DeMarne: Stick with concrete sealers for driveway |

DeMarne: Stick with concrete sealers for driveway

Question: I have a couple of questions for you:

First, will products such as Round Up or other such weed killers be effective against moss or algae growing on roofs? I know there are many other solutions, such as copper strips or roof cleaning companies. Also, does the sun get rid of moss on roofs? I noticed a few spots on my roof, but after the sun came out, they seemed to disappear.

Second, can I use products such as Flex Seal to seal driveways effectively where cracking exists? I’ve used different caulking products. They work for a while, but the cracking seems to come back in the spring months. I’ve often wondered why commercials for such products as Flex Seal never show concrete being sealed.

Thanks for any help.

— via email

Answer: An extensive exposure to the sun is a good deterrent to the formation of algae and the growth of moss or lichen on any surface. But on surfaces receiving little or moderate sun exposure, the sun is not likely to get rid of any growth that has developed.

To get rid of moss and algae, try Wet & Forget Outdoor. Check out Wet & Forget’s website at, click on “Store Locator” on the right and enter your city to find the nearest store selling the product.

Flex Seal is marketed as “rubber in a can.” I have had no experience with it, but it seems to me that it is only a surface coating film. I can see no use for it on a concrete driveway.

There are several concrete crack sealers sold in hardware and big-box stores. The kind to use depends on how deep the cracks are and their shape.

If the cracks are very small, such as shrinkage cracks, and your concern is aesthetics, you can make a slurry with Portland cement or a vinyl-reinforced concrete repair product and work it into the cracks.

For successful repairs of larger cracks, widen small cracks to a quarter-inch wide, using a cold chisel and a hammer. The cracks should have vertical sides or, better yet, inverted V sides.

Remove any loose pieces of concrete from the cracks and thoroughly clean the cracks with a steel brush.

For cracks no wider than one-fourth inch, you can use Quikcrete Concrete Repair, a flexible repair product that can stand small movements in the concrete. It comes in tubes for use with a caulking gun. Apply it to the cracks, and tool it with a putty knife.

For cracks wider than one-fourth inch and up to a half inch, use Quikcrete Concrete Crack Seal, which comes in its own pressurized container.

Overfill the cracks with the product to allow for settlement during curing, which takes about 24 hours.

If the cracks are deeper and wider than a fourth inch, you will need to apply several “lifts,” allowing 24 hours between each.

Be sure to follow the directions for a satisfactory result.

Driveway sealer

Q: Our concrete driveway is about 50 years old. The expansion joints are breaking down. I bought a bottle of Quickrete blacktop crack sealer.

I filled up the cracks with dry sand. Then I used the Quickrete as a sealer. Before I do any more joints, is this the right thing to do? Is there another product I should be using? Will doing nothing cause harm to the driveway? The driveway is in good shape otherwise.

— via email

A: The only potential problem with doing nothing is water filling the joints, freezing and damaging the concrete. But this is only conjecture, as original expansion joints are not waterproof anyway.

The method you are using is acceptable, but consider the following for a more durable solution: Remove as much of the failing expansion joint as you can.

Insert a backer rod of the correct size into the crack, pushing it gently to the bottom of the crack with a blunt tool or stick to avoid breaking its fragile skin.

The object is to have a caulking space shallower than wide, so you may need to put some sand down to leave enough space to top the sand with the backer rod.

Backer rods ensure that the caulking compound only adheres to the opposite sides of the surfaces to be joined because, for elastomeric caulking to perform as intended, it must not adhere to a third surface — the bottom of the crack.

Get as many oversized tubes of Sika Pro Select concrete crack flex sealant as you’ll need to caulk the cracks. You can buy Sika products in construction-specialty stores, some big-box stores or through A.H. Harris’ stores or website: Backer rods can be purchased in the same outlets.

Be sure to follow the directions for a satisfactory result.

Send questions to Henri de Marne at his blog,

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