Why would the deck be a problem; has it deteriorated in some way? There shouldn't be anything wrong with wood boards if they are in good shape.

The only plywood roof sheathing I have ever heard of delaminating was because the plywood was fire treated. You really don't need to worry about this issue for a house.

The necessary area of attic ventilation is usually provided in the state or local building code. I think it's usually 1:300 but can be 1:150 if the vents are at the eave and ridge or there is a vapor barrier at the ceiling below. But you should check it out yourself by calling the local building department.

Attic ventilation has little effect on the temperature of asphalt shingles and even less effect on their longevity but manufacturers often use the issue to avoid paying legitimate warranty claims. Not all shingle manufacturers prohibit unvented roofs and many only require it if you want their higher level warranty (warranties are all about marketing) so you should read the actual warranties rather than relying on conventional wisdom.

Who/what is telling you that the roof needs replacing? Slate can be replaced/repaired in small sections for a much more reasonable cost by someone who knows and actually works on slate. I live in a community with many slate roofs and I've never heard of 1 less than 80 years old that needed work.

If your son is getting his advice from a company that only works on new roofs, I'd keep looking for an old house roofer.

Please suggest that your son consider repairing and replacing the damaged slates rather than re-roofing. This is often not only the best solution (retaining a very high-quality and fire-safe roofing material), but also the cheapest way to go.

The only people who advocate tearing off slate are those who want one to spend big bucks installing another material. The beauty of slate is that individual slates can be replaced whenever necessary over the roof's very long life.

Repairing slate is easy, cheap and extremely effective.

The only hard part is finding a qualified slate worker to make the repairs. But I'm sure that in an area such as DC there are such persons. (And now that housing madness in DC has settled down it should be possible to get someone to actually have the time to do the work.)

Last summer I learned how to replace slates and repair slate roofs. I have since embarked on rehabbing all the slate roofs in my care (and that's ten separate buildings). I'm not advocating your son learn to do it himself, though it is quite easy to undertsand. It does take some specialized (though not expensive tools) and ladders.

Even though DC houses may have been roofed in the softer PA slate, a roof installed in 1941 ought to have more years of life in it, particularly if the failing slates are replaced. With asphalt this a big trouble, with slates, it's often more work to set up the necessary ladders than remove and install the new slate.

The boards (decking) under the slate may or may not be suitable for an under surface for asphalt. In my much-older buildings they are separated by large spaces, not the continous underlayment asphalt requires.

I suggest you check out John Jenkins Slate Roofing site, and perhaps borrow or buy a copy of his Slate Roof Bible. It's all you ever need to know to install, repair, or just supervise same, and hang on to your luxury roofing material in a world of dubious copies.

Regarding heat: I used to own a house in Rappahannock County, and I think there's nothing really to be done with heat build-up in that climate. You might consider adding some additional venting, but it's still going to hot as Hades under that roof (probably is even now,in late winter).

Molly~

Thanks for all the replies, comments and advice - all appreciated. Our family roofer (we have a slate roof too) has done slate roofs for years - he is in his late 50s and his 70 plus yr old father is still roofing - but as you might guess is not a strong advocate of keeping up slate roofs once he perceives they have "had it" thinking asphalt a lot easier. Each year as he does the gutters and inspects the roof he tells us and our neighbor that our 1930s roofs are soft but we tell him to just replace broken slates etc. Our house is substantially better than our sons starter house and in our son't neighborhood his is the only existing slate roof as the roofer pointed out!.

We bought the slate roof bible when it was first published and my husband as a student in England worked summers for a slate roofer and did a one-story porch roof at our house a few years ago so we do appreciate slate roofs. Our son initially asked for an estimate for the ridge which needs redoing which was when the roofer told him that the back half of the slate roof is shot but the front is still OK and gave him estimates for the ridge work and a new asphalt one. He is a meticulous roofer (my husband has worked with him) so we totally trust him to do the work correctly and well but you are right that he does, like many other roofers, have a predisposition for replacement with asphalt.

We will reconsider just getting the ridge done: $385 I think the quote was vs a very reasonable $2800 for the new roof.

If your son goes ahead with changing out the slate , which I think is a huge mistake, make sure he salvages the slate and resells it rather than to dump it in the landfill.
Which BTW once he changes over to asphalt will become a added cost to repairing any future roof. Even if he's the only one in a ten block radius with a asphalt roof he's still miles ahead if he stays with it. There are a good number of people out there who truly appreciate the beauty and fire safety a slate roof provides.

Slate has a lot of fans and I am one of them but it doesn't last for ever and this Bangor slate (PA) - one of the worst kinds - looks very flakey and at the end of its life. The decision to make in any particular case is how long it makes sense to go on repairing.

I'm a long time lurker on this forum, just thought I'd add my 2 cents of knowledge & opinion.
Our street has all 75 yr old tudor revivals on it and they all started out with slate roofs. I'd have to say about half to 60% of the slate roofs have been replaced recently with either regular or architectural shingles. IMHO, it does not compare to the look of a real slate roof.

That being said, one of our neighbors actually had their slate roof redone entirely in slate this past summer. Of course a bunch of us were out and started speaking with their roofer to see what a job like that would cost since we all have exactly the same size roof. Our houses are not large, and the cost of a total tear off and replacement was approximately $20k-25k!
That compared to what our next door neighbors paid to tear off and replace with architectural shingles was about $3k. I see why people have been tearing off the slate.
However, I still think people may have been too quick to replace the roof entirely instead of patch it. That's just IMO.

We have quite a large, quite old home. It started out with a wooden shake roof, and somewhere before my lifetime had been replaced by a slate roof. I have had more than a few houses with a slate roof, and just sold one, actually. That roof was ancient and still in decent repair. But anyway back to the present house...LOL.

The roof is steeply pitched, and was leaking in several places and we did have a roofer with slate experience who said that the slate was at the end of its life and we went with shingle. There was no underlayment upon which to lay a shingle roof. Just widely spaced planks. We were obliged to put down a surface on which to nail the shingles. No, shingles shouldn't be discarded. There is always someone in historic communities who needs slate for repairing their roof and we let our roofer have any he felt he could use. I still have a huge stack of them off the old barn I am keeping.

My last husband,now deceased, was at one time a roofer and did slate. He told me that the usual reason for slate repairs is that people walk on them without due attention when making repairs. He also said when slate starts sliding, it's often nail failure. The nails rust, the slate moves.

That being said, I was pretty skeptical about having our slate roof removed and looked them over well as they were being taken down. It was very chalky and brittle, and the edges were worn and weathered. Not healthy looking slate, IOW. I think that roof was about 75 years old. So, I do think all slate isn't created equal.

Our roofer said a ridge vent would be adequate, but I am suspecting now that it isn't, and am going to get somebody in soon to give it another look-see. I'm suspecting moisture build-up.

The nails rust, the slate moves.

Slate is properly installed with stainless steel or copper nails.

New slate roofs are very rare here, showing up only on homes in the multi-million dollar range.

Here is a link that might be useful: Inspecting a Slate Roof

It's not unusual to see common hand-forged nails however. The ones in this roof weren't copper and certainly not stainless.

Unfortunately not all slate is made equally.

Depening on the type and hardness, it can last pretty much indefinitely, or it can have a lifespan of 50 years or less.

The normal course of action is to roof right over the slate with asphalt.

If you strip the slate off, you're normally left with boards that are spaced out, with gaps between them. Unfortunately, you normally cannot lay an asphalt shingle roof on spaced decking. You need a solid decking surface to support the shingles or they will fail very quickly.

Kframe thanks for replying. I am not sure how good roofing with asphalt shingles over slate would be. You would think that slate would provide a pretty uneven surface and that plywood would be better if necessary. If we go the asphalt shingles route our roofer plans to use the boards if they are tight enough and adding to them otherwise -he did this on a side porch of our own house which was too low slope to take slate successfully (my husband had retiled in slate 20 years before not realizing it was the slope/slate combination causing water in the porch). Anyway the boards there were tight enough to take asphalt.

Back to son's roof we are going up on a ladder to take photos of the slate today and submitting them to the Joe Jenkins forum to check if the slate is really shot although one side from the ground looks so. Then, if so, the issue becomes how, if at all, to vent a new asphalt roof (replacement with slate is out of the question). There is no possibility of a soffit vent which is normally associated with ridge vents to get air flowing. So perhaps better not to have anything other than the one gable vent there already although even with the board and slate and associated air gaps the area is very very hot. I would love to hear more about venting these retrofitted old roofs.

The benefits of a vented roof assembly are greatly overrated.

Sealed attic retrofit.

Here is a link that might be useful: Unvented Roof Assemblies for All Climates

"I am not sure how good roofing with asphalt shingles over slate would be."

It works just fine.

This house was reroofed around 1989 with 30 year shingles over top of slate that was starting to decay. At the time it belonged to my Grandparents, it now belongs to my Mother.

The old slate was not stripped off, the shingles went right down over top of the slates and were nailed in place.

I went up the year before last to reattach a section of spouting, and the shingles are in excellent shape.

i just can't imagine nailing shingles into slate..
am I misunderstanding this???

One other thought to add to the "save it if possible".

If the roof really needs to be replaced, look into the faux slate roofing that is made out of rubber (or something). I've seen a few of those, and while not as good looking as slate, they are looking a lot better than asphalt shingles. While the cost is more than asphalt, it's a lot closer to that than to slate since installation is about the same in difficulty.

LOL. I am the original poster and 11 years later during which my son kept his slate roof and did nothing he now actually has a leak. So this time he is getting an estimate of repair or replace from a very experienced DC slate roofer. We will see.

A follow up to this it was a hole in the gutter and water running down the wall and not the rough so the 1940 slate roof remains to see another day! He repaired the gutter himself by putting in a new piece and no more leak through the living room ceiling near that outside wall.

@mahnrut what did you end up doing? We’re having the same dilemma now and we’re also in DC!