We’ve decided to bite the bullet and pull the old (36″ OC 1 x8 supported) built-up roof off the building and replace it.

In the retained portion of the building, this will let us top off our currently balloon-framed walls and add a top plate.  In order to get the strength that we need, the space for mechanicals and stay within our approved building envelope height, we’re using engineered web trusses which we’re having built with a low slope (1/4″ per foot).   It will also mean that our roof should be strong enough to take Solar which will allow us to increase the amount of Solar we can have on the project significantly.

The original plan was to seal in the roof void (close off the soffit vents), and put a TPO membrane over the existing roof, and add R38-R40 (code required) of ocSPF under the roof deck.  This was something of a compromise.

As we start thinking about a new assembly, it looks like ideally we’d have all of the insulation ‘outboard’ above the roof deck, add a coverboard above this (e.g. DensDeck or similar to make the deck walkable and add fire protection; we’re in a part of California where there are some occasional fires!).

This is where the challenges start.  Most notably, R38 would add about 6.5″ (or more) above the roof deck, which creates a potential height issue for us both aesthetically and per our permitting (there are a few other inches creeping in here and there!).

So we’re looking at a hybrid approach where we put R6-R8 above the deck, and then add the remainder below the roof deck (interior).  We’re not sure on the external insulation (rigid foam board, rockwool, etc.,) externally, but it looks like we should add at least 2″ of ccSPF below the deck, and then use either ocSPF or batts to build up the rest of the ~R40 we’re looking for.

The design / detailing of these kinds of systems appears to be an area where there is little I’ve reached out to a few building envelope architects/roofing & waterproofing specialists and haven’t yet found someone local with the right skillset; although several have told me that in this CZ we shouldn’t use EPDM roofing membranes, and that TPO should only be used if fully adhered, and that they (still) prefer built up roofs to TPO which surprised me.

This leaves us with a few questions:
1.  Does R6-8 (or 12-15?) above the deck; 2″ of ccSPF below, and then either rockwool or ocSPF below that for a total of R40 seem like a reasonable approach?  My back-of-the-envelope math tells me that it should be fine in terms of where our condensing surfaces are, but I’m not a ‘hygrothermal modeler’!  Our exterior temperatures range from the mid 40’s to the low 90’s annually, with the low for our three coldest months averaging 45F.  The average annual external RH is 75% (a high of 80% in Jan, a low of 70% in October).  I expect to maintain the house c.40-50%.

2. If we add insulation above the deck, can we keep the air barrier against the roof deck and (belt and braces against leaks) put a WRB down before the insulation, does anyone have any suggestions as to good products (someone suggested to me that if it’s just a backup under the insulation, with the main barrier being on top of the cover board, then a vapor semi-open SA house wrap would work?).

3. Do folks have any thoughts or advice on what seems to be a vast range of opinions on single-ply membranes (TPO/EPDM) and more traditional ‘torch down’ built-up approaches?   Initially, the more reflective nature of the TPO option was appealing from an energy perspective, but the main priority I have for the roof is waterproofing and durability!

Welcome any thoughts.  This doesn’t seem to be a topic area where the internet has a lot of answers!