To shingle on top of existing shingles there are a few things you want to make sure. 1st and foremost your existing roof needs to be in reasonable shape. No missing shingles, no gaps, no holes in the roof, no soft spots because of water damage. If you have ANY of these things then you need to REPAIR THEM FIRST. Dips, gaps and holes will cause your new roof to deteriorate very quickly and fail, so don't cheap out, fix the problems.
OK, enough said about that. Next thing to do is to calculate your roof area in square feet (sqft). The best way is to get up there with a measuring tape write down length and width in feet of each roof area. The area is length x width. So for example if one area is 30 x 60 then that section is 1800 sqft. If you have some triangle shaped areas then measure the 2 short sides of the triangle then multiple and divide by 2. For example 20 x 40 triangle is 800 divide by 2 is 400. You roof will be combinations of rectangular and maybe some triangular shapes. Calculate all the areas and add them up for your total roof area. Once you know how many sqft your roof is you then need to figure out how many bundles of shingles you will need. 3 bundles does 100 sqft. A full skid is 42 bundles and covers about 1400 sqft. The other parts you will need are valley flashing for any valleys where 2 roof sections meet and form a valley where water flows to the eaves. You will easily see these on your own roof as they will be the only sections not covered by shingles but will have metal (aluminum). You will need starter strip material for all the edges of the roof. Last you will need ridge shingles for the tops of all the roof sections where they meet and form a ridge. Measure all the existing ridges, measure all the valleys and measure the edges where the eavestrough is to determine the amount of aluminum flashing, starter strips and bundles of ridge shingles. You will also need roofing nails, I recommend a roofing gun which runs off a compressor for any job of more then a few hundred square feet.
The shingles you will want to be as accurate in your calculations as you can be. They weigh a ton and you will want them delivered as close as possible to where you will be bringing them up to the roof. Most shingles now are longer lasting higher quality shingles that weigh about 75 lbs per bundle. Hauling that up a ladder is UNSAFE and not recommended. If you're a gorilla and can handle it I still say it's not safe. Do not have them boomed up onto your roof as 42 bundles at 75 lbs each is more then your roof can handle.
Step 1: Tools You Will Need
There are a few specialty tools that will make your job easier but you can make do with hammer, roofing nails, metal straight edge and utility knife with a HOOK blade for cutting shingles and starter strips. Make sure you have extra blades. If the shingles are cold they are harder to cut and you will go through more blades.
To remove existing ridge shingles some kind of pry bar or wrecking tool like the one in pic 2 will make the job easier. A compressor powered roofing air nailer. I bought one since renting 1 for a week was almost as much as buying a new one. $125 to rent and $160 to buy.
If you are doing any valleys then you will need metal snips, a caulking gun and several tubes of roof repair.
The yellow thing is an eaves trough saver. This will prevent your ladder from crushing or denting your eaves trough.
Step 2: Getting Ready to Shingle!!!
So here are my shingles, my boxes of starter strip (33 ft in each box), scaffolding to make the job of getting material onto the roof much easier and my great shingle elevator.
As you can see I have the skids on top of each other so I can stand on top and easily transfer each bundle to the scaffolding. Then onto the scaffolding to move them onto the roof. Once the distance between the top of my material and the scaffolding is too much I will use the lift. If I had a winch on any of my vehicles I would have used that but this works as well. Those pulleys are from a clothes line and there is a 3rd pulley attached to the bottom of the scaffolding. If anyone out there has a better way to get shingles onto the roof please make a comment & I will update this section. Shifting bundles of shingles is by far the worst part of shingling.
There are 2 types of shingles you will need. Regular roofing shingles and ridge shingles. The last picture is a ridge shingle. They are flat, no contours and perforated at the slits so you can easily separate the pieces.
Step 3: Remove Ridge Shingles, Place Starter Strips and Install Valley Flashing
If you are planning to do the roof all in 1 go with helpers and no rain in the forecast then remove all the old ridge shingles and nails. If you are doing you roof in stages, as I am, then you can peel back the lower parts of the ridges as needed.
The starter strip is very easy to install, it just rolls out and you can pull the back from under it as you go along. Make sure the tar edge is at the roof edge, slightly overhang the previous shingles. I do the starter strips in sections as I need to and then immediately install my 1st line of shingles to lock it down.
The valleys are installed right on top of the old ones using aluminum nails or staples. Only tack down the edges of the valley flashing, do not nail or pierce the flashing anywhere else. Begin your flashing at the lower edge working your way up the roof. I used a 50 ft x 12 inch roll and cut it as needed. There are also 10 ft sections that are available and perhaps easier to handle for some people. Minimum width for valley flashing is 12".
When shingling over the valley flashing you can precut the angle or cut in place. I prefer to cut in place. Be careful when you are nailing the shingle in place that you do NOT pierce the flashing, nail close to the edge but never into the flashing. You will want your shingle to overhang the flashing by at least 2". Trim the tips of the shingles to ensure that any water hitting the edge of the shingle is directed back into the valley.
Step 4: Shingling, Shingling and More Shingling...
So not the grunt work. Hauling hundreds of pounds of shingles onto your roof, laying them out and nailing them down. Since you are installing on top of an existing roof there are contours to deal with. If you try and nail between the top edge of the lower new shingle and the bottom edge of the old shingle when they are close together your nail will most likely just punch through your shingle. So try and install your nails into the old shingle edge or at least an inch below the edge. You can place your nails anywhere in the upper portion above the line. Try to nail closer to the line, but never right up against it, 1" above minimum. 3 nails per shingle is sufficient. A roofing nail gun will make this job much easier. I tend to lay out an entire line of shingles then nail them all down. The nails in the 1st pic are about as close to the line as you want to get.
As you get close to ridges, edges, flashing, etc you will need to trim the shingles. Start a line with a full shingle and trim the last one. Use a metal edge and your utility knife with the hook blade. I put and extra nail at the top of the shingle I'm about to cut to help hold it in place. Then start the next line from the opposite side with a full shingle and trim the other end. Keep going like this, back and forth, to complete a section. If you have completed a section that needs a line of ridge shingles then break these apart and start at the lower end of the ridge. If the ridge is a roof line then start at the end away from the prevailing wind direction. The 1st piece remove the bottom part below the tar line and use this as your starter piece. The bottom of a ridge shingle extends about 1 1/2" below the tar line.
If you are done for the day and have a ridge exposed, like the 3rd and 4th pictures. Cover the exposed ridge with a temporary cover by dropping a shingle over the ridge and lightly nail in place.The 4th picture has some old ridge shingles still in place with the last temp shingle inserted under the end of the old ones.
Step 5: Tricky Edges, Flashing, Roof Chimneys, Pipes and Vents.
So 90% of the roof consists of straight runs with a trim at the end of the run. The other 10% is pain in the butt trim jobs to get into, under and around the things on your roof. The 1st two pics are typical things on your roof, the 1st is an attic vent and the other is a plumbing vent (nasty smells come out of this). As you approach things sticking out of your roof you will want to have a good look at how they are installed. Typically the bottom edge of a ventilation flashing sits on top of the shingles and the upper edge is covered over.
The next series of pics are the removal and reinstallation of the rubber flashing/cover for a plumbing vent pipe. You need to carefully remove the ventilation or flashing piece then shingle to the opening. Cut the existing shingles and pry up carefully. Remove the cover and then nail the old shingles back down again. If you nick or tear the old flashing piece then go buy a new one. Install new shingles to the lower edge of the opening. Reinstall the flashing. A bead of roof sealant will go around/under the flashing edge as well. Pay very careful attention to these items as they are a primary source of water penetration and roof damage. Use lots of sealant as this is so much cheaper then a repair later on. Continue your shingles covering the upper portion of the flashing piece.
The next 3 show an attic ventilation piece and the sealant around the edge prior to the upper shingles being installed.
Step 6: More Flashing, Sealing, Trimming and Finishing Off Your Roof
Getting your shingles under existing flashing can be tricky. The next two pics are where a roof ridge meets another roof section. Make sure you get your shingles under the flashing and that you continue a ridge run right up to the edge.If you can easily remove or lift the flashing then do that. At least get your new line of shingles right up against the flashing edge and use your sealant, then drop the last trimmed piece on top and seal the top edge very well.
You will want to seal your shingles at the valleys using a double line of sealant. The 1st line up near the top edge of the valley flashing and the 2nd line closer to the lower shingle edge. Use a continuous strip of sealant to ensure no water will get out of the valley and under your shingles. When water is pouring down off your roof it can have a lot of force when it hits the valley so don't skimp on the sealant.
The final ridge line finishes the peaks. Always start a top ridge away from the prevailing winds whenever possible. As you can see we overlap the final line of roof shingles. When the ridge is installed you will have a triple thickness of shingles along the entire ridge.
I will also go along my lower roof edge after I'm done and use PL glue to fill any small gaps under the starter strip, between the old shingles and along the eaves where there are larger gaps visible. I will also finish with a bead of sealant along the entire lower edge to further lock down the bottom edge and prevent water or ice from getting under my shingles and into my roof. Over the years the original shingles can start lifting and exposing the edge flashing, especially on the sides that are exposed to prevailing winds and sun. In North America that would be South and West sides.