How To Thin Roofing Cement

Roofing cement can be put to use in a wide variety of roof repair jobs. Since it can be used in different ways, many skilled homeowners and roofing contractors make it their go-to material. If you're wondering how you can thin roofing cement, here's what we gathered after thoroughly researching.

By adding water, paint thinner, or mineral spirits you can thin roofing cement. Thinning ensures the cement will evenly coat and adhere properly to the surface.

There are specific procedures involved in thinning roofing cement. Continue reading to learn more as we would discuss the step-by-step process in the later parts of the article.

A roofer's hand in white mittens holding a brush with bituminous mastic, How To Thin Roofing Cement

Roofing Cement: Repair & Waterproofing Compound

The compound known as roofing cement can be utilized in a wide variety of different kinds of roofing applications. It can repair and insulate the roof while also acting as a waterproofing barrier. It can also be thinned to use as an adhesive; useful for affixing shingles.

How To Thin Roofing Cement

You'll want to thin roofing tar during the application process. To do this, select your method: water, paint thinner, or mineral spirits.

The roofing cement should be warmed (100 degrees Fahrenheit) and thoroughly mixed before getting started. Follow the manufacturer's instructions for the appropriate ratio for your purpose.

Add your thinner to the roofing tar mixture, in small parts. Test the viscosity. Continue to apply, or add more thinner depending on your result.

Alternatively, you can wipe the thinner onto the roofing cement using a saturated rag. Again you'd want to wipe until the cement becomes spreadable for your purpose.

How To Apply Roofing Cement

Putting roofing cement on your roof is as easy as spreading it over the surface with a trowel. If your roof has a steep pitch, you should use an edger instead. Here are the steps for applying roofing cement.

Bitumen primer for a flat roof sealing

Step 1 - Remove Debris

Remove any debris, dust, or dirt from the area that needs to be fixed. If at all feasible, work when the weather is dry.

Avoid watering down the cement. If you must patch areas in wet or rainy weather, use formulas that are designed to adhere to damp or slippery surfaces.

Step 2 - Fill in the Gaps

Use roof cement to fill in any gaps or cracks that you find. Utilizing the trowel or the putty knife, you should work it into the hole or crack. If you are using formulas designed for wet surfaces, make sure that the cement is thoroughly worked into the surface around the area that needs repairing.

Over the patch, smoothly apply a thin surface coat with a thickness of no more than 1/8 inch. If additional coats are required, wait 12 hours before applying a new layer.

When you are repairing a roof blister, you must first open the blister and give it ample time to cure completely before adding roof cement. When sealing the flashing, apply cement in a layer that is up to a half-inch thick.

Step 3 - Patch Large Holes

Use polyester or fiberglass fabric, or rolled roofing, to patch larger holes and cracks in the roof. Apply a thick layer of roof cement all over the patch.

In the region that was damaged, firmly press the patch into place. Apply one or more surface coatings of roof cement to seal the surface. When applied at a thickness of 1/8 inch, one gallon of roof cement will cover approximately 12 square feet.

What Is The Difference Between Roofing Cement and Roofing Tar?

Roofer worker painting black coal tar or bitumen at concrete surface by the roller brush

The most significant distinction between tar and roofing cement is that tar is resistant to water, but roofing cement can form a barrier that stops water from penetrating it.

Roofing cement emerges victorious when pitted against roofing tar as the superior roofing material. Its impenetrable barrier can prevent any moisture within the roof, such as precipitation such as rain or snow.

On the other hand, roofing tar has only moderate water resistance. That indicates that the tar can offer some degree of protection, but only for a limited amount of time.

Your roof will be protected from the damaging effects of rain if you use roofing cement. When using roofing tar, it is possible for leaks to eventually reach inside and cause water damage.

Although roofing cement is initially going to be a more expensive choice for you, it will end up being the more cost-effective choice for your roof in the long run because you won't have to worry about water damage.

What is Roof Cement Made Of?

The majority of it is composed of premium ground asphalt that has been polished. The remaining components of the cement, include a variety of minerals, solvents, stabilizers, and fillers.

Roofer's hand holding a brush with bituminous mastic

The product can tolerate temperature extremes without becoming brittle, breaking, or sagging and retains its shape. It can be applied using a trowel or a caulking gun.

After it has been applied, it can be smoothed out with a putty knife to provide more protection against leaks and to improve its visual appeal.

Does Roofing Cement Dry Hard?

The rooftop cement does not require a long curing time and is instantly ready for shingles to be applied. If the weather is favorable, you should be able to complete an asphalt repair in approximately a day. When left unprotected from the outdoors, roof cement has the potential to harden over time.

Roofing tar does harden up, with the curing process taking anywhere from 8 to 24 hours.

How Long Does Roofing Cement Take to Dry?

In general, it takes between 8 and 12 hours for roof cement to dry, and it takes around 24 hours for it to fully cure. The disparity in period might be affected by the meteorological conditions. Because of the dew and the high humidity, the drying time for the cement will be extended.

The coating will be able to dry at a significantly faster rate if the temperature is high and the weather is dry. When roof cement is dry, you will recognize this by the fact that it is entirely solid and no longer moist. Try denting the surface with your finger to see if it does.

If the cement on the roof can be dented, it has not yet fully dried.

Can You Apply Roofing Cement in The Rain?

Yes, you can apply cement in the rain. Use caution for your own safety while working in rainy conditions.

If you use a rubberized tar that is designed to be applied specifically in the rain and the surface area you will be applying it to is clean shortly before you apply it, then the tar can stick to the roof.

Can You Use Roof Cement on a Metal Roof?

Metal roof with waterproofing layer

It is possible to apply roof cement to a metal roof, and many people do so to stop or prevent leaks in valleys, roof-to-wall junctions, and plumbing vents.


There are times when roofing cement can lose its ability to adhere to surfaces or become brittle, which can result in cracks appearing on the surface. These are indicators that the substance should be reapplied at some point.

The most challenging aspect of applying roofing cement is thinning the substance so that it may be absorbed more easily and strongly adhere to its target.

For more articles on roofing, we recommend reading these posts:

Are Roofs Waterproof?

How to Patch a Touchdown Roof