I'm doing some new townhouses and this is the detail I've used in the northeast before. I'm new to the southwest California, so I'm modifying my townhouse roof detail for the new location. I'm also going to be more involved in the construction, so I have a few questions.
I don't remember how this detail was implemented by the framers in the northeast, but I don't know how you would set the heights of the roof joists so that you get a 1/4" slope for a single ply like TPO. The roof joists are bearing on the party walls of the townhouses, and that header probably wants to be flat.
It seems like it would be easier to build it flat, and then somehow overbuild / cricket the slope. I need to look into it more insulation wise, but I'm not sure I need the foam. Either way, why not add a third piece of plywood to the detail that is shimmed above each joist to the proper height for the slope?
Not sure what's the best technique here, your input would be appreciated. Any other suggestions on insulation in this forgiving climate, or a good and cost effective walkable roofing product would also be welcome.
awaiting_deletion
Aug 18, 16 10:25 pm
is the question - why slope the entire roof instead of building a flat roof with another structure on it that is sloped?
BulgarBlogger
Aug 18, 16 10:30 pm
I don't get it.. why not just taper the insulation?
Tyler Wallace
Aug 18, 16 10:38 pm
@ Olaf: sort of. I wouldn't say "another structure", but more of a shimmed layer of plywood / cricket.
@Bulgar: It's my understanding that tapered insulation is expensive and difficult to coordinate, so I don't want to go that route. Also, it's more difficult to replace/repair down the road and could crush.
awaiting_deletion
Aug 18, 16 10:52 pm
Tyler, you just answered your questions I believe.
Tapering is tough/costly, but if the entire roof slopes - Snap a line and done.
Now to your first point. Right now @ 16" O.C. you have joists I presume. If you shim, what are you shimming with and will the contractors shim directly over the installed joists? Will the shims be at 16" O.C.? Let's say you shim have way, what happens to the plywood under point loads?
The issue is, as I see it, by adding that layer of plywood over a perfectly good roof structure - you add one more location for possible waterproofing failure (although could be structurally induced). Let's imagine the water gets through the layer of plywood over the roof, where will it go? What happens in the winter if the water gets in?
proto
Aug 19, 16 2:42 pm
pick a strategy and draw it - then point out your concerns to GC in bidding
the GC will tell you which way works best for him
discuss ramifications
choose with builder's advice in mind
JeromeS
Aug 19, 16 2:54 pm
I don't get it.. why not just taper the insulation?
I second that emotion
cadomestique
Aug 19, 16 3:22 pm
Nope.
Best practice in this case is to have pre-engineered trusses with the slope already buit-in and any other truss heels you may need. Nothing beats that, everthing else is cumbersome, takes longer and prone to additional mistakes. Roofing ply goes directly above the roof sheathing although I've seen some mfgs prefer a thin layer of polyiso as a "cushion" for their membranes.
In California, typically you pass T24 with r30 for roof insulation, however some cities (Sacramento, for instance) are requiring and additional 10% which it's not a bad deal if you wish to substitute NFPA 13 for filling cavities with batts.
geezertect
Aug 19, 16 3:58 pm
I'm trying to get past the "walkable roofing". Is this going to be a deck that people go out on?
If not, sloping the insulation is the typical way they do large "flat" roofs. The subcontractor does shop drawings, each piece is designed and labeled, and it is installed per a layout drawing. It's no more cumbersome than shims, etc.
shellarchitect
Aug 19, 16 4:33 pm
if it's a single slope, say to the back of the building, sloped trusses with crickets at the back are the way to go. Seems like a lot of thought going into a simple thing.
Sloped insulation over the entire roof is a little nuts, in my opinion.
maybe the issue is scale, for example I'm working on a building that's roughly 60 ft. long at the moment, roof designed as stated above. Perhaps tapered insulation makes more sense on smaller jobs?
I've also never used wood trusses, due to scale....
awaiting_deletion
Aug 19, 16 5:47 pm
by the way how are the joists attached at the sides? are we pocketing them, hanging them, sitting them on top of...what are the sidewalls?
gruen
Aug 19, 16 6:29 pm
If joists are on top of sidewalls, just slope the top of the sidewalls and then you have your roof slope. Tapered does cost more. But you are overthinking this. Only two options-slope the structure or slope the insulation. Tapered trusses are also a good option.
citizen
Aug 19, 16 6:50 pm
Rip strips are a common answer in framing low-pitched roofs: adding a shaped member atop a flat one.
This image shows a symmetrical condition, but of course "ripping" can be done for a single pitch, too.
go do it
Aug 19, 16 8:30 pm
the answer here is pre-engineered sloped roof trusses
geezertect
Aug 20, 16 8:18 am
gruen: Per the drawing, there is living space below. If you slope the bearing walls, the ceiling in the room will have a soft pitch as well, which will look weird.
bowling_ball
Aug 20, 16 12:28 pm
What go do it said.
mightyaa
Aug 21, 16 12:11 pm
Slope the structure. It is not hard and a lot cheaper than taped insulation. Like Olaf said; snap a line and be done with it. Essentially, you slope the main drainage plane, and use tapered for the crickets to push to your drains. So think outside walls set at 10' brg, inside bearing line on like a hallway at 9' brg. Structure slopes to center where you collect in a roof drain. btw; I recommend real roof drains in the center versus scuppers and collectors. Freezing is an issue. So also be aware of drains that sit in the shade of a parapet. Frozen scuppers & downspouts won't drain. Internal plumbed roof drains don't have that issue.
The most common method is a truss roof system. The 'flat trusses' are actually more tapered shaped so the top chord is sloped. The trick with these is the continuation of the rated wall assemblies into the truss area or deal with it with the trusses (like a end wall truss they can apply gypsum to in the attic).
I'm doing some new townhouses and this is the detail I've used in the northeast before. I'm new to the southwest California, so I'm modifying my townhouse roof detail for the new location. I'm also going to be more involved in the construction, so I have a few questions.
I don't remember how this detail was implemented by the framers in the northeast, but I don't know how you would set the heights of the roof joists so that you get a 1/4" slope for a single ply like TPO. The roof joists are bearing on the party walls of the townhouses, and that header probably wants to be flat.
It seems like it would be easier to build it flat, and then somehow overbuild / cricket the slope. I need to look into it more insulation wise, but I'm not sure I need the foam. Either way, why not add a third piece of plywood to the detail that is shimmed above each joist to the proper height for the slope?
Not sure what's the best technique here, your input would be appreciated. Any other suggestions on insulation in this forgiving climate, or a good and cost effective walkable roofing product would also be welcome.
is the question - why slope the entire roof instead of building a flat roof with another structure on it that is sloped?
I don't get it.. why not just taper the insulation?
@ Olaf: sort of. I wouldn't say "another structure", but more of a shimmed layer of plywood / cricket.
@Bulgar: It's my understanding that tapered insulation is expensive and difficult to coordinate, so I don't want to go that route. Also, it's more difficult to replace/repair down the road and could crush.
Tyler, you just answered your questions I believe.
Tapering is tough/costly, but if the entire roof slopes - Snap a line and done.
Now to your first point. Right now @ 16" O.C. you have joists I presume. If you shim, what are you shimming with and will the contractors shim directly over the installed joists? Will the shims be at 16" O.C.? Let's say you shim have way, what happens to the plywood under point loads?
The issue is, as I see it, by adding that layer of plywood over a perfectly good roof structure - you add one more location for possible waterproofing failure (although could be structurally induced). Let's imagine the water gets through the layer of plywood over the roof, where will it go? What happens in the winter if the water gets in?
pick a strategy and draw it - then point out your concerns to GC in bidding
the GC will tell you which way works best for him
discuss ramifications
choose with builder's advice in mind
I don't get it.. why not just taper the insulation?
I second that emotion
Nope.
Best practice in this case is to have pre-engineered trusses with the slope already buit-in and any other truss heels you may need. Nothing beats that, everthing else is cumbersome, takes longer and prone to additional mistakes. Roofing ply goes directly above the roof sheathing although I've seen some mfgs prefer a thin layer of polyiso as a "cushion" for their membranes.
In California, typically you pass T24 with r30 for roof insulation, however some cities (Sacramento, for instance) are requiring and additional 10% which it's not a bad deal if you wish to substitute NFPA 13 for filling cavities with batts.
I'm trying to get past the "walkable roofing". Is this going to be a deck that people go out on?
If not, sloping the insulation is the typical way they do large "flat" roofs. The subcontractor does shop drawings, each piece is designed and labeled, and it is installed per a layout drawing. It's no more cumbersome than shims, etc.
if it's a single slope, say to the back of the building, sloped trusses with crickets at the back are the way to go. Seems like a lot of thought going into a simple thing.
Sloped insulation over the entire roof is a little nuts, in my opinion.
maybe the issue is scale, for example I'm working on a building that's roughly 60 ft. long at the moment, roof designed as stated above. Perhaps tapered insulation makes more sense on smaller jobs?
I've also never used wood trusses, due to scale....
by the way how are the joists attached at the sides? are we pocketing them, hanging them, sitting them on top of...what are the sidewalls?
If joists are on top of sidewalls, just slope the top of the sidewalls and then you have your roof slope. Tapered does cost more. But you are overthinking this. Only two options-slope the structure or slope the insulation. Tapered trusses are also a good option.
Rip strips are a common answer in framing low-pitched roofs: adding a shaped member atop a flat one.
This image shows a symmetrical condition, but of course "ripping" can be done for a single pitch, too.
the answer here is pre-engineered sloped roof trusses
gruen: Per the drawing, there is living space below. If you slope the bearing walls, the ceiling in the room will have a soft pitch as well, which will look weird.
What go do it said.
Slope the structure. It is not hard and a lot cheaper than taped insulation. Like Olaf said; snap a line and be done with it. Essentially, you slope the main drainage plane, and use tapered for the crickets to push to your drains. So think outside walls set at 10' brg, inside bearing line on like a hallway at 9' brg. Structure slopes to center where you collect in a roof drain. btw; I recommend real roof drains in the center versus scuppers and collectors. Freezing is an issue. So also be aware of drains that sit in the shade of a parapet. Frozen scuppers & downspouts won't drain. Internal plumbed roof drains don't have that issue.
The most common method is a truss roof system. The 'flat trusses' are actually more tapered shaped so the top chord is sloped. The trick with these is the continuation of the rated wall assemblies into the truss area or deal with it with the trusses (like a end wall truss they can apply gypsum to in the attic).
+1 Go Do It.