Bitumen Roofing Sheets: All You Need to Know

Corrugated bitumen roofing sheets are a cost-effective alternative to many other roofing options, especially when it comes to smaller DIY projects.

By comparison to corrugated steel panels, they are much lighter and easier to manipulate and install.

They are highly durable and waterproof, and since they can neither rust nor corrode, they can last for many years even decades without requiring replacement.

Although bitumen roof sheets are commonly available and widely used in the UK and Europe, they are less popular in the United States.

While almost all home improvement stores in the US carry corrugated PVC and steel panels, many do not keep bitumen (or asphalt) sheets in stock.

Nevertheless, they can be ordered from both domestic and UK manufacturers and distributors.

Bitumen roof panels are not only a viable alternative to PVC and steel but, in some applications, preferable.

If you are planning on installing corrugated roofing, it is well worth your time to consider whether asphalt sheets would meet your needs.

What is Bitumen?

Bitumen is a naturally-occurring substance; however, most present-day bitumen is synthesized from crude oil.

In both its natural and synthesized forms, it is a non-volatile, highly viscous substance, which becomes solid at normal, ambient temperatures, and it makes an excellent waterproofing compound.

It is one of the primary components of naturally occurring asphalt which, in addition to the bitumen, contains various minerals.

Natural bitumen is produced as part of the same process that ultimately results in the formation of oil and natural gas.

As organic matter settles into an underwater sedimentary basin it is subjected to bacterial decomposition and low-temperature molecular transformations. The resulting compounds then undergo a process known as thermal cracking.

Bitumen is produced both before and after the production of oil in tight rock formations, and these different types of the substance are known, respectively, as “pre-oil bitumen” and “post-oil bitumen.”

When naturally occurring, it is found as a residual substance that needs to be extracted using volatile solvents such as carbon disulfide.

Because it hardens at ambient temperatures, natural bitumen, such as that recovered through oilsands extraction, must be transported in a heated state, either through short-distance pipelines, which requires that it be diluted to reduce its viscosity, or in heated railroad cars.

Bitumen has been used for hundreds of thousands of years. Researchers have determined that it was used as an adhesive to bind flint to handles and other shafts at least as early as 180,000 years ago. In ancient sources, it is often referred to as “pitch.”

For example, in the story of the deluge from the Hebrew Bible, Noah uses pitch, or natural bitumen, to waterproof the wood of his ark.

Today, bitumen substances have an extraordinarily wide variety of uses, including within the construction, agricultural, paving, and electrical industries, to name only a few.

In addition to its roofing application, which we will be examining below, there are over two hundred and fifty other uses, including the production of carpeting, body armor, reservoir, and canal linings, electrical insulation, and marine enamels.

Bitumen emulsions are the most common form of binding agent used in cold paving, which is the form of paving used on low-volume secondary roads.

Types of Bitumen Roofing Products

Three bitumen products are commonly used in roofing applications:

• flexible membranes
• corrugated panels
• bituminous (or asphalt) shingles

Modified bitumen membranes are manufactured from a combination of polymerized plastic or rubber and asphalt, which is then reinforced with fiberglass. They are most commonly applied to low-slope or flat roofs.

Because the seams are melted together, they create a solid, impermeable barrier, and the two- or three-ply versions can last more than 20 years when properly installed. They are most often used in the construction of commercial buildings.

Bituminous shingles often referred to as asphalt shingles consist of a fiberglass substrate that is coated with bitumen on both the back and face.

They are inexpensive, durable, and available in a wide variety of colors, making them the most widely used type of shingle in residential construction.

There are three types of bituminous shingles: regular three-tab shingles, which lay flat when properly overlapped; dimensional shingles, which imitate the appearance and texture of a wood shake roof; and architectural shingles, which look like thick slate shingles.

Corrugated bitumen sheets or panels, which we will examine in greater detail below, can be used in a variety of applications, including both residential and commercial construction.

However, owing to their light weight and their solid panel design, they are often preferred by DIYers for smaller construction projects, such as chicken coops and sheds.

Corrugated Bitumen Sheet Design and Manufacturing

Corrugated bitumen sheets are an inexpensive and durable form of roofing material, which are particularly well-suited for garages, carports, sheds, workshops, and awnings.

They are manufactured by impregnating an organic fiber underlayment with bitumen and thermosetting resin. This infused fiber is then pressed and formed to create parallel corrugations that run the length of the panel.

Production standards stipulate that the bitumen content of such corrugated panels be greater than or equal to 40% of the total panel content, as determined by testing procedures articulated in the European Standard EN 534:2006+A1:2010.

The bitumen content makes these corrugated panels impermeable, and most manufacturers guarantee at least 15 years of waterproof use when properly installed.

Bitumen-saturated natural fiber panels have a similar design to corrugated sheet metal; however, they are much lighter.

A typical bitumen panel weighs approximately 3.5kg per square meter or 0.7 pounds per square foot, whereas 26-gauge corrugated sheet metal weighs about 5.52 kg per square meter or 1.13 pounds per square foot.

Twenty-gauge sheet metal is substantially heavier, weighing 11.23kg per square meter or 2.3 pounds per square foot.

The lightweight construction of bitumen panels reduces shipping costs and makes them easier to handle and manipulate than their steel counterparts.

Moreover, unlike corrugated steel, bitumen panels are much more soundproof, they do not have sharp edges, and they cannot corrode.

Specifications of Bitumen Roofing Sheets

Physical and Mechanical Properties

Bitumen roofing sheets have high tensile strength and a superb strength-to-weight ratio.

In keeping with European Standard EN 534:2006+A1:2010, the panels have a tear resistance greater than 200 newtons and they are impermeable to water, as determined by a 48-hour permeability test.

Moreover, they do not resonate noise, nor do they radiate heat. Unlike sheet metal panels, they do not have sharp edges and they cannot rust or corrode. Some manufacturers offer products with an added fire-retardant surface layer.

The crests and troughs that constitute the corrugations of bitumen panels provide structural rigidity along the length of the panel while giving the width of the panel a measure of flexibility.

This allows the span to be secured to either flat or curved supports. The flutes of the corrugated panels are intended to be installed running parallel to the slope of a roof and perpendicular to the eaves, thereby allowing rainwater to drain quickly.

Standard Dimensions

The dimensions of bitumen roof panels can vary. However, in the United States, sheets tend to range from 26 to 38 inches in width, which accommodates the industry-standard 24-inch spacing of rafters, trusses, and joists while also allowing for overlap.

Slimmer panels can be cut from standard-sized panels using either a saw or knife and wider panels can be custom ordered from manufacturers. Panels are typically between 6.5 and 8 feet in length, though they can be as long as 24 feet.

The big-box home improvement stores that stock such panels typically do not offer sheets longer than sixteen feet. In most cases, the panels are 1/8 inch thick.

In the UK and Europe, panels are standardly 950-970 mm wide and 2000 mm long, with a thickness of 3mm.

Availability

While a variety of brands of bitumen roofing panels are readily available in the UK and Europe, they are somewhat less commonly available in the United States.

The big-box retailers that do stock them often carry Ondura-brand panels, and they are typically marketed as “corrugated asphalt” panels.

Installation of Bitumen Roofing Sheets

Corrugated bitumen roof sheets can be installed directly to longitudinal purlins that have been placed between the trusses, or to a solid sheathing, such as plywood.

For sheds and agricultural structures, it is often unnecessary to use an underlying plywood sheathing.

However, any residential application will need to conform to local building codes, and in some jurisdictions, sheds and outbuildings must also meet code requirements.

Like all corrugated roofing, bitumen roof panels are easy to install. In fact, since they are composed of impregnated fiber rather than metal, they are easier to install than corrugated steel, which makes them ideal for DIY projects.

The following installation guidance is general in nature; always refer to the manufacturer’s instructions when installing your roofing panels.

Preparation

Bitumen panels can be affixed to an existing frame or structure with minimal tools.

If the panels do not need to be cut, you will simply need a hammer and nails or, in the case of screw fasteners, a screwdriver. You should use either galvanized ring-shanked nails or #10 x 2-inch screws with polycarbonate washers.

(Most manufacturers of bitumen panels also sell fasteners appropriate for their product and which match the color of the panels you are installing.)

If your panels require cutting, you will need either a handsaw, angle grinder, or circular saw.

While the panels can be cut lengthwise with a utility knife and straightedge, it is more difficult to cut spanwise (across the corrugations) in this manner. For projects requiring multiple panels, a chalk line tool is also helpful.

To determine the number of panels that your project requires, you will need to calculate the square footage and the pitch of the roof.

For roofs with a pitch less than 10 degrees, the end lap of the panels will need to be approximately 12 inches (300 mm), whereas, for pitches between 10 and 15 degrees, the lap needs to be about 8 inches (200 mm).

When affixing bitumen panels to roofs with greater than a 15-degree pitch, the overlap only needs to be 7 inches (approximately 178 mm).

In addition to end laps, you also need to account for side laps. As with all corrugated panels, bitumen or asphalt sheets should have a single ridge overlap. In other words, the sides of two panels should overlap by one full corrugation.

Once you have calculated the number of panels required for your roofing project, purchase at least one additional panel or increase the quantity by 10% to ensure that you have enough material to complete the job.

Before positioning and fastening the panels, determine how many panels you will need in your first row. Remember to account for the overlap on the sides of the panels.

It is highly likely that you will need to cut one, or perhaps both, of the outermost panels to achieve the correct total width. If only one or two corrugations need to be trimmed off, you can cut just one panel.

However, if more than this needs to be removed, you should distribute the excess that is to be trimmed off over the first and last panels in the row.

For example, if the first row will require a total of 5.5 panels, you should trim a quarter off both the first and last panel, rather than simply cutting the last panel in half.

As you decide where to cut, keep in mind that you will need to stagger the second row of panels so that the seams of the second row are offset from the seams of the first row. Ideally, they should be positioned halfway between those of the first.

Anticipate this factor when determining how to trim the panels in your first row, and plan ahead: figure out how the second row will need to be cut.

Subsequent rows will simply repeat the arrangement of the first and second rows.

Positioning

To properly position the first panel, you need to know which direction wind tends to blow across your structure.

To prevent strong gusts from lifting the panels, the first corrugated sheet must be laid opposite the direction of the prevailing wind.

The prevailing wind is the direction in which most often blows in a specific area. While they change with the seasons, low-latitude regions typically have an easterly prevailing wind and mid-latitude regions tend to have westerly prevailing winds.

However, the terrain and elevation, as well as other factors, can result in variations on, and exceptions to, these general rules.

Once you have determined the direction in which the wind most often blows, begin laying the first panel on the opposite side of the roof (i.e., the leeward side).

You will then lay subsequent panels so that the wind blows over, rather than into and underneath, the overlap (i.e., you will work windward).

The first panel needs to be laid at the corner of the roof and positioned so that it projects beyond the eave by one or two inches. This will ensure that water does not seep into or behind the fascia, which can cause damage.

Fastening the Panels

Attach the panel by driving fasteners in appropriate locations. Whether you are using screws or nails, your fasteners should be inserted into a ridge (not a valley) at a 90-degree angle to the corrugated surface.

If your panels did not come with predrilled holes, you should predrill them yourself using a bit appropriate for the diameter of the fastener.

While manufacturer’s instructions may vary, generally, fasteners should be placed as follows:

• For the rake edge, place a fastener twelve inches on center along the entire length.
• For the eave edge, place a line of fasteners three inches up from the edge. A fastener should be placed at the ridge (not the valley) of every corrugation for the first row on each panel.
• Wait to install fasteners in the side lap corrugation until the second panel is in position. It should overlap the first panel by one full corrugation and the fastener will secure the edge of both the first and second panel.

The second row of fasteners will be positioned either 12 or 24 inches up. Since you already installed fasteners along the length of the panel, the first one is already there.

If the starting panel has an odd number of corrugations, skip one corrugation (or ridge) and then drive in a fastener in the third ridge.

Continue by skipping a ridge and driving another fastener. If the starting panel has an even number of corrugations, drive a fastener in the second ridge (remember: there is already one in the first ridge) and then proceed by skipping a ridge and driving another fastener.

Continue adding rows at either 12 or 24 inches.

Position the second panel so that it overlaps the first by one full corrugation and attach fasteners in the manner described above. Repeat until you reach the end of the first row.

Drive fasteners twelve inches on center along the entire length of the panel, parallel to the windward rake.

As mentioned above, the second row of panels needs to be offset from the first row so that the seams are positioned halfway between the seams of the first row.

Place the first panel of the second row so that the end properly overlaps the first panel of the first row. Recall, the length of overlap is determined by the pitch of the roof:

• For less than 10 degrees, overlap by 12 inches (~300 mm)
• For between 10 and 15 degrees, overlap by 8 inches (~200 mm)
• For greater than 15 degrees, overlap by 7 inches (~178 mm)

Attach the fasteners to the second row in the same manner used for the first row. You will duplicate the positioning and fastening of the first and second rows for any remaining rows.

The final row of panels, which will run parallel to the ridge line, will likely need to be cut to length. Once both sides of the roof have been paneled, you will then need to attach a ridge cap.

If you require more than one length of the ridge cap, you will need to overlap the sections in a manner similar to the panels: begin at the leeward side of the structure and proceed to the windward side, overlapping sections by seven inches.

What to Expect When Installing Bitumen Roofing Sheets

Compared to other forms of roofing, corrugated bitumen sheets can be installed fairly quickly, especially if you have measured and cut the panels and predrilled holes prior to installation.

Since they are lightweight, they usually do not require a second set of hands, though the process will go quicker if you have help.

You will find that asphalt panels are much easier to cut than their steel or fiberglass counterparts. They are also much safer since they do not have sharp edges.

You will find that a simple handsaw is often sufficient, especially for smaller jobs. When sawing, be sure to wear appropriate protective gear, including a dusk mask.

Cost Comparison

When it comes to corrugated roofing, bitumen sheets are well-priced. They are less expensive than corrugated sheet metal, polycarbonate, and PVC panels.

At most big-box home improvement stores, such as Lowes, a 22-square-foot bitumen panel (one of the most common sizes) retails for approximately $16 (USD).

For comparison purposes, metal, polycarbonate, and PVC are typically sold in 17 square-foot panels, which retail for approximately the following (in USD):

• Steel: $21
• PVC: $19
• Polycarbonate: $27

Final Thoughts

While corrugated bitumen sheets are lighter and less expensive than metal panels, they are less durable.

As any drive through the countryside will reveal, the corrugated metal roofs on barns and outbuildings can last many decades and, in some cases, well over a hundred years.

Bitumen panels are, of course, very durable and can be expected to last a long time, but it is difficult to compete with galvanized steel on these counts.

However, if you want to roof a studio, patio, or garage, it is worth bearing in mind that bitumen sheets absorb rather than resonate sound. During rain or hailstorms, sitting under a metal roof can sound like you are sitting inside a snare drum!

For smaller applications, such as rabbit hutches or chicken coops, the dimensions of the roof will ultimately determine whether an inexpensive PVC or bitumen panel will be most cost-effective.

While there is only a minimal cost difference, the dimensions of distinct.

PVC panels are typically 2.2 feet by 8 feet, whereas the most widely available bitumen sheet in the US is approximately 3 feet by 6.5 feet.

In light of this difference, you may find that you can cut what you need from a single panel of one, but not the other, material, thereby making it the best choice.