. But that assumes that the prints are correct, there are actually side elevations drawn, that the elevations are drawn
b correctly,
that you can see all the roof elements on the prints,the homeowner/contractor hasn't changed the roof pitch or configuration, added a couple of feet on the side, etc.

3. I'm glad you do 100 roofing jobs a year, and come out accurately on your estimates. But don't you suppose anyone who does it that often ought to ?

4. You aren't coming to the Breaktime get-together, are you ?

(post #169945, reply #16 of 31)

Ron,Red Dog,Pete,Blue Eyed Devil and anyone else I may have P.O'd,you are right. I am absolutely something of a smart -[JOBSITE WORD],and it is a serious charecter flaw. I admidt it.

However,since the topic under consideration is one I work at every day I take great interest in it.Now if I was to mention I wanted info on a VERY simple carpentry say coping quarter round shoe mold,andI proposed doing it in a needlessly complicated and unreliable method ,you would all let me know.And I assume that If I insisted that some way I read in a book was the bee's Knee's in accurracy and would point out to me that the real world application of the miles of quarter round shoe mold you have installed,.... has proved to you and every other carpenter you have met( and even their rawest apprentice) that there is a simpler,faster, more accurate method.And you would be just as incredulous ,as I am about THIS topic.

Also,....When I mentioned putting this method into practice 100 plus times a year I certainly did not mean to imply a scale of operations or an insulting manner.That figure ranges from the VERY small to some pretty nice sized jobs.I think I have been pretty open and honest over the last year or so about the types of work I do,how I price it,market it,and produce it.And I have several times pointed out that I am nothing special in this regaurd.

Ron,I think you are the person who mentioned that if I am estimating 100 some jobs a year then I should be pretty good at it,and not so smug.(I am working from memory here and my paraphrase may not correctly convey your meaning)You are absolutely correct about that. In fact,that is kind of the whole point isn't it.If I put my own money on the line so often each year,....every year,Don't you think a simple,effective,method would be in place?Remember we are talking about actual money here....the bottom livlihood,not theoretical results.If my method did not work,it would have been financially impractical and discarded a long time ago. Is not that the kind of advice you would want when faced with an situation unfamiliar to you.(what is the simple, effective,profitable way to accomplish this task....based on numerous tests of workability)

Blue Eyed Devil,...when you say using those simple factors on a complicated roof is sure to produce "flawed results" you are both correct and incorrect.The estimate may be a little light on one area of the roof,a little heavy on another area of the roof.what makes the method effective is that over the whole roof it all seems to average out.Also ,as I mentioned if the roof shape is somewhat complicated to figure,I simply break it down into a couple of simple,easy to grasp shapes.

by the way,I want to mention that my math skills are very,very ,very poor.( Almost as poor as my people skills)There is always the possibility that I have just been very lucky using a heavily flawed system all these years.Of course taking in to account my limited math skills and stunted roofers intellect....If my method works it must be pretty near idiot proof....It has sure worked for me.

Regaurding Pete's Summertime Slapfest.I have good news and bad news.

Bad News)I only live about 30 minutes or so away from Pete's my attendance, which would apparently offend so many of you,would be at least geographically possible.

Good News)I was not going to commit one way or the other untill a date was set since one of the dates will find me out of town already.If my invitation has been revoked ,of course I will stay away. but I will still wish you all good luck and lots of laughs.

good luck all,stephen

(post #169945, reply #17 of 31)


I was surprised by the response you got from Pete. I'll give him the benefit of the doubt on that one because it was his chart that got criticised. The "whiny prick" remark was totally uncalled for and I don't think there are many who read this thread that would agree with it.

As for Blue, I took his remark to mean he was in agreement with you. I don' think you pissed him off.

your analogy with the shoe mold is solid, and your remark about your "stunted intellect" was ironic. Anyone who is familiar with your posts knows that that isn't true.

I am amazed that anyone would want to measure some other object and use a conversion factor in an attempt to make those measurements closer to the original object. You waste your time measuring the wrong object and then add another step and another chance for error by adding one more calculation to your efforts.

Stymied, John

(post #169945, reply #18 of 31)


I meant "whiney prick" in a nice way. I was hardly mad at you you. It was more like a gee whiz, settle down thing. You were seeming a bit overreactive to his asking for the info. Actually I made the chart based on somebody else's info in part. When I made it, there seemed to be a lot of people who wanted it and I had too much time on my hands so I did it.

This is just another one of those cases where verbal tone would have said more than text. You can say the same thing ten differents ways with as many different meanings when you are talking in person.

You are invited to my house anytime you wish. Any weekend you might be in town I'd be glad to have you over and meet you.

No offense meant.


(post #169945, reply #19 of 31)

JRS,I would like to thank you for taking the time to stick your neck out and back up my position.Your writing and reasoning was much clearer than mine and I appreciate you being a stand-up kinda guy and stepping forward. Thanks again.

Pete,you and I have had discussions in the past(and probably will in the future).........Some issues we agree on,some issues we are totally opposite on............( like a lot of political issues). But I am always favorably impressed with your ability to come straight to the point and say exactly what you think----You do have a way with words. I can hardly complain that in this case I was the one being skewered.Please continue to say what you think,as you see fit....It is that kind of interaction among peers that makes this kind of discussion valuable(and entertaining)

By the way....I don't think there is actually anybody who would ENJOY being called a " whiney prick". But if I am not able to take a shot like that and keep coming,then I have no business being IN business.In fact if I was that thin skinned I would be better off taking a job in a factory and letting someone else make my decisions.

So anyhow,thanks again JRS....and harm,no foul

good luck all,Stephen

(post #169945, reply #20 of 31)


I haven't been monitoring this thread so I hadn't seen the heated discussion. Generally, I think that using your method of LxW of each inclined rectangle or the other method of taking square footage, adding overhangs, and multiplying by a factor that is function of the roof pitch yields essentially the same results.

Undoubtedly, your method is the way to go for someone who does it every day doing reroofs or otherwise working on already constructed homes, but for us who don't get on a newly constructed 18:12 roof or who do most of our estimates as part of entire building project takeoff, using tables or slope factors may be the most particle.

Besides, by reading this and some of your other posts, I kinda thought you were probably just having a bad day/week.

(post #169945, reply #22 of 31)

Reality check.....Using a chart "to ease" the estimating task...So you now have to keep track of a chart for the rest of your life!!!!! I call that stupid, iritating and more....I'll stick with my secret methods which somehow were copied by Steve...

Kiss it, by the stream,


(post #169945, reply #23 of 31)

........yegods...and I was just about to take a nap...

I used to do all those complicated things too, and then I hired a roofer as my foreman..he couldn't read or write...sure could roof (and drink)...he always came out right on the money ..but his labor estimates were all over the map...

anyhow....walk the roof whenever you can...or put a ladder up and take a looky- loo....

if you're doing takeoffs from prints measure the lengths half way up the roofs ..then you don't have to average the top & bottom... and measure the height of the roof from the elevation that shows the pitch..

if you're out in the field ...take your looky-loo but you still don't have to physically measue the can get a pitch viewer from your roofing supply (clear palstic card with pitches imposed on it and a base line) Owens Corning makes one , there's others.... you can count tabs and courses.. you can do a little of all the methods just so you don't leave any thing out (easiest is half the roof....oh yeh...)
I don't do near as many roofs as Steve, sometimes I get too cute and wind up a bundle short....that's expensive... always be over, not under.

get your area.....add 10% for gable and as much as 20% on cut ups for hips , ridges, waste etc

and , here's the one I always leave out...add your starter course (that's the bundle short)

digital cameras make life easy too....we don't walk slick roofs..

(post #169945, reply #24 of 31)

Nice image...what camera do you have?...I have yet to buy a dijital and am still shooting loads of jobsite film to the tune of $80 in processing recently....

near the stream,


(post #169945, reply #25 of 31)

......that's a Kodak DC215 Zoom..running about $350 with a 4meg can get 16 meg cards, and 32 meg.

my 4 meg at highest resolution gives me about 25 pics..and I down load to my computer with a card reader by SanDisk, that had the software incl.....then you wipe the card and start over....I thot I was pretty slick with a 1-hour Photo that was developing for about $6 a roll, but this beats hellout of that..and I still got my 35 MM for the pretty ones, just don't use it much because I can't manipulate the photos like I can w/ the digital....and I don't know squat about them yet....

anyway...goin past 300 shots now and process cost is ZERO !

(post #169945, reply #27 of 31)

I can't believe I am stepping back in on this but I forgot to mention a few things.If you use the method, which I now find that I un-knowingly stole from Adirondack Jack,you will also enjoy the following benefits.( Clearly Mike and JRS are already familiar with them)

1)Figuring the roof area as described also gives you the dimensions needed to figure Drip Edge consumed.We will usually have just under one stick left over.

2)You will know how many lineal feet of icegaurd you will need,and since you know how long your favorite brand is---you know how many rolls to order.

3)You will know how much felt is needed----AFTER taking the icegaurd into account.

4) If gutter is part of the job you will have an excellent idea of its lengths. Not exact,but definitely close enough.Supplying gutter at the same time as the roof is an excellent way to add several hundred dollars profit to your job with very little increased effort( You,your equipment,and your labor will already be on site and set up)

5)Instead of looking at the estimating process as an algebra problem,you have looked at it as an actual job situation.You have considered each roof plane and the logical ,efficient work sequences have made themselves apparent.You know which guys you want doing which roof section,you even know if it would be best not to do a particular area on certain days----like monday,or more likely friday.(You may even know that you want a certain guy to flash a certain area and that work would be done late in the day.You also know that same guy has to leave early each thursday to take his daughter to ballet,take his dad to chemo,meet his parole officer----whatever. You can plan around it)

Anyhow,all that information is readily available to you when using the same simple dimensions and a little common sense at no additional effort. Using the tables will still require you to go back and figure all of that out bit by bit.But I could be wrong.

Hey Mike,If you want to make sure that you don't run short of shingles(forcing you to run out and buy a bundle from a different color batch and wasting a lot of time) make sure your crew knows that if there is atleast one full bundle left over you will buy the crew a 12pack.The crew is usually a lot less wastefull and will save individual tabs for cap instead of just throwing it away.

Like it or not,beer can be an effective motivator.

good luck All ,Stephen

(post #169945, reply #28 of 31)

......spoken like a true but a 6pak...and not til we're standin around with our [JOBSITE WORD] draggin on the ground........

That guy told you he was goin to his parole officer...he was moonlightin for me....

We had one form company... except for the Owner and a foreman....every guy was always out on work release...he kept a tight leash on those guys...

But yeh... I use the measuring / estimating part to do the job in my head and figure my crews... and who's gonna do detail work while someone else is runnin courses....that 's why I never estimate using the man hour per square method , I just use it as a reality check....

(post #169945, reply #29 of 31)

...Steve... I just tried to send you an email to the address on your home page..and it came back...I wanted to ask you about skylights ..