I agree with Sophie, this is not the time to play with mixed materials. You have come this far and why muck it up now.

If the addition was of a different style (as you sometimes see in additions in more urban settings) a different roof material could work. In this case where you have gone with a seamless integration of new with existing it requires the same type of roof. Sophie W. suggestion of utilizing the garage slate is the way to go. Thankfully with careful removal you should have enough material to salvage from the garage for the addition. PS is that bare plywood I see? It should have a synthetic membrane on there right now. These are generally rolled out immediately to protect the plywood before shingling starts.

Here some different trade names...

http://www.gaf.ca/Roofing/Residential/Products/Roof_Deck_Protection/TigerPaww

https://www.iko.com/products/residential/accessory-products/ice-water-protectors/stormshield/

Best of luck!

Have you considered recycled rubber shingles made to look like slate? http://inhabitat.com/euroshield-eco-friendly-roof-shingles-made-from-recycled-tires/

You have a source for the slate you need - use it. Rusty Empire is TOTALLY correct in that the new plywood should have been covered immediately upon install.

Perhaps you don't understand the purpose of shingles / tiles / shakes / slate? The roof decking (plywood or boards) are covered with a waterproof material; rolled roofing paper / tar paper / hot asphalt / membrane. THAT material is what protects the roof from water incursion. The shingles etc. are only to protect the underlying material from being damaged by the sun / rain / wind / ice.

A slate roof is a "forever" roof...how I envy you!

Whew, thanks for clarifying. When you remove shingles from garage - which I hope you do - then you can weigh your choices for doing that building.

heres another source for shake options

http://www.davinciroofscapes.com/products/slate/bellaforte/

Is the addition structure designed to support the weight of a slate roof?

Whatever is on the existing house should match what is on the addition, even if you have to change what is on the existing house.

I have to agree with the suggestions you have been given to carefully remove the old slate from the garage roof to use on the addition and then use a different material (maybe a gray metal?) roof on the garage. You can console yourself, however, that even if you could have afforded it, new slate would not have matched the old slate you have. We have an original slate roof from the 1920's, much like yours. Recently, a house down the street put on a new slate roof (paid for by insurance after a fire), and I am amazed at how different in color and texture the new slate looks. It is much more uniform and a different shade than the rest of the old slate roofs on our street. It would look like a completely different roof material if we put it on an addition and used it next to our older slate.

Evil, very good points - but it sounds like our homeowner was set on synthetic from the start.

Yes, looking for validation. Now has a new dilemma up on same. We did not provide the strokes!

perhaps in different areas the available slate wears down faster/differently than in others? i've been taught that slate roofs DO in fact last forever; and what starts to fail after a century or so is the nails holding the slate on begin to rust out. to that end, in areas where slate has been used extensively (as I would guess has been the case in the area in question), slate roofs are often removed and pitched in a bin...making it somewhat easier and cheaper to access used slate than one might have expected.

it appears evil that you have much more knowledge surrounding this than i, seems we're on the same page regarding salvage slate being the way to go though. i also raised my eyebrows at the sentiment that shingles don't do the job of keeping water out.

i think a standing seam metal roof that matches the color of the existing roof could work.

Cchute has a real life dilemma - someone started this addition (and perhaps it broke them) before he got here. His budget is a real issue.

Architecturally, keeping the carriage house historic and intact is not as big a priority as keeping the house intact. Architecturally, and value wise for the home, if the slate has 25 years left on it, moving the slate from the carriage house over is the best answer. Now, what if that is cost prohibitive or the new structure wasn't engineered to carry the slateweight? Once again we are looking at the downside of DIY (and I don't mean you cchute) - this is a house that deserved an architect. Whoever designed this addition wasn't too thoughtful - they tucked the new roof line in under so the eave bounces down, the stucco is actually an easier material to match /more appropriate architecturally than siding / no more expensive / even less expensive .. so not sure what that was about. Builder addition?

Must be because cchute has inherited it and trying to make some good decisions.

If you can move shingles from the garage (and no doubt it can be done, if they wait until spring, then you can probably get away with a slate look alike over there.Go to the lightest gray option if you do so. My guess is that any good roofer would want to keep that slate going - and finding some reclaimed to patch the garage roof back after you scavenge what you need might be theticket (or it might be a ticket to the poorhouse in cchutes mind).

Sadly, the architecture doesn't lend itself to using the slate for part of the roof and something else for part of the roof. I mustagree that if you can't afford to pull the roof from the garage, or it isn't engineered for that, then you need to pull the whole roof off (talk about expense). Then standing seam metal (still an expensive choice) is probably best. Even if you have to cover the roof with a tarp for the winter (and it is winter now most places), take your time making this decision. Investigating what you have will be worth the trouble. Could be the underlayment on the carriage house was about to go and you would be back in there in the next 5 years anyway. This might turn out to be a blessing in disguise.

I have very little experience with slate roofs. I'm pretty sure Evilfij has sufficient first hand experience with slate roofs that his advice should be well heeded.

I have much experience with slate roofs after working on dozens of Yale's gothic dorms and other buildings, which are fabulous treasures. The slate bible mentioned above is the best reference around. There is also a Slate roofing organization you can google, call, and feel free to get pro unbiased advise. They love to help!

As for underlayments, Certainteed, manville, and others, but grace makes a new product that is meant for preventing ice damns and to be left exposed for months. You really should not pinch pennies on the roof. Above all do not mix asphalt or composite, rubber or other types of shingles. It is hard enough to get slates to match. If you can find where the original source is, call them. The quarries are usually Vermont, other new england states, pennsylvania and i believe north dakota.

Many original slate roofs have no underlayments and the old guys (installers) will tell you if installed correctly slates do not need anything below the furring strips. The whole industry has become more complex because of energy codes and "experts" who really are not. Take the time to track down an experienced architect in this field. Or call a roofing consultant (i can name a couple for you if you like) who only do that... provide details, specs and inspections on slate roofs.

I agree with the idea of considering changing material of the addition to metal. Red copper would be my first choice, as it will patina wonderfully. Yes it is $$$$. On a house like yours consider it an investment and it will pay you back. If you cut corners and mix inferior grade materials your home value will plummet!

SLATE ROOFS is a book by the national slate association is another great guide book along with the bible. They just issued a revised edition of the bible, by the way.

National slate association: 866-256-2111 You can get the SLATE ROOF book there.

814-786-9085 is where the author of the SLATE BIBLE can be found.

Slateroofcentral.com ; Traditionalroofing.com ; Slateroofers.org and Joseph-Jemkins.com are all great references. The last one is that of the author.

Make sure your designer has allowed for the extra weight of slate. It is far heavier than the heaviest asphalt composition shingles. Usually, requires double the size of normal rafters, or if your home has an open attic, use it as a guide. Check the spans and your areas code requirements for snow loads too.

The best thing you can do is to get sound advice from one who knows!

I hope I did not put anyone to sleep!