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David Zimmerli
Posted on Sunday, December 29, 2002 - 02:13 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Hello,

I'm going to be replacing valleys and am
wondering whether I need cant strips along them.

The book "Copper in Architecture", put out
by the Copper Development Association, has
diagrams on page 4.3.11 showing wood or copper
cant strips running the length of the valleys,
positioned on each side just under the edges
of the overlapping slates. It says the cant
strips are used to "break contact with the
copper surface which minimizes line corrosion".
I think I've also read that this helps prevent
capillary action leaks.

However, the Slate Roof Bible doesn't mention
this detail, so I'm wondering if anyone has
any thoughts on whether these cant strips are
necessary. (The copper book also shows cleats
on the valley edges, which the SRB says aren't
needed).

Regards,
David Zimmerli
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Walter Musson
Posted on Tuesday, December 31, 2002 - 08:51 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

The cant strips you mentioned are never used in our area of working.As the slates overlap the cut edge along the valley exposure line doesn't rest on the copper,it's held slightly above it.It seems to me capilary action would cause the wood to rot prematurely and be a maintainance issue down the road.Also the strip would have to be nailed thru the copper which wouldn't be good.The cleats you mention are really a matter of personal preference,not used much in our area.There is nothing wrong with using them however.
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admin
Posted on Wednesday, January 01, 2003 - 02:03 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

Cant strips should not be used along valley edges. I don't know who writes these books (such as the copper development assoc. book) but I can tell you for a fact that they don't know what they're talking about. I don't know where they come up with these ideas, but I think they just sit in an office somewhere and make them up because they sound good.

Slate laid directly over metal is a technique that has been used since the beginning of slating hundreds of years ago and it has never been a problem. You must overlap the valley metal with the slate by a minimum of 5" and you should also pay close attention to other normal roofing factors, such as slope, and opposing slopes (if asymmetrical, use an inverted "V" groove down the center of the valley (etc.).

I have replaced hundeds of valleys, literally miles of valleys, on every conceivable type of slate roof, and have never seen a cant strip. It's just unheard of and unnecessary, if not a total mistake. Also, on those hundreds of valleys, about 1% of them have been folded and cleated along the edges, the other 99% were simply nailed. All failed for the same reason: metal deterioration, and the cleated ones DID NOT last any longer. So cleating is an option, but not necessary. It's a waste of time, as far as I'm concerned, unless the metal has a lot of soldered joints and you want to try to minimize stress on those joints. Virtually all of the cleated valleys we have replaced have been on high-end buildings such as institutional buildings and mansions.

I think that at one time, roofing firms with their own sheet metal divisions tried to convince their customers that proper valleys should be cleated. This,of course, involved extra cost and extra time and material, and I think that was the intention (extra $). This was compounded by the fact that old instruction books always show the valleys cleated, again, a problem similar to the CDA book - information published not by experienced roofers but by someone else (architect? engineer?) who didn't know enough about his subject matter.
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TSMartin
Posted on Sunday, January 12, 2003 - 11:46 pm:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

This may be off-topic so I apologize before hand. I was considering getting the CDA book but from this thread it appears the info in it may not be correct. Where can a person obtain the correct info for installing flashings, drip edges, ridge caps and such? Thanks
TSMartin
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Walter Musson
Posted on Thursday, January 16, 2003 - 09:13 am:   Edit Post Delete Post View Post/Check IP Print Post    Move Post (Moderator/Admin Only) Ban Poster IP (Moderator/Admin only)

The Copper Development Association manual has a lot of valuable information on the subject of copper in architecture,however the detailing on some of the field work is not totally accurate.I've re- read the piece on valley battens and agree with Joe that it was written by someone in an office who never has done much roofing.Their premise for the use of the battens is flawed.Line corrosion and capillary action are not causes of coppers deterioration at valleys,rather the effect of water dripping from the shingles onto the soft copper over many years simply wears thru the sheet.Look at where the worn thru spots are on valley sheets and you'll see bright areas where the copper wears during each rain event.Better yet ,observe the way water travels during the rain by actually watching it firsthand.
The CDA book has a lot of merit as a whole,its just you have to use your gut instincts on some matters related to field work.

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