Best roof assembly on a budget?
Thank you for your website and for the exceptional information available here. I am an active student of GBA and a big fan.
I live in Central East Texas, Hot Humid, and am in process of remodeling a mid-century modern home with low sloping roof lines: 1.5/12 to 3/12. Essentially the entire home and roof assembly will be a vaulted ceiling setup since the interior ceiling is the actual roof deck. Roof is about 50 square.
We have budget constraints given the local economy, real estate market and neighboring home values but Im willing to spend what it takes to get everything right. Local builders and contractors are also not aware of many of the techniques discussed here which adds a few kinks to the initial plan.
The roof I envisioned starting from the interior is 5-7 inches of open cell spray foam on the current 1×6 tongue in groove roof deck (circa 1956) that is still in good shape. On top of that would be full Ice and water shield coverage. Next would be 1.5-3 inches of poly-iso insulation in two layers with staggered and taped seams. Then 1×4 or 1×6 purlins strapped on the diagonal to hold the poly-iso down and create 3/4 vent space. This would be topped off with Peterson 24 gauge standing seam 16 inch panels.
Fascia would have Coravent intakes at the eaves built into the trim, but no open upper ridge vent given the lower pitch sections (1.5/12)
My questions are:
Given the low pitch and inability to safely vent the ridge, should i abandon purlin/vented approach altogether and just deck over the poly-iso with 3/4 CDX plywood and call it good? That would make the details much easier for our roofer who has never tried this before.
If I applied the upper decking Plywood and properly taped the seams with something like Siga tape could I also avoid the more expensive step of ice and water shield on the initial original roof deck? As I understand it, air migration here is key and skipping this step may cause significant problems if it is not controlled properly elsewhere.
If the plywood decking approach is used, do you need a framing structure below the upper panels that is blended with the poly-iso sheets to keep the assembly straight and even. One concern from my contractor was that over time the upper plywood panels may become uneven given the more pliable nature of the foam.
My general thought now is that keeping the assembly simple may minimize potential errors and ultimately be the most cost effective.
Any advice you may offer would be greatly appreciated.
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