Fall, 2003: Issue #3:

Readers Write: Slate Roof Savoir-Faire

Plywood, Condensation, Ice Guard, Costs of Slate

Reader: "Have you ever heard of anyone running into any problems using plank board decking? I am having a small roof addition built and I want to install new slate. I'm trying to convince the contractor to use 1 inch plank board for the decking, but right now the job is spec'd using CDX plywood. I have shown him the info you have about plywood vs plank decking, but the contractor still believes the plywood will work! Yorktown, VA

The Roof Savant: No, I have never heard of a problem concerning plank board decking related to the material. I have heard of lots of problems regarding contractors. Plywood on slate and tile roofs is used strictly for the convenience of the contractor or architect and for no other reason. It has never demonstrated the phenomenal longevity necessary for a traditional slate roof (100 to 200 years). It will work temporarily, but it's not worth using when better materials are available for less money. It's your money and therefore your call — downgrade your roof construction to satisfy the convenience of your contractor, or build it right — once.

Reader: I read your newsletter some months back when I was shopping for a new roof. I saw your opinion on plywood and thought, man that guy's full of it — what a blowhard! Plywood lasts forever! I've just had my mind changed — by a floor, not a roof. I recently stripped the finish flooring in my ca. 1915 house, which had an addition put on in 1965. The original subfloor is 1x6 pine, and it is still in outstanding condition. On the other hand, the addition had a plywood subfloor. Though it is still sound, you could tell it is less flexible and much more brittle than new plywood, and there was some evidence of delamination by the outside door. In a powder room in the addition, they used plywood to shim the sheetrock out, and it is brittle like an old newspaper — completely worthless. The state of the plywood all around was pretty sorry considering it was younger than I am and had been living under ideal conditions (warm, dry, and out of the sun). I can't imagine what it would be like if it had had to endure the extreme temperatures and hot/cold cycles a roof does. I guess my point is just to say thanks for such a nice and informative newsletter; you've convinced a skeptic that what you say is correct. I hope some day to own a home worthy of a fine slate roof, and you can bet it will have a lumber deck if I have anything to say about it!

Reader: Can you tell me what the approximate cost per installed square would be in Vermont using a middle of the road slate? Architectural shingles run approximately $58.00/sq installed.

The Roof Savant: Costs are affected by various criteria, including the removal of old roofing. If yours is a new home, the slate material will cost maybe $400/square, delivered. Installation will be that much again, or more, depending on the style of roof, amount of flashing, etc. Slate is a one time deal, though, if done right.

On the other hand, cheap temporary roofing such as "architectural shingles" cost less, but you get what you pay for. Would you buy windows for your new house that would last only 20 years if they were cheaper than lifetime windows, then replace them every couple of decades at ever increasing cost? I doubt it. Yet, on the urging of the roofing industry, people routinely buy cheap roofs — the most important part of the house — and replace them over and over (which makes lots of money for roofing contractors). The wisest approach in the long run is to put a lifetime, beautiful, natural roof on your house in the first place.

Reader: I own a slate roofed home built in 1954. When we experience rapid changes in weather (common here in northern Ohio), the roof gutters have frozen and ice damming has caused roof leaks at the eaves. I have installed snow melt in the gutter area, but this is not fully effective. I have been contemplating removal of the shingles to install iceguard at the gutter line to prevent this problem in the future. It appears that you object to the use of iceguard. My question is — do you believe it is detrimental to the slate to use it and if not, what are your objections?

The Roof Savant: I don't think ice guard is the solution to your ice-dam problem. If it was, every older home with a slate roof would have chronic leaking problems along the eaves because none of them have ice guard, yet, this problem does not exist. When a properly insulated, designed and installed slate roof leaks, there is probably a specific problem in the roof (faulty old repair, nail hole, cracked slate, etc.) where the water is penetrating. Better than putting ice guard under your eaves slates, in extreme circumstances you could remove the slates, then re-install them with greater headlap. You would need to add a row of slates in order to do this. You would only expect to have to do something like this if the roof had been installed incorrectly in the first place (slope too low, headlap insufficient, etc.), which is unlikely. A simpler method of fortifying a drip edge is to slide copper bib flashings (6" wide) under the slots along the eaves in order to artificially increase your head lap. Again, this remedy, although simple, cost effective, and long lasting, is rarely needed. I have only needed it when the roof was low slope, such as on a shed roof dormer eaves where the ice and snow could not get off quickly enough and no other faults were visible. A defective slate, invisible from above, could leak in the slot during ice damming, and the copper bib flashing remedy will cure it (which is most likely what is happening in your case). Ice guard has become the panacea of plywood roofers. A good slate roofer will never rely on temporary underlayment to keep the roof waterproof.

Reader: I recently had installed a standing seam copper roof over my home. The contractor installed the copper in the traditional way. Now that it is winter, water condensation tends to collect under the copper in the attic (which is not heated). What is the best way to deal with this? Would a de-humidifier make sense?

The Roof Savant: Your best bet in dealing with attic condensation is keeping the warm air out of the attic. As long as warm air is coming in contact with the cold metal, you will have condensation. Try either better ventilating the attic space so it stays colder in the winter, insulate the attic floor so no warm air penetrates into the roof space, or insulate the roof itself under the metal. Always install a vapor barrier (heavy mill plastic) interior to the insulation.

 

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