Without a Vapor Barrier, Metal Roofs are installed over Purlins

Without a vapor barrier, metal roofs are installed over purlins, which is one of the issues constructors face while installing a metal roof.

As a metal roof is not enough alone to be installed, purlins, vapor barriers, and underlayments are some essentials that, if done correctly, can increase the roof’s life.

Purlins are of two types, steel or wooden; they are more effective than wooden purlins due to their unmolded features, whereas wooden purlins can get fuzzy or moldy over time.

Moreover, the vapor your carrier works around prevents water from gathering in the walls of the roof.

The article below discusses the importance and purpose of purlin, vapor barrier, and underlayments for the longevity of the metal roof.

Stay tuned to read till the end to learn some beneficial facts about the topic under discussion.

Metal Roof Over Purlins No Vapor Barrier

Having a metal roof over purlins and no vapor barrier is a good combination but not the best. To have a purlin installed below the meta roof is the most recommended option, as purlins increase the roof’s life by lifting it.

However, having a vapor barrier is also recommended as these barriers prevent water from accumulating in the walls and let the moisture have its proper place.

Having no vapor barrier may lead to sagging and damage to the roof over time due to water accumulation in the roof walls.

Proper sealing techniques should be used to prevent moisture infiltration when installing a metal roof over purlins without a vapor barrier.

What Is Meant By Purlin?

Purlin plays a vital role under metal roofs, and it works as the direct support to hold the panels of the top as purlin is designed in a horizontal shape so that the whole is over purlin.

Purlin could be made from plywood, metal sheeting, or wooden panels. The primary purpose of purlin is to hold the weight of the metal roof and to provide support in case of any falling of the top.

Purlin is usually installed with a vapor barrier, or it can be installed with roof underlayments with synthetic or felt underlayment.

Types Of Purlin

Two main types of purlin are used under metal roofs and have been discussed in detail below;

Steel Purlins

Steel purlins are mainly preferable due to their feature rigidness, and they cannot mold, shrink, or get out of their shape for any reason or weather condition.

This unmoldable feature of steel purlins makes it the first choice of customers who believe that any rain, storm, or other weather conditions cannot easily affect the steel purlin.

Also, they don’t have to spend repeatedly on the roof underlayments with purlin if they go for steel purlin. 

Steel purlins are also very light in weight, making them easy to install and hold the capacity to bear heavy weight over them, which seems very convenient to the constructors while using steel purlin for their construction.

Wooden Purlins

Wooden purlins are the first preference of users with a low budget, as wooden purlin and cement sheeting make the best combo purlin to be used under a roof.

However, this purlin is not widely used by professional constructors as they believe wood can get out of its shape quickly, as water or storms cannot be borne by the wooden purlin for long durations.

Moreover, wood gets moist quickly, which leads to unwanted smells and the risk of sagging. Customers use wood purlin but not so frequently, and it is also not so famous for professional use.

Is It Necessary To Have A Vapor Barrier?

A vapor barrier is used to clear out the water that gets stuck in the walls, and a clearance is required to save the wall or purlin from getting rotten, molded, or saggy, especially where there is a wooden purlin.

A vapor barrier is a must. However, roof underlayment or purlins will not work according to the requirements without a vapor barrier. 

Water will get stuck in the walls, and moisture will not stay for longer durations, which will cause issues in longer runs.

Due to the exceptional features of a vapor barrier, it is advisable to install them to work better with other equipment.

Why Is Underlayment Important Under Roof?

Underlayment plays an essential role under the roof. It enhances the roof’s life by immunizing the water, moisture, and dust that can directly affect the top if no underlayment exists.

In contrast, purlins and vapor barriers support underlayments as purlins support the roof by holding its weight, and the vapor barrier reduces the chance of water storage in the walls, which can damage the material of the top and can adversely affect the roof.

Thus, two prevalent types of roofs are popular in the market and known for their unique features.

Synthetic Roof Underlayment

Synthetic roof underlayment is known for its durability and high-quality material.

The polythene or plastic used to make the synthetic roof underlayment makes it a market leader. The demand for synthetic underlayment is very high because of its durability and longevity. 

However, synthetic underlayment is one of the most expensive underlayments available.

According to the set of rules and regulations drawn by construction authorities, not every area is eligible for synthetic underlayment, which further restricts the usage of this underlayment.

Felt Roof Underlayment

Felt underlayment is the best option to go for when you are looking for a budget-friendly underlayment.

Felt underlayment also has a quality material that enhances the roof’s life, but compared to synthetic underlayment, this underlayment fails to provide such long years of life.

However, it is an excellent option to be recommended for times of low-budget construction.

Best Underlayment Over Roof

Numerous underlayments are available in the market; the most commonly used, synthetic roof underlayment and felt underlayment, have been discussed above, with their respective pros and cons, to be precise.

Perhaps to choose the best, one should look at their budget.

First, high-budgeted constructors should go for synthetic roof underlayment, and low-budgeted constructors should opt for felt underlayment.

Consequences Of Not Having A Purlin Or Vapor Barrier?

The purpose and uses of Purlin and vapor barrier have been discussed above.

However, a brief description of the consequences of not using both options is here;

  • Purlins without a vapor barrier beneath metal roofing can increase condensation, possibly leading to corrosion or rot.
  • Purlin increases the life of the roof as it holds the top over it and lets the roof stay for years
  • Steel purlin is preferable over wooden purlin, with a view that wood gets rotten or moldy quickly when compared to steel purlin
  • Purlin is very lightweight, which makes the installation of purlin far more accessible than any other material
  • A vapor barrier works to stimulate the effects of moisture in the wall of the roof by not letting the water stay in the walls


Although metal roofing can be directly installed on purlins without a vapor barrier, some professionals advise against it due to potential moisture problems.

In light of the above article, a conclusion can be drawn that having a metal roof over purlins with no vapor barrier is not ideal, as a vapor barrier should be installed to enhance.

Through the roof’s life, draining water accumulation in the walls of the top. However, purlins lift the weight of the roofs and give them complete safety back. Steel purlins are more recommended than wooden purlins.