Value of 1x4 Lathing over Plywood Deck

Guest User
7/8/2015
We are in the process of removing the old composition shingle roof and having a clip-on standing seam roof installed. I think our installation is fairly simple and the contractor is competent, so our issues are minimal. But there are a couple of things we don't understand and need expert help on. We have a very simple gable end house with a 7/12 and 5/12 pitch roof on each side. The attic has free flowing ventilation using continuous soffit vents and a ridge vent. The deck is plywood and has a foil backing on the attic side. This foil was a specially applied item on the plywood at the time of the house construction. The existing roof is an Elk composition/asphalt shingle at 22 years of service and is finished. We live in humid Texas near Houston where the dewpoint is almost fog in the mornings, and there are trees and pollen coats everything in the Spring. The 1st question involves the use of 1x4 lathing on top of the deck so that the metal is not in contact with the plywood deck. Our assumption is that the space will reduce the heat gain into the attic. Is that correct? But, have we created an air gap and more condensation that would affect the wood? The lathing is not treated lumber and against the metal. The lathing is installed crossways along the roof so there is not a free flow of ventilation. The underlayment is Titanium 30. So, the question is, is it BEST to use the lathing or not use the lathing, long term? The 2nd question involves the color choice for the roof. The contractor recommended the uncoated galvalume since it is the most energy efficient. However, we do not prefer a shinny roof, but we do not know if the shinny roof will turn a dull gray over time. But, if we bought a painted light gray coated roof, would it be significantly less efficient on the heat gain compared to the dull-shinny roof over the long term. Doesn't the reflection of the "shine" work even if it is below the paint coating? So, the question is how less efficient is a light gray coated roof compared to a shinny roof?
Eric Novotny
An informed customer is the Best Customer!
7/8/2015
Primary decking would still have an underlayment on it prior to putting down the purlins. Condensation on the backside of the roof should be minimal at that point and any that does accumulate would just run down the decking and out the bottom.
Guest User
7/8/2015
Don, thanks for your interest in metal roofing. It can be the best investment you will ever make. First the underlayment. The Titanium underlayment is the best product to use. You reference '30'. They offer PSU 30 and UDL 30. PSU 30 is a high temperature Peel and Stick that should be used along the eave. The UDL 30 is a synthetic underlayment. If you are using UDL 30, I would recommend upgrading to the UDL 50 because it a heavier product. These should be installed directly to the deck. Above Sheathing Ventilation is a good idea. First install lathing vertically (preferable attached through the plywood and into the trusses) then install horizontal lathing. Studies have shown that this will reduce heating and cooling costs. To reduce lathing read through make sure you use a panel that is either stucco embossed or has striations in the flat area. Embossed is also good for roofs that are in hail zones. if you are expecting a lot of debris on your roof particularly in the valleys. It is recommended to use aluminum to prevent corrosion. The Solar Reflectance Value (SRI) of Galvalume Plus is 58. Slate Gray is 73; Dove Gray is 51. For reference black is 2; Ascot White is 87. Long answer is to go with your first choice of a gray roof. let us know if you have any further questions.
Guest User
7/9/2015
Thanks very much for the information. I understand what both experts have said and can make the necessary adjustments in the plans. There is one thing that is not clear and I do not know if it is terminology of the industry or a mis-type. It is the term " 'reduce lathing read through' use a panel that is either stucco embossed or has striations in the flat areas". Are your referring to the potential rot of the wood lathing from condensation of the underside of the steel panel, and this can be reduce by having a slightly irregular contact surface? Does that mean that untreated wood installed on top of the underlayment and in direct contact with the sheet metal roof may tend to rot? Thanks very much for your information and getting me lined up to replace the roof.
Dick Bus
7/9/2015
I am referring to the read through or 'dents' that may (maybe not) occur when stepping on the panel directly at the end of the lathing.

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