Post Number: 2
|Posted on Wednesday, March 12, 2008 - 02:18 pm: ||
The project is in Pennsylvania. Snow load is 30 PSF on 10:12 pitch roof. My biggest concern at this point is allowable deflection of the supporting members since the applied loads (including the slate and snow load)are approaching 80 PSF.
Stephen J Taran
Post Number: 46
|Posted on Thursday, March 06, 2008 - 11:46 pm: ||
Where is this job located? My thought was extra weight besides the slate such as snow loads??
Post Number: 231
|Posted on Saturday, March 01, 2008 - 01:05 pm: ||
1" solid board roof decking is traditional and was widely used under slate roofs for generations. However, 2" t-i-g boards can have more strength, durability and nailing depth, and were not uncommon on larger buildings such as churches or institutional buildings, especially with open ceilings. The slate was nailed directly to the 2" lumber, however.
The longevity of the slate roof system depends, in part, on the fasteners and what they're fastened to. Other factors affecting the longevity include the type (quality) of slate, the slope of the roof, and today, materials that impede the ventilation of the roof system (exterior vapor retarders can adversely affect a slate roof system, for example).
The substrate, or roof deck, should last longer than the fasteners because it's the substrate that holds the fasteners in place. Glued, laminated materials have not shown the longevity of a slate roof or its fasteners, which would be a minimum of 100 years, but more reasonably 150 or 200 years, especially if the fasteners are copper or stainless steel and the slate is a good quality material. 1/2" Vermont slate, sea green, unfading green, purple, unfading gray, etc., can reasonably be expected to last 150 years if it's made from good quality rock. Therefore, any slate roof system should be designed with that longevity in mind.
This means that the slate fasteners should be attached to solid wood whenever possible. If you're using 2" exposed ceiling lumber and covering it with foam, you need to figure out how to finish it with a board roof deck. This may require installing sleepers parallel to the rafters, then sheathing over them with boards, for example. Of course, a good grade 3/4" plywood deck will probably last 75 years, which is long enough for most people. But installing a slate roof that will only last 75 years is a waste of time and energy when the longevity of the roof system could theoretically be doubled by simply using a tried and proven material for the roof deck.
Regarding "typical" construction details for slate roofing, typical slate roofs were composed of 1" boards on rafters with maximum 24" centers and 30 lb felt underlayment on a slope greater than 4:12. This simple system has been proven to last 150 years, or I should say that there are 150 year old roofs still functioning well with this construction system. Other details vary from job to job, including flashing details. Today, however, many of the latest generation of roofing contractors, architects and engineers aren't interested in typical slate roofing details because traditional details have never included laminated roof decking, peel and stick underlayments, etc., which are now popular, at least among asphalt shingle roofers and general roofing contractors.
Post Number: 1
|Posted on Thursday, February 28, 2008 - 10:54 am: ||
I am a structural engineer involved in a new construction project with a slate roof (planned to be Vermont structural slate, 1/2" thickness).
Starting from the inside out, the architect indicates the planned construction to be: exposed heavy timber trusses, exposed heavy timber purlins, exposed 2X tongue and groove timber deck. There will be a 3/4" plywood deck on the 2X decking for a roof diaphragm. Rigid insulation will be provided on the plywood.
From my research, it appears that the recommended slate substrate (to be located on the rigid insulation) is a 1" sawn lumber in lieu of a plywood substrate. Are there any comments or concerns on the planned roof assembly regarding the slate roofing?
I have not been able to find any "typical" construction details for slate roofing on the web or from slate suppliers. Can you recommend a place to look?
My last question is regarding deflection requirements for the supporting structure. The applicable building code (IBC 2006)indicates the most stringent deflection requirements for roof members to be (span/360 for live load) and (span/240 for dead + live load). I have not been able to find any references specific for slate roofing that defines deflection criteria for the supporting structure. I want to make sure the supporting structure is "stiff enough" to accommodate the slate. Is there any reference that indicates deflection criteria for slate roofing or are the requirements of the building code satisfactory for design?
Thanks in advance for your help.