Get 3 or 4 Roofing contractor Estimates
As a Roofing contractor myself I'd say get as many estimates as possible, they are free. Rotted wood can be scary and I'd say unless you've had many years experience, stay off the roof. I've fallen through rotted sheeting before. A barrel roof is not going to be any easy job for the un-experienced person or even roofer. If you insist on getting on the roof, first thing to do is establish a fall protection system by bolting an anchor on the peak of the roof and connecting a lifeline to the anchor and the other end to your body preferably with an Osha Approved harness and rope or strap. Don't play with your life, its too easy for things to break loose and your gone. Anchors are cheap, harness and rope $75. Preferably bolt to a beam if possible, after roofing you can re-install and flash around it so its always there if you need it later.
Rotted wood seems to always leed to more rotted wood. Rotted plywood leeds to Rotted rafters or truss's. Guess what? You can't nail new wood to rotted wood, then the rafter needs repaired or scabbed...so then it all starts adding up quickly. As said before, sometimes you never know until the old roofing is off. Sometimes a heavy loaded roof can be removed and the plyood bows from the stress of the weight being removed. Wood is flexible.
As far as the elastomeric coating goes, I have used coatings commercially for about 15 years. Todays coatings are better than ever if used to the manufacturers specs. Coatings are great for .5 to 3 Pitch with proper drainage. In this case I'd say its a no go, you could probley find one that would do the job but the cost would be outrageous for the life you'd get...In other words, not cost efficient. You'd think some of these coatings are made of liquid gold with the prices they charge, I've seen some costing $300/Gallon.
If it were mine I'd probley strip the roof, fix all rotted wood and then maybe install a 25 - 30 year Architechtural shingle roof.
Its all exspensive, anyway you go, Sometimes you can get alot for the old shingles as mentioned before, do some research on the web to find a buyer or even run an auction on ebay. Sometimes you can't get alot for them and it might not be feesable to handle with care when removing them because it takes alot more time to stack them, clean them, store them and ect. You never know. We charge $100 for the first sheet of plywood replacement and $65 per sheet afterwards on a standard roof for 1/2 inch plywood. We replace 1/2 sheet minimum sizes since replacing a whole sheet for a small rotted corner would be silly and a waste of resources, instead of replacing the whole sheet we'd just replace half. Sometimes depending on the structure a whole sheet may need replaced if just the corner is rotted but only in very limited circumstances. Even if just a half sheet is replaced, it is the same charge as a whole sheet. We usually charge $2 - $5 per linear foot when scabbing a truss, since alot of times you may need to remove even more plywood to install it. It can get to be a big circle, this supports that which supports that which supports this. On a barrel roof it may be more difficult to scab rafters with the arch, just depends where the ones that need scabbing are and if you'll need scaffolding to work from the inside or can it be reached without scaffolds or ladders. Make shure plywood clips are used if possible, they let the plywood exspand and contract without bowing later on, if not possible to use them..leave a 1/8" gap between sheets, do not butt.
After all wood replacement is made.....
I think your cheapest route is going to be a 35-30 year architechtural shingle.
First install self adhering storm shield directly over the clean plywwod decking for a good bond, starting from the bottom going up overlap each following row (going sideways) 6 inches over previous row and then at the end laps.
Bring up both sides (right side building roof and left side building roof) till you get to the top middle then run your last row down the middle on the peak overlapping both of the last right and left side runs. Storm shield will seal around any nails you put in after this step.
Then install 30 lb. roofing felt in the same manner as the Storm shield using
1-1/4 incch roofing nails..hand nailing the felt is prefered.
Start the felt by cutting in half (down the middle long ways as rolling out),
start with a half sheet to offset with laps of the strom shield, so there will not be lap on lap which will cause a thicker build up and may be noticeable after roofing.
Then, your ready to start laying shingles baby.
I would use 3 tab shingles for starters up the edges and on the starter row (1st bottom row across) for a crisper cleaner look, Might look into drip edge depending on the situation where you are. As the shingles are nailed the Storm shield automatically seals around the nails and that will give alot of protection up top where its sort of flater. The directions to installing the shingles are on the package, I wouldn't use 3 tabs as the slits will make easy way for leaks in 10 years. Use the architech shingles..not that much more exspense.
Remember, never nail your shingles too high as they will start falling off if the roofing gets direct sunlight and 90 degree tempretures where you live.
This is because of the steepness on the sides of the barrel roof, so don't nail high. Also, not seeing the roof I might be cocerned with the capping and may run another run of 30 lb. felt before capping. On the Package of shingles it may indicate to offset each shingle 6 inches (stagger your roofing shingle joints 6 inches from the previous row) but I would stagger 12", makes it harder for water to find a way in in very windy conditions.
I'd think $85/ per square should cover nails, stormshield, felt and shingles..all but wood replacement and labor. Labor will probley run about $100/ per square or $1 per square foot for tearing off and installing all but wood replacement.
You might want to look into the building codes in your area as they differ from region to region, call your local building standards dept and run it by them, just make shure if in doubt..also check with your neighborhood association if you have one. Just some suggestions for ya.
C Jollie
C & M Contractors
Charlotte,NC