What is a Warm Deck Roof? (What You Need To Know)

If you were to lay out a detailed blueprint on a warm roof and set it next to an equally detailed blueprint of a cold roof design, you would assume that the warm roof is the more complicated design of the two. The truth is, it’s the exact opposite and a warm roof also comes with its own advantages.

A warm roof requires no ventilation and the layers of insulation sit directly below the roof deck, which sits below the tiles. Of course, there’s usually a vapor barrier as well but the insulation sits above the joists, so you won’t see any insulation sitting in between your attic joists.

The warm roof design is a far better design for warmer climates, especially warm climates that are also high in humidity. A warm roof also contains that climate within your home, without allowing it to radiate up, through the roof, and out into the atmosphere.

There are a lot of advantages to a warm roof and some disadvantages; both of which we will cover below.

How is a Warm Roof Designed?

If you were to go up in your attic—and you will find this design occasionally in flat roofs as well—you will see that the joists along your roof framing are empty in between, as in no insulation between them. The insulation barrier rests above the joists and below the roof deck.

In fact, a roof deck made out of wood is sometimes optional, though you will often find that there is one. Above the roof deck is your roof covering and above that, the tiles. The vapor control layer sits directly underneath the insulation but above the joists.

There is typically a “sub-roof floor” that separates the joists from the above insulation. It’s usually plywood like you would expect directly underneath the tiles. There is no ventilation required in a warm roof.

The roof deck is additionally sealed with a waterproof membrane. You will typically find that there is a nonporous cover, along with an additional insulation cover. All of this design and structure results in a roof structure that is closer in temperature to that of the inside of your home.

Advantages of a Warm Roof

There are several advantages to the warm roof design, many of which are not things that you would normally consider if you don’t have any experience in carpentry or in any kind of construction trade.

Reduced level of thermal bridging or its eliminated altogether
Lower energy bill
Lesser amount of moisture accumulation within the home
Dew point is only relevant outside of the home
Warm roofs are simpler to construct
Warm roofs are less expensive than cold roofs

Since there is minimal or no thermal bridging involved (ventilation isn’t required for warm roofs) the roof space and attic above the home remain close to the inside temperature of the home, which reduces cooling costs over the summer.

Reduced energy bills are the direct result of the heat and the cold on the outside having very little in the way of accessing the living spaces inside, so your HVAC system doesn’t have to struggle to maintain the ambient temperature.

You also don’t have to worry about humidity within the home nearly as much. Since there are no combatting temperatures from above and below, the only dew point that you have to worry about is the one that greets you when you step outside.

Despite looking more complicated on paper, a warm roof is far less expensive than its cold roof counterpart. The design is deceptively simple and the materials that go into the construction of a warm roof are simpler and less expensive.

Disadvantages of a Warm Roof

The disadvantages of a warm roof are far fewer than the advantages. With a warm roof, it’s probably best to install either a harder watertight membrane or an additional deck. This is because a completed warm roof is soft without either of the additions.

It also places a level of restraint on how high you want to build the overall house or building, because of the higher roof depth.

If you happen to live in an area with heavy snowfall during the winter months, the falling snow is likely to melt and then refreeze. The problem is, when it refreezes it usually does so on top of or at the edges of your eaves.

The additional weight there is well above the specs for the roof and it can cause damage. That’s not to mention the fact that you don’t want to be standing underneath it if it decides to collapse under the weight.

It can also be enough weight to damage your gutters if you happen to have any and most people do. But the most prevalent damage occurs as the snowmelt refreezes on the eaves and more snowmelt continues to add up, running its way up to your roof.

As the refrozen snowmelt continues adding up, it can get up underneath the tiles, cracking them or breaking them off altogether.

Does a Warm Roof Need a Vapor Barrier?

We’ve mentioned the vapor barrier that typically goes into a warm roof above. However, in an inverted version of a warm roof, there is no vapor barrier required. An inverted roof is just like it sounds, flipping everything upside down.

The insulation is on the top, with the watertight membrane sandwiched between the insulation and the roof deck, removing the necessity of a vapor barrier.

An inverted roof is supposedly more environmentally friendly, so you’re starting to see more and more of them as environmentally-friendly construction methods gain popularity. Since a straight-up inverted roof loses more heat than a traditional, warm roof, it is often built over the existing warm roof instead.

All Things Considered

A warm roof is simply a construction method that is applied by placing the insulation above your joists, not between them. There are also two versions of the warm roof with the inverted version playing second fiddle to the standard version.

There are a lot of advantages to having a warm roof, especially if you are in a more tropical and warm climate.

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Take a look at our other most recent blog post below:

What is a Warm Deck Roof? (What You Need To Know)