Author 
Message 
Anonymous
 Posted on Tuesday, May 16, 2006  05:38 am: 

I was wondering if someone could do some math for me. We are going to build a garage and will use slate remaining from the house slate job. The remaining slate is as follows: 1) 18" x 12" slates = 1835 of them 2) 18" x 10" slates = 2130 of them 3) 156 ridge slates What size of roof, squarefootagewise, can we make the garage roof? Thank you. 
TonyEriePA
 Posted on Tuesday, May 16, 2006  10:25 am: 

I hope this is right!? The 18" slates should have a headlap of 3" (or 4" if a low slope roof)which means they'll have an exposed (visible) length of ~7.5". The visible widths will be 12" and 10", respectively. The visible area for each slate will be the product of visible length times width to give you sq inches which, when divided by 144 sq inches (for every sq foot), will give you the area in sq feet for each slate (averages ~0.6 sq feet). Then you'll multiply that by the number of slate, excluding the ridge slate which you'll need the 156 or more elsewhere. So that'll give you 1146 sq ft of coverage with the 12" slate and 1109 sq ft of coverage with the 10" slate, so a total of roughly 2250 sq ft are possible, or roughly 22.5 squares, maybe 510% less to allow for starters, breakages and cutting, etc. That would be a mighty big garage if my math is correct! 
Anonymous
 Posted on Tuesday, May 16, 2006  10:38 am: 

Thanks Tony for the calculations. We put roughly 30 squares of slate on the home. I need to keep some slate for repairs, replacement, etc, but I don't know what the percentage would be for that. So out of your calculation of 22.5 squares, some of that must be held back. Any idea what percentage is used as a rule of thumb? 
TonyEriePA
 Posted on Friday, May 19, 2006  09:41 am: 

5% would be reasonable 
