|Posted on Saturday, March 27, 2004 - 08:27 pm: ||
I am having a hard time understanding how the installation of felt below the slate does not reduce or remove the ability of the roof to breathe.
As I look at felt, it seems to be asphalt impregnated, so how is its behavior any different from that of asphalt shingles in terms of breathability?
|Posted on Sunday, March 28, 2004 - 12:24 pm: ||
Asphalt shingles have self adhering tabs behind them that intentionally cause the shingles to stick together once they heat up in the sun (to help prevent them from blowing off the roof in a high wind). They also lay about 3 layers thick, with the overlap. That, combined with a plywood roof deck, creates a roof that cannot breathe.
Thirty pound slater's felt is one thin layer with a 3" overlap. It does not adhere to anything, but simply lies on top of the roof deck with some air space underneath (the felt wrinkles a bit after installation). When used over a board deck, there is much better opportunity for the roof to breathe. Eventually, the felt dries up and disintegrates.
The felt is not necessary for the proper functioning of a slate roof. It keeps the rain out while the slate is going on. After the slate is on, it is obsolete. We re-slated a 120 year old cathedral last year (I was the consultant) - it had no underlayment on the roof - just slate on wood. Virtually every slated barn roof we have worked on (dozens of them, at least) have not a square inch of felt paper or any other underlayment - just slate on wood. These are century-old slate roofs that do not leak (unless there is a broken slate or bad repair or deteriorated flashing). Never had any underlayment and never needed any.
I do recommend 30# felt for new installations today - even barns. But that's because it makes it easier to chalk the roof and keeps out the rain until the roof goes on. If you put the slate on without the underlayment, it will work just fine.
|Posted on Thursday, August 12, 2004 - 01:12 pm: ||
joe- I have some guys putting a vintage slate roof on.. they put 30lb felt paper up about 90 days ago.. there are some large areas on portions of the roof where the felt is ripply etc.. i think from the rain which we have had a ton.. i dont think there are any tears but clearly when the slate goes on, it could create bends and maybe present areas where water could be unable to flow down if a slate broke?
Net- should i be concerned about the ripples? Do i have it removed and replaced w/ new felt? do i just add another layer of felt etc..
|Posted on Saturday, August 14, 2004 - 11:41 am: ||
The ripples should not be a problem. Felt ripples when cold and flattens out when hot. You should be able to slate right over the ripples.