Roof Computations for an Overlay and Preparations for the Job
If you are doing the work yourself, you need to order your materials. There is only one way to find out what you need and that’s to get up on the roof and measure. But first, if there has been a problem with leaking, go up in your attic and check for rotten sheathing (plywood or planks) and rotten rafters. If an area of the roof is bad, you’ll see it from the attic and won’t get any surprises (like falling through) when you walk on the roof.
The “feet” on your ladder should be anchored firmly in the grass at a distance from the wall equaling approximately one-fourth the height to be climbed. Don’t set up on a concrete sidewalk or patio if you can help it. A smooth, hard surface increases the chance that the feet will kick out at the worst possible time.
If you are using an extension ladder, pull the slack out of your rope through the pulley. Tie the rope to adjoining rungs on both sections of the ladder. If the rope used to raise the ladder is gone, tie two adjoining rungs on each section together securely. You don’t want the catches to fail and the extension to slide down with you on the ladder.
If your home has gutters, set your ladder up in the rear or on some other less visible section of gutter.
If the ladder scrapes or damages the gutter, it won’t show as much. Center the ladder over a spike and ferrule (a long nail and a spacer sleeve around the nail). This is the strongest point on your gutter. Make sure three or four rungs extend above the gutter. The additional length of ladder will give you something to hold on to when swinging from the roof back onto the ladder. On the first trip up, tie the ladder securely to the spike and ferrule to keep the ladder from shifting or blowing down. Hook a bungee cord over the spike and ferrule, loop it around your ladder, and hook the far end of the bungee back to the same spike and ferrule. Your ladder won’t go anywhere.
If your home doesn’t have a gutter, tie the top of the ladder off to a chimney, plumbing vent, etc. If nothing else is available, you can drive a couple of strong nails partway into the fascia (the edge above the gutters) and tie off to the bent-over nails.
PITCH OR SLOPE
Shingle roofs slope up from the horizontal. Roofers don’t talk about this slope as a vertical angle. They don’t say, for instance, that the roof is at a 45° angle; they’ll say the roof is a 12/12. The slope is measured by the rise (vertical distance) over the run (horizontal distance). Let’s say that in a 1-foot (12-inch) horizontal run, your roof rises 4 inches. This is referred to as a 4/12 roof. To find out what your slope is, hold the end of a bubble level against your roof surface and measure 12 inches. Take a vertical measurement down from the 12-inch point on your level to the surface of the shingles. For example, if your roof drops down (which is the same as rising) 5 inches in 12 horizontal inches, you have a 5/12 roof.
If your roof is steeper than 6/12, you will be smart to let a professional roofer do it for you. Professionals refer to up to an 8/12 as “walk-able,” but part of that is macho bragging. True, you can walk and work on such a roof without scaffolding. However, you can fall near the peak of a 7/12 roof, claw and grab as you slide down, and still pick up speed all the way to the ground.
Walk around on your roof, checking its general condition. The 2 x 4, 2 x 6, or heavier supporting rafters underneath your plywood or plank sheathing are probably set on either the standard 16-inch centers or 24-inch centers (measured center to center of the rafters). Walk the roof, checking for areas that give or feel soft underfoot. If you feel something giving too much, get your feet over the rafters quickly. If the sheathing gives way completely, you could find yourself back in your bedroom in a hurry.
BASIC ROOF LAYOUTS
There are five basic types of roofs. They are:
1. Lean-to: The lean-to roof is a sloped roof that rises up to tie into a wall. You see this type on screen porches, carports, etc.
2. Saddle: The saddle roof is a roof that rises straight up the front to a ridge line then drops straight down the back. The measurement of both legs of roof from the ridge is usually the same. The gables at the end of a saddle roof rise straight up from the end walls to a point at the ridge. Roofers call a saddle roof an up-and-over.
3.Hip : The main part of the hip roof is an up-and-over but instead of having gables, a triangular section of roof slopes up from the top of the end walls to the ridge of the roof. Some people call these “Dutch” roofs.
4.Gambrel: This is a roof having two distinctly different sloping sections on both the front and the back legs of the roof. The lower roof sections have a steeper slope, and the upper sections of the roof have a flatter slope. The gables on each end are vertical. Some later model townhouses have the gambrel roof with dormer windows projecting from the lower, steeper sloped section of the roof.
5.Mansard: A roof having two slopes on both front and back with the lower slope steeper than the upper one.
Make a “bird’s-eye” sketch of your roof. Show the complete outline of the roof at the fascia. I have basically drawn my figures to scale (1″ = 10′).
Your roof is sloped but draws it as if it were flat. Show all valleys, ridges, plumbing vents, pot vents, metal chimneys, brick chimneys, skylights, power ventilators, etc. Don ’ t leave anything out of the sketch, or you might forget to order something.
If you have a “straight up-and-over,” you are in luck. Your roof goes up the front and down the back with no additional wings, porches, or dormers to play with. This is the simplest kind of roof. Even if your roof is this simple style, you should make a sketch of the roof showing everything on it.
. The length of your roof along the peak or “caps” is 50 feet. Your measurement up the slope from the fascia (at the gutters) to the peak is 20 feet.
To figure the squares of shingles (10′ x 10′ of roofing) you will need, just add an additional foot to the measurement up the slope to cover the starter course along the fascia and the cap pieces. For the normal up-and-over roof, the measurement down the back slope (the other half of the roof) will also be 20 feet. Don’t ass/u/me this; measure it. Add an additional foot to the measurement of the rear slope too. The math is: 50* x (20 + 10 + 50′ x (20 + 1′) =2 x 50’x (21′) = 2,100 sq.ft.
2,100 sq. ft./100 sq. ft. per roofing square = 21squares
The standard three tab shingle comes one-third of a square to a bundle, or three bundles of shingles equal one square. 21 squares x 3 bundles per square = 63 bundles OK, we’re rolling, except that there will be a few mistakes, and some of the shingles will be damaged when you open the bundles. Let’s order an extra square of shingles just to be on the safe side. (You can always take your receipt and turn in any unbroken, undamaged bundles and get your money refunded, minus probably a 15 percent handling fee.)
So now what do we order?
21 squares + 1 extra square = 22 squares or
22 squares x 3 bundles per square = 66 bundles
Note: If you are ordering dimensional shingles, which more or less simulate the texture of a cedar shake roof, you will find that these shingles are heavier than standard three tab shingles and are packaged four bundles to a square.
Nail: If you haven’t measured and computed the area of your roof yourself, your roofing contractor can get you. He can tell you that you have a 30 square roof when in truth you have a 25 square roof.
He will give you his price for roofing 25 squares, while telling you that your roof is actually 30 squares. He knows you might want to do a quick check on him and may find another roofer willing to give you a rough price over the phone. You use your first roofer’s figures and report that you have a 4/12 straight up-and-over that is 30 squares. The second roofer’s quote over the phone is automatically going to be twenty percent higher for a 30 square roof than his quote would be for your actual 25 square roof. Your first roofer has edged out his competition, and you feel good contracting with the first roofer because his price is lower than the one you got over the phone.
You ordered an extra square because you don’t want to go back to your supplier for additional shingles. The bundles of shingles may all come with the same color code on the end of the bundles, “Star White” for instance, but one production run may come out cream white and another production run will come out chalk white. The production run number (or blend number) is on the end of the bundle below the color code number, or it may be on the opposite end of the bundle from the color code number, depending on the manufacturer. Tell your supplier you want not only the color code numbers but also the blend numbers on all the bundles to match.
If you get mixed blend numbers delivered to your roof, you won’t be able to tell the difference as you’re laying the shingles, but it’s very possible you’ll be able to see it from the ground. To put this in perspective, ask someone who knits what it would be like to use different dye lots of yam in one sweater. Then brace yourself for the answer.
Of course, if your supplier slips you a few odd numbered bundles, and you also have a section of roof nobody will ever see, just use your odd shingles on the hidden section.
If you order a 50-pound box of 172-inch galvanized roofing nails, you may have enough for your roof.
If your house is massive, you can always go back and get another box of nails.
If you already have a compressor and you want to buy a pneumatic nailer (also called a nail gun) for $400 to $450, here are some figures pertaining to a specific brand. Bo-stitch nails are made for Bo-stitch guns. A box of Bo-stitch coil nails will do twenty squares of roofing. There are sixty coils to a box, so each coil will nail 1h square of roofing.
22 squares x 1 box of nails/20 squares =1.10 boxes
In a case like this, I recommend buying a box of the Bo-stitch nails and finishing up nailing the shingles with a small box of regular hand-dipped galvanized roofing nails. If you have, for instance, a 32 square roof, it’s worth it to go ahead and buy two boxes of the Bo-stitch coil nails. (Suppliers won’t accept the return of a partial box of coil nails.)
Now let us suppose your house has a wing or is L-shaped. A triangular area of your main roof in the rear is now nonexistent: it is covered over by the two triangles that tie your wing into the main roof. This makes your math more interesting.
Main House: 2 x 50 x (20 + 1) = 2,100 sq. ft. The roof area of the wing is broken down into two rectangles and two triangles. (The area of a triangle equals one-half the base times the height: A=72BH).
Wing: [Area of two rectangles] + [area of two triangles]
[2 x 20 x (15 + 1)] + [2 x 72(15 + 1) x (14’8m)] = (Convert inches to hundredths of a foot by dividing by 127ft 8712” per foot = 0.66′)
 + [2 x 8 x 14.66] =  + [234.56] = 874.56 = 875 sq. ft.
Now, subtract the triangular area of the rear roof of the main house that is nonexistent due to the triangles of the wing tying in above. You can sight down from the ridge and “guesstimate” the base of the triangle that has been covered over. You can also measure up the roof “guesstimating” the height of the covered triangle.
[1/2(28’2m) x (15’8M)] = [72(28.17) x (15.66)] = 220.57 = 221 sq. ft.
So how many squares of shingles do we order?
Main house =2,100
Wing = 875
Minus area = < 221>
Total = 2,754 sq. ft.
Now after all this exactness, we need a fudge factor.
Notice you have two 20-foot valleys where the wing roof joins the main roof. No matter how you finish a valley, it’s going to gobble up a lot of shingles. With valleys that long and with the probability of some damage in shipment or of making one or two errors, you should order 2 extra squares of shingles.
2,754 sq. ft./100 sq. ft./roofing square = 27.6 squares
You will need: 27.6 squares + 2 squares = 29.6 squares
Remember that roofing comes in xh square bundles. 29.6 squares x 3 bundles/square=88.8 = 89 bundles
Now let’s suppose you own the two-story home. In addition to the rear wing, you have three dormer roofs on the front and a lean-to porch roof tying in partway up the left side of the back wall. There is a brick chimney. A sunroom with two skylights ties into a side wall.
First let’s decide what we are not going to worry about. Let’s figure the dormers as if they were three simple rectangles instead of two even smaller rectangles and two very small triangles:
[3 x (5 x 4)] = 60 sq. ft.
The two squares for the skylights are:
[2 x (2 x 2)] = 8 sq. ft.
The brick chimney is:
5×2= 10 sq. ft.
Three dormers: <60> sq. ft.
Two skylights: < 8> sq. ft.
Chimney: <10> sq. ft.
Total <78> sq. ft.
78 sq. ft. x 3 bundles/100 sq. ft. = 2.34 bundles
You already know that roofing the dormers —“cutting into” the walls of the dormers, skylights, and chimney—will take extra shingles. If you have small items such as skylights and chimneys on your roof, just figure the roof as if they weren’t there. If you have valleys, figure your areas and add a couple of bundles rather than trying to figure everything out to the nearest hundredth of a square.
Here we calculate the whole thing:
Main House: [2 x 50 x (20 + 1)] = 2,100 sq. ft.
Rear Wing: [2 x 20 x (15 + 1)] + [2 x 72(15 + 1)
x (14’8″)] = 875 sq. ft.
Minus: [72(28,2″) x (15’8″)] = <221sq.ft.>
Screened Porch: [25 x 25 + 1] = 650 sq. ft.
Sunroom: [two triangles + trapezoid]
(The area of a trapezoid equals one-half the sum of the two bases times the height A = 72(B 1 + B2) x H)
[2 x 72(14) x (7’9″ + 1)] + [72(30+ 16)x(14’H” + 1)] = [122.50] + [366.16] = 488.66 = 489 sq. ft
TOTAL 3,868 sq. ft.
3,868 sq. ft./100 sq. ft./square = 38.68 squares
Round it off to 39 squares = 39 squares
But remember, we added a couple of squares for the valleys on the back. We should figure another extra square for the dormers and skylights and chimney altogether. Since you are new at this and the roof is a bit complex, add another 2/3 square.
39 sq. + 2 sq. + 1 sq. + 2h sq. = 422/3 squares
422h x 3 b/sq. = 128 bundles
This is not an exact science, but you will be surprised how close you’ll come. The secret is always to figure to the high side, so you won’t come up short at the end of the job.
If you or your roofing contractor have a partial bundle left over, stash it away somewhere reasonably cool and dry just in case a shingle fails or is torn in a high wind later on. If you have a couple of unopened bundles left over, return them along with your receipt to your supplier. Remember, you can get a percentage (usually 85%) of your purchase price back.
VENT COLLARS OR FLASHING
Your plumbing has vent pipes that go through the roof to get rid of methane and other gases from the sewer system. You can see the vent pipes through your roof above the kitchen, bathrooms, and laundry room. The plumber may have run one vent to adjoining bathrooms or say a laundry room and kitchen. These pipes stick up several inches to afoot above the roof.
The standard vent flashing is a neoprene (rubber) collar mounted on a galvanized metal base. These are the easiest to install; you just slide them down over your vent pipe and weave the base right in with your shingles. However, there are problems with these neoprene collars. Many of them fail before the roof needs to be replaced. This allows water to penetrate the roof. You can use them if ten years from now you want to start getting on your roof occasionally and caulking around them with roofing cement (asphalt mastic) or silicone caulk.
Commercial all-lead flashing is the best option. It consists of a 12″ x 12″ flat lead base with a lead pipe soldered solidly around the hole in the center of the plate. This type is slightly more expensive and a little more time-consuming to install, but it will outlast your new roof without requiring any additional maintenance.
Plumbing vents are measured by their outside diameter. A 2-inch vent has a 2-inch outside diameter. Okay, let’s suppose on your home you have a 2-inch vent and a 3-inch vent.
All-lead vent flashing:
1 2-inch collar = 2 x $ = $
1 3-inch collar = 1 x $ = $
If your house has valleys, you need to pay special attention to them. Valleys are one of the most leak-prone locations on a roof.
If you have an aluminum or copper valley which has been trouble free, you will probably want to reuse it.
You can also “double weave” the valleys, alternating the shingles as you lay up both of the roofs on the main house and the wing at the same time. You can “double weave” or “California Cut” right over an existing copper or aluminum valley.
The California Cut means you lay the main roof and extend the shingles through the valley from one direction. You then run the shingles from the other roof through the valley also and cut the second, or top, layer of shingles off along a chalk line.
More details on these methods later. Reusing the existing metal valley is the most durable and best looking way to do your valleys. Double weaving may give a slightly rougher appearance than the California Cut, but it is considerably stronger and less likely to give trouble.
Nail: Most roofers will do your valley with a California Cut regardless of w hether your metal valleys were giving trouble. It is the quickest and easiest thing for them to do. If it should leak or blow loose, your old metal valley is still in place and will work, assuming your roofer didn’t put any additional holes in it.
If you plan to reuse your existing metal valleys, you will need a caulk gun and some tubes of roofing cement to seal the cut edges of the new shingles on both sides of the valley to the old shingles beneath them. Don’t drive a bunch of nails into new shingles in the area of the valley. You will need a tube of roofing cement for approximately each 15 feet of cut edge you need to seal down.
Roofing cement 2 valleys x 20′ valley x 2 sides = 80’
80715′ per tube = 5.33 tubes = 6 tubes of roofing cement
From here on, roofing cement will be referred to as mastic.
Step flashing would also be required where the triangular sections of the sunroom tie into the wall. The standard piece of step flashing is a piece of 5″ x 7″ aluminum, which is bent L-shaped to give 2-inch and 3-inch legs to the 7-inch piece. More about step flashing later, but now you need to know how much to order. For every 5-inch course of shingles, you need one piece of 7-inch step flashing. It is easiest to figure the number of pieces per foot:
12″/foot/5 “/course = 2.4 courses/foot= 2.4 pieces of step flashing/foot
We will tie in two 7’9” lengths of wall for the sun room:
2 x 7.75′ x 2.4 step fl./foot = 38 pieces
Step flashing along the sides of the chimney:
9 y V y 9 4 ctp.n fl /foot = 24 nieces
Step flashing along sides of skylights:
2 x (2′ + 2′) x 2.4 step fl./foot = 20 pieces
Step flashing along the walls on the sides of the dormers:
3 dormers x 2 walls x 4.5′ x 2.4 step fl./foot = 64.80 pieces
Total =147 pieces
If you are using aluminum, these pieces are pretty cheap. Go ahead and order 175 pieces of step flashing, also order 1-inch aluminum siding nails. (The lV2-inch aluminum nails bend easily when you are driving through heavy stuff.) Aluminum nails are best, because you don’t want to mix dissimilar metals. Electrolysis between dissimilar metals speeds up corrosion.
The vertical leg of step flashing slides into place behind siding. On a brick wall, it goes behind the counter flashing or what I call “skirt” flashing. Skirt flashing is sealed along its top edge with caulk. To make skirt flashing, you will need a roll or “coil” of aluminum. Coils come in 50-foot lengths and are either 12 inches wide or 24 inches wide. The coils come in two different gauges (thicknesses). The heavier .027 gauge (.027 of 1 foot) costs a few dollars more a coil and is preferable to .019 gauge.
These coils are also used to make metal valleys. If you are only going to use the aluminum along your walls or chimney, 12-inch coil will probably be sufficient, depending on how high the existing skirt flashing is and what old caulk will be left visible when you replace it. Retrace what we did on the step flashing and see how much coil you need. Add up the skirt flashing along porch walls, sides of chimney, sides of skylights, triangle roofs on sunroom, and sides of dormers.
15 + [2(7*9″) + 16] + [3(4) + 6(3)] = 76.5′
In this case, you would round up to 100 feet because the coils are 50 feet long. You also have 2 20-foot valleys. Consider purchasing three 20-inches wide coils (150 feet).
Having done your own computations, you go to the roofing supplier with a complete list of materials. This tells the roofing supplier something. He knows that you know what you are talking about, and he will treat you differently than he will a homeowner who obviously knows nothing about what he is doing. Remember that he doesn’t know you; he’s going to quote prices in the higher range. Let him know you are shopping with a couple of suppliers and then do so. You are doing the hard work any- way, and serious bargaining can save you hundreds of dollars.
When you pick your supplier, he will want to be paid in advance. There is certain to be a delay even if all the material is right in his warehouse. He will want your check to clear before he delivers anything
to your roof. A cashier’s check can speed up this process. This is a little inconvenient, but the supplier is a battle-scarred veteran too.
Suppliers may refuse to deliver to your roof, or “load the roof,” if the pitch is over a 6/12. There are two reasons for this refusal. The first is the danger of bundles sliding off the roof, doing damage to gutters and shrubs, and the liability if they should fall and hit someone on the ground, causing serious injury or even death. The second consideration is the increased risk of falls to his men.
Your supplier won’t deliver to your roof if cars are blocking your driveway, if he has to back up onto your grass, or if tree limbs or power lines prevent him from swinging his conveyor boom around from its cradle over the cab of the truck.
There are proper and improper ways to load the roof. The shingles should be in smaller stacks, spaced out on your roof. Each stack should contain no more than sixteen bundles (1,200 lbs.) Putting more weight than that in one spot on your roof is courting disaster. There is also a proper way to stack the shingles. The delivery person should lay one bundle just below and horizontal to the caps. The next bundle should rest with one edge up the roof and the other edge resting just over the top edge of the first bundle. This “locks” the two bundles in place. Two bundles are laid in the same fashion on the opposite side of the caps. Three bundles are then laid across the caps and rest on the upper surface of the two “locking” bundles. The first bundles and locking bundles should be spaced so that the first layer of three bundles bridges across the cap and lies flat on the locking bundles. The second layer of three bundles should then be laid lengthwise along the caps over the first layer. Alternating the direction of the layers of shingles “locks” the pile. Four three-bundle layers over four locking and leveling bundles is plenty of weight in one place.
[(4 x 3) + 4] bundles x 75 lbs./bundle = 1,200 lbs.
It would take an extremely high wind to displace bundles stacked in this manner.
Nail: An uncaring supplier or contractor can nail you by stacking your shingles wrong. The shingles can be stacked by just piling all the bundles across the cap and letting the bundles bow down into a U- shape. It is tougher to lay these bowed shingles and get a nice finished job.
There is an extra fee for loading the roof as opposed to stocking everything on the ground. In our area the suppliers charge $1.00 to $1.50 additional per square.
66 bundles x 75 lbs. per bundle = 4,950 lbs.
4,950 lbs/2,000 lbs./ton = 2.48 tons
It’s worth the $22.00 to save lugging two and a half
tons, 75 pounds at a time, up a one- or two-story
ladder. If the supplier doesn’t have a way to deliver
to your roof, find another supplier.
Work to Do Prior to Roofing
You may have work to do before you lay the first shingle. Do the dormers or divider walls above sections of your roof need to be painted? Paint first and save yourself the risk of splattering your new shingles. Also, paint any metal chimneys or metal vents prior to roofing. (Auto supply stores have aluminum and black “high heat” paint in spray cans. It stands up well on the outer metal surface of double-wall metal chimneys.) Painting before you lay the new roof means you will overlay or tear off any overspray or drips.
REMOVE OVERHANGING LIMBS AND TREES
Remove limbs that can damage your roof during high winds or snows. Remove any leaning, dangerous trees before you reroof the house.
Cupolas are small, usually decorative, venting structures. Remove, repair, and paint the cupolas. If the cupola is functional and removing it leaves a hole in the roof, patch the hole with plywood and roofing felt or polyethylene. Replace the cupola when the new roof is complete.
CLEAN GUTTERS AND DOWNSPOUTS
Cleaning gutters and downspouts can be a messy job. Go ahead and give them a basic cleaning before you begin laying shingles. An overlay will narrow the space between the lower edge of the new shingles and the outer lip of the gutter. This will make a gutter full of leaves and dirt harder to clean. If you are going to tear your roof off, it is still a good idea to clean the gutters first. You want the gutters to catch as much stuff as possible as it rolls off the roof.
You will be amazed at the nails, grit, and pieces of roofing that the cleaned gutters will catch. You’re going to clean the gutters when the job is completed anyway, and it’s much easier to clean the gutters a second time than to pick nails and other trash out of the flower borders and yard.
If you have a brick chimney, check its top. Inspect the concrete chimney cap around your flues carefully. If the cap is cracked, use silicone caulk or roofing mastic to seal the cracks. If the cap is shattered or loose, chip it off with a hammer and chisel. A premixed concrete such as Sakrete is good for achimney cap. Add water and mix it a part of bag at a time in a wheelbarrow. Go easy on the water; make the concrete thick or stiff, not soupy. Carry the concrete up the ladder a couple of gallons at a time. (A 5-gallon bucket full of concrete is too heavy to fool with on a ladder.) Trowel the new cap in place. Taper it down from the flue liners to the outer brick faces of the chimney. Keep the concrete moist as it cures over the next twenty-four hours.