Flat Roofing Tapered Insulation Questions (from an email conversation with a customer)

This post is actually an email conversation with a customer, regarding Tapered Insulation and our flat roof installation in Cambridge, MA done in the summer of 2010. This email conversation is posted as is with the customer’s permission – the only editing on this post was done to remove any personal information, and spelling :). I thought this would be helpful to our readers, who are interested in tapered insulation and/or have ponding water problems with their flat roofs.

Here is the original roof video, so you have a better idea about this roof:

Original email question about tapered insulation

Peter
Hello,
We live in Wellesley. We were referred to you by a friend. I was reading your article on flat roof installation and was confused by the calculation of roof pitch/slope on the flat roof you had installed in Cambridge. The page I’m referring to is: https://www.coolflatroof.com/flat-roofing-blog/flat-roof-cambridge-ma/
In the second paragraph you mention that you sloped the roof at a rate of 1/4″ per foot and that the span was 24 feet.

You mention, and I quote:”we started with a new 1/4″ per foot slope tapered insulation system, spanning over 24 feet run, which gave us a 3 inch difference between the highest and lowest points of the roof, and eliminated ponding water.”

If I calculate the run of 24 feet and the slope of 1/4″ per foot, the highest point I come up with is 6 inches not 3 inches… For a 24′ run to rise 3 inches that would equate to 1/8″ per foot and not 1/4″ as you state in your documentation. I was told that 1/8″ per foot isn’t adequate to quickly shed water and snow off a flat roof. Is there something I’m missing?

I would like to get started on a flat roof installation possibly with your company but I would like to understand your calculation before I go ahead with the estimate.
Would you please let me know what I’m missing here?

Thanks

Peter.

Leo:

Hello Peter,

You are correct – we did use 1/8″ tapered insulation on that job – I just checked my calculations. The highest point is in fact 3 inches – not 6. For some reason this did not cross my mind when I wrote the article, as we made the decision to use 1/8″ tapered ISO when we first saw the roof in June and this fact never crossed my mind again.

On this job we actually could not use 1/4″ tapered insulation as that would raise the roof level too close to the window sills above it, and not allow for adequate roof to wall flashing (see attached photo).

We did this job in the summer, while the job review was written just before Thanksgiving, and I forgot the exact numbers by then. An honest mistake I suppose.

As for as 1/8″ not being sufficient:

If I’m not mistaken on this one, the state building code requires 1/4″ slope for all new construction flat roofing. For retrofit, it is not required. On this job, we had a 2″ deep puddle in the center of the roof (see attached photo), which is why we used tapered insulation, and a 3 inch rise was sufficient to level it off and eliminate ponding.

1/8″ slope is sufficient for the water to run off, if the roof is perfectly leveled, while in most cases roofs are not leveled and there will still be very small puddles right after the rain, even on a 1/4″ slope. Water will run to the lowest point, so 1/8″ is enough, though 1/4″ is preferred.

As far as snow, it will stay on the roof even if the slope is 1-2″ and will slowly slide off 2″+ slope. Snow will stay on the flat roof until it melts.

Bottom line, I always prefer to use 1/4″ tapered insulation, unless roof conditions require something more or less than that. In this case 1/8″ tapered ISO was tho only right solution. And one more thing – IB roof, which we use for all our flat roof installs, is actually not affected by ponding water and warranty is not affected either. It is however best to eliminate ponding water, if possible.

As far as your roof, if you can send me the details and pictures, we can work out the technical details and have a quote for you.

PS, would you mind if I posted this conversation as comments in the article? I think this will be interesting to other readers.

Peter:

Thank you Leo for the quick reply and yes you can use this thread in your blog if you’d like.
Concerning our roof, I’m meeting with the architect today to come up with the specs and to finalize the decision on what type of roof we’re going to be building. Our architect is strongly against any type of flat roof; he’s pushing for a gable roof not just for waterproofing concerns but also for aesthetics. Our main goal in building a flat roof is to have a nice cedar deck on top of it. The size is about 14’x18′ where the 18′ side would be the runoff. So one side would be facing the street, another side would be facing the side yard, a third side would be butting against a wall and the last side would be parallel to the bedroom wall where we would open up for a french door leading to the deck.
It is actually a one story living room addition. The side that butts against an exterior wall we’re free to go as high or as low with the pitch as we’d like. The issue we’re facing is that the floor of the bedroom needs to be higher than the finished floor of the deck. With the pitch, the insulation and the decking materials, it’s becoming a challenge to accomplish this task. We need to be flush with the existing ceiling on the inside and lower than the bedroom floor on the outside. The floor of the bedroom is 12″ engineered I joist. We’re looking to install 18′ LVL beams, each 1 3/4″ thick x 9 1/2″ wide. If we pitch the roof @ 1/4″ that would cause us to cut off the LVLs at the end of the 18′ run by 4.5″ which would leave us with all the LVLs tapering from 9.5″ to 5″. The architect is against that for two reasons. First, for the stability of the room since the ends of the LVLs would be considerably weaker. Second, towards the front of the house, at the tapered side where the thickness is 5″, we will not be able to install R30 fiberglass insulation in the joists, which would push us for a more expensive alternative by spray foaming…
I’d appreciate any ideas you can throw at me in building this flat roof.

I’ll let you know if we elect building a flat roof. Oh, just so you know, my wife is siding with the architect; I’m the one that’s pushing for the deck off the master bedroom. Let’s see who wins.

Thanks again Leo

Peter

Leo:

Hello Peter,

Right off the bet I can suggest two things, which I’m not sure you will like. However I need to know this:

I understand you don’t plan to use any tapered insulation, or any roof insulation for that matter – only insulation between the joists.

Second – as you see it right now or as you want to see it, what should the clearance between the door sill and surface of the deck be? Also what is the total clearance between the door sill and roof sheeting, before the insulation and deck are added.

Deck itself: What would be the total thickness of the deck? Assuming decking boards are 1″ thick and slippers below it can be 1 1/2″, (2×2 PT wood) 1 3/4″ (2×4 laying flat) or 2×4 standing up? – if it’s the last one, you can shave a couple of inches off.

Now here are my suggestions:

I would not recommend having LVL trimmed down to 5″ either. Would it make sense if you lowered the ceiling height in the room under the deck, and sloped the LVLs without trimming them – this would drop your ceilings by 4.5″ or you can have the ceilings sloped with the LVLs. This is technically more than doable, as architects make contractors build all kinds of weird stuff.

Second – do you absolutely need to have a cedar deck? Would you consider IB DeckShiled as an option? It is our PVC roof made for decks and has a walkable, slip-resistant surface. It will eliminate you clearance concerns. With DeckShield you could raise the tops of LVLs up by 4.5 inches (if there is enough clearance) without trimming the bottoms down.

Here is more info: IB DeckShield Flat Roof Deck material.
PS, here is a job profile of virtually the same project as you want to have done, also in Wellesley: https://www.coolflatroof.com/flat-roofing-blog/flat-roof-installation-wellesley-ma/

I’m attaching a picture from that job, which shows about 4.5″ clearance between the roof and the door sill. After they added about 2.5″ of deck, they had about 2 inches of clearance left. Is that something like your roof?

Peter:

Hello Leo, and thank you for your response, I really appreciate it.

The Architect wasn’t available today, so we postponed the meeting until next week when he gets back from Michigan.

Regarding your questions; well to be honest with you I do prefer both, tapered insulation and fiberglass batting to be installed in order to provide maximum insulation and comfort. I just figured that with the limited clearance we are faced with, the tapered insulation wouldn’t be a possibility.
Also regarding the clearance between the future door sill and the deck surface, I prefer the inside finished floor to be level with the surface of the deck (granted that the deck would be 100% waterproofed and able to shed water away from the door) this way when approaching the room, the deck would seem on the same plane and would give the amazing feeling of grandure.
I know the norm is to have the living plane higher than the deck plane, but that step down is really a horrible brake visually and physically. It’s awful to see it and it’s also awful to have to step down to the deck or step up from the deck in. The architect is proposing a 2″ drop down.
Now concerning the sleepers, the architect is proposing 2×4 PT laying flat wide side down and glued to the EPDM membrane. He’s also proposing to have the decking screwed to the sleepers using blind screws in order to hide the screws from the surface.
Regarding lowering the ceiling, that’s not an option because as I said earlier we absolutely have to have all the ceilings new/existing blend together even-though there’s plenty of room to lower the ceilings since they’re 10′ high, but still I promised my wife that I definitely will never lower the ceiling in the new addition…
Regarding the IB Deckshield this product would definitely not complement our house. Our house is not modern and in fact is historic. The look of cedar (which I forgot to mention would be 1 1/2″ thick and not 1″ as you thought) is what I’m aiming for in a deck if we build one.

Regarding the tapered insulation, is this something you sell or do you use something like Firestone insulation and EPDM?

Thanks

Pete

Peter:

Hello Leo,
In addition to the points I made and the questions I proposed in my previous email I’d like to ask if you are able in addition to waterproofing to frame a 2″ floor in the bedroom/bathroom level.
I received an email today from the architect proposing to raise the height of the bedrooms level by 2 inches by laying 2x4s wide side down on top of every joist and by installing 3/4″ plywood on top of that, which would raise the height of the floor by 2 1/4″. Our bedroom level is 9′ high and we don’t mind losing height there.
I would also encourage any other suggestions.

Thanks

Pete

Leo:

Hello Peter,

Without knowing the clearance between roof sheeting and the door sill, I’m not sure what to actually advise. I understand there is limited clearance there, and not much roof to give it a good slope.

I’ll go back to 1/8″ slope. If you put a deck on it and it is properly waterproofed, it will shed the water with minimal ponding and will give you back 2.25″. This way you could trim down your LVLs to 7.25″ at the outer edge, which will make them much stronger (versus 5″) and if that is still a concern, you could add more LVLs (less space between them). LVLs are much stronger than 2x wood, so you should not have much to worry if ends are 7.25″.

As far as tapered ISO. I’m not sure I correctly understand your question if we sell it. We don’t. We use it when we install the roof. We don’t use firestone insulation or EPDM. As far as insulation – to me it’s all the same, as it usually has the same R-value and we don’t really care who makes it (usually we do use Atlas). As for EPDM or rubber roof – we do not use it at all, as it does not meet our quality standard, and here is why – https://www.coolflatroof.com/rubber-roofing.php

We use PVC roofing for decks and regular flat roofs. The main advantage of PVC vs rubber (EPDM) is the hot air welded seams – once you weld them, they won’t come apart. Rubber seams are glued, and glue will break down sooner or later, and the roof will leak. Deck Shield is the same PVC as we use for roofs, only it has a special slip-resistant surface / texture. I understand DeckShield is not for you – I just wanted to clarify this.

I suppose we can frame out the floors in your bedroom / bathroom. Although framing is not our specialty, we often have to do it on our projects, and have a pro framer who is now a pro roofer also, so we can do that.

Basically I’d suggest to use a 1/8″ slope with 80-mil IB roof for waterproofing, and a floating deck on top. We always add strips of membrane to where the sleepers will be to separate deck from the roof. The deck will not go anywhere off the roof, so there is no need to glue sleepers to the membrane. Also the railing posts will hold the deck in place.

As for insulation, you could fill the space between the rafters with more insulation to increase the R-value. Fiberglass is easily compressed and you could use 3 rows of regular R-13 between rafters, or use the insulation for attic space.

I heard that to provide maximum efficiency, fiberglass insulation should expand fully. Though this does not make sense to me. I think if you use 3 courses of r-13 and compress it to 7.25″ at the end of rafters, you should still have about 39-R value. I might be wrong here.

Taking a better look…

Actually after reviewing the pictures of the job and paperwork, I concluded that we did in fact use 1/8″ tapered insulation on this roof in Cambridge, as we needed to remove 2 inches of ponding water from a roof run of about 24 feet long, and with 1/8″ tapered insulation we dropped (or raised) the roof level by 3″ toward the drip edge. If we used the 1/4″ insulation, the roof would rise too much – above the window sills directly above it.