Precautions for installing a steel roof

Q.  We are thinking of putting on a steel roof and wondering if we must remove the shingles or put strapping on before the roofing is applied. Any help would be welcome. The present roof is in good condition with no leaks. Standing-seam-roof

A.  It depends on what type of steel roof you plan on installing. If you are considering a standing-seam steel roof, you need to install it on a smooth surface. Otherwise, the unevenness of the shingles will be telegraphed through the metal over time. However, if you are contemplating a screw-on metal roof, strapping is sufficient.

Keep in mind that with any metal roof, you need a water-repellent material underneath, as there will be frosting and condensation forming under the metal. Your present shingles will provide that. Because the strapping will get wet from melting frost and condensation, you would be wise to use pressure-treated wood or to coat plain wood with a wood preservative.

Another consideration is the added weight the metal will put on the rafters. If your roof is steep, this is unlikely to be a problem as the snow is likely to slide right off. But with a lower-pitch roof, or one with protrusions such a skylights, chimneys, plumbing vents, etc., snow may not slide off. It is wise to consult with an experienced contractor or structural engineer to make sure you will not be overloading the rafters.

Cleaning algae from roof

Q.  This past year, we noticed black markings coming down the roof. Would you have any idea what is causing that, and will a power-washer clean it up? I’ve noticed several homes with shingles on their roofs with the same black markings. Do you have a solution?

Algae on Roof
Algae on Roof

A   Do not use a power-washer to clean roof shingles; it could dislodge the mineral granules that protect the shingles from UV damage. The black streaks are caused by a type of algae that grows on the mineral granules of the asphalt shingles when they do not dry fast enough after rain or after snow has melted off the roof.

There are several ways to remove algae. You can buy special solutions that are sprayed on the roof; you can have the cleaning done by firms that specialize in cleaning roofs, or you can do it yourself. If that is your choice, spray the shingles with a solution of equal parts fresh Clorox bleach and water. Use a plastic garden sprayer and apply the solution on a windless day at the rate of 1 gallon per 50 square feet of roof. The purpose is not to overspray and have the solution run off the roof for a long time.

Zinc Roof Strips
Zinc strips to control algae on roof

Wear rubber gloves and old clothing, as well as eye protection. Spray the solution from a ladder, as it is dangerous to walk on a roof, which may void any warranty left on the shingles.

If you have metal gutters, run water in them while applying the solution because it is corrosive. Soak all vegetation below the roof before spraying, cover it with plastic and rinse it thoroughly when you are done. Make sure there is no longer any solution dripping from the roof. It will take several weeks for you to notice any results.

Installing special zinc or copper strips at the ridge (or replacing an existing ridge vent with one specially made for this purpose) should prevent recurrence.

Getting best value when replacing siding and roofing

Q.    I am soon having my split-level house re-roofed, while also installing new siding and gutters. The enclosed picture (below) shows my home, which faces South.

904 Ironwood

When the contractor strips off the old (original) aluminum siding, he will be injecting Airkrete into the wall cavities all around the house (also the brick). The lower roof on the right (East) side of the house covers a cathedral ceiling which is over the front living room and a few steps up, to the dining room. The far North side of this lower roof covers the kitchen which has a regular 8-foot ceiling.

The contractor will also be injecting the Airkrete into the cathedral ceiling, and adding additional blown-in insulation above the kitchen. There are two can vents over the kitchen area. Airkrete will also be injected into the walls and ceiling in the attached garage.

On the top roof, you can see the front gable vent, there is also a matching gable vent on the North side of the house. There are also four can vents on top of roof. I have a 1500 cfm. powered attic fan on the East side of the top roof.

Fifteen years ago, I re-insulated the attic using Owens Corning R-25  batts (I think it was called Easy Touch. Has a thin sheathing all around the batt). First layer laid in the floor joists, 2nd layer laid perpendicular over 1st layer.  I also installed soffit vent chutes in all rafters at the soffit area.

For the siding, my contractor will be installing Mastic Structure foam-backed Dutch lap siding over Tyvek housewrap. For the roof, he will be installing GAF Timberline Architectural shingles with 6 feet of ice and snow shield.  The 4-inch gutters will be replaced with 6-inch gutters and downspouts. The wood railing and posts above the garage will be replaced with Azek board material.

I would greatly appreciate your opinion and suggestions to make sure this is being done correctly so I can get the most value for the money I am investing in this major renovation. My location is a Northwest suburb of Chicago. Thank you.

A.    Everything you are having done seems fine. The only comments I would offer are as follows:

  • Power roof vents have serious drawbacks. Unless there is enough net, free ventilation area (NFVA) in the attic itself to satisfy their CFM rating – which is seldom the case – they will draw conditioned air from below. This is a waste of energy both summer (assuming that you have central air-conditioning) and winter.
  • A better approach is to insulate the attic as you did, and make sure that you have adequate passive ventilation by means of continuous soffit and externally-baffled ridge vents. You also must have an uninterrupted air flow between the soffits and the ridge.
  • However, if there are no moisture problems in the attic, it would indicate that the two gable vents are doing the job, in which case there is no need to change anything.
  • Roof can vents are not effective, and it is best to eliminate them in favor of the passive ventilation described above. But, as mentioned above, if you have no problem, don’t change anything.

Otherwise, you are doing fine. I am glad that you are replacing residential gutters and downspouts with commercial ones; it’s a winning combination.

Ice dams on roof of heated garage

Q.    I have a problem with ice damming on my garage roof. I use a heater during the winter in the garage. The snow melts and ends up in the gutters, and at night freezes and ice forms in the gutters.

The roof in the garage is not insulated. Any suggestions other than shoveling the roof when it snows, to getting on a ladder and trying to remove it manually? Install heating cables? Insulate the attic? Thank you.

A.    If you are putting your cars in the garage, be aware that heating the garage activates the salt that accumulates on them during the winter, and it encourages rust formation — so it’s best to keep garages unheated. However, if you use the garage as, say, a shop, then you should treat it as a lived-in space and insulate the roof. Insulating the walls exposed to the outside as well would save you a lot of energy.

I would not recommend getting on the roof to shovel the snow or using a roof rake from the ground for the purpose; besides the personal risk of injury, it is not good for the shingles. Heat cables have some use but would not solve your problem.

Aluminum flashing won’t protect against algae on roof

Q.    I have the black algae problem on my roof that you have mentioned on several occasions. I wonder if using strips of aluminum flashing under the ridge cap would be as effective as the other metals you have mentioned. I am 83 years old and can’t do very much but I have a grandson who helps me a lot.

A.    Sorry, but it does not work. The strips need to be copper or zinc-plated. One good brand of zinc strips is Shingle Shield. Their Web site is The strips are installed under the first course of shingles below the ridge cap shingles.

Power ventilators not a solution to ice buildup

Q.    I need some advice regarding attic ventilation. I live in a large 12-year-old-home that was built with steep hip roofs — totally inappropriate for New England winters as the amount of ridge vent available is inadequate for proper winter ventilation.

This, in addition to a south-side cathedral ceiling, results in large amounts of ice build-up. I’ve had leaks, rot and huge icicles. Over the 10 years I’ve owned the home I’ve used heat tapes, redone portions of the roofing materials to include storm shielding (as this was not used originally), and I still am not satisfied that I’ve addressed the problem properly.

I believe I have to force air through the attic to achieve the least amount of melt-off. So I have come to the conclusion that I should install power roof ventilators. Do you have any recommendations?

A.    I would not recommend power ventilators, as they are likely to make matters worse by drawing make-up air from the living quarters. Ventilation is an important part of roof health but there are other ways to prevent the problems you are experiencing if ventilation cannot be provided.

The most important thing to do is to make sure there are no convective paths from the living parts of the house into the attic. Be very careful in that examination. Years ago, I worked on a case where extensive rot of the rafters had occurred in an unventilated attic from a drywall tape that had separated at the peak of a cathedral ceiling; warm, moist air entered the rafter spaces and wreaked havoc.

There should be no recessed light fixtures, no cracks anywhere. Seal all wall and ceiling electrical boxes with closed-cell gaskets throughout the house. If there is a scuttle hole to the attic, make sure it is weatherstripped with closed-cell tape. If there is a folding attic stairway, install a specially-made insulated cover for it.

Make sure that all bathroom and kitchen fans are vented to the outside and not into the attic. Same goes for the dryer vent.

Once you have made sure that there cannot be any convection into the attic, look into adding insulation in the attic over flat ceilings. Several inches of cellulose blown in would be ideal — don’t hesitate to have two feet of insulation.

The cathedral ceiling is another matter. The best way to deal with it is to fasten 2-inch thick extruded polystyrene or polyisocyanurate rigid insulation to the ceiling (tape all the joints) and cover it with new drywall.

These steps should reduce, and perhaps even eliminate, the melting of the snow cover from heat loss.

How to cool a south-facing enclosed porch

Q.    Many years ago I built a porch that faces south, so it is in continuous sun all day. I have been trying to determine how I can cool off the inside but I have been unsuccessful. I have a 52-inch ceiling fan and a 24-inch two-way fan (exhaust and intake) to circulate the air but it doesn’t appear to be working. The temperature is constantly 90 to 100 degrees on sunny days.

I thought of putting a roof vent thinking this might release trapped air. I also thought of insulating the ceiling. I would like your opinion on these ideas and any other suggestions you may feel would help.

A.    The only thing the ceiling fan does is move hot air around. The exhaust fan draws in outside air that is also hot on those days and, since the ceiling assembly has become radiant under the sun, the fan does not do much for you.

If the roof is covered with a dark membrane or dark roll-roofing, the best thing you can do to improve the situation is to put on a white or reflective roof covering. Roll roofing comes in white and there are also white synthetic rubber and PVC membranes (Sika Sarnafil and Johns Manville are two manufacturers of such roof membranes). They cost more than roll-roofing but will last a lot longer. However, you may need to clean them from time to time, as air pollution may dull them and diminish their reflexivity.

If there is space to lay 2-inch thick rigid insulation on top of the roof sheathing before installing the new roofing, that is the ideal way to insulate the roof. However, if there isn’t, this is the best way to go: If the rafters are exposed, you can nail or screw 1-inch by 2-inch furring strips to the sides of the rafters and against the roof sheathing, tack 1- or 2-inch thick rigid polyiso insulation, with the aluminum reflective film facing up, to the bottom of the furring strips and fill the rest of the rafter depths with fiberglass.

Install a ceiling finish. Install Air Vent, Inc. Flash Filter Vent where the porch roof meets the house wall and off-the-shelf soffit vent strips in the porch roof overhang. This will provide some ventilation and a reflective surface to reflect solar heat back out.

Metal roofing usually should have ventilation

Q.    I am nearing completion on a new house that I have built myself and am interested in installing either metal roof shingles that are stamped and colored to look like slate, or metal roof shingles that look like conventional, colored, standing-seam roofing installed in 5-foot or 3-foot lengths.

The roofing would go over stressed-skin panels that are currently covered with deteriorating 30-lb. roofing felt. The objective is an attractive, long-lasting roof that I can install myself to save money. I have a number of questions:

  • Do you recommend this type of roofing or is there a better alternative?
  • Is ice and snow shield on the peak, in valleys, around penetrations, and over the bottom three feet of the roof adequate?  (I’ve been seeing roofs that are completely covered with this membrane before shingles are installed).
  • Is any strapping required or can the roofing go directly over the ice and snow shield and roofing felt?
  • Anything else I should know?

A.    Metal roofing is generally a good choice, although some of the manufacturers of the types you describe will sell only to licensed contractors.

I would be concerned about installing metal roofing directly over stressed skin panels without some ventilation between the panels and the roof deck. These panels are so energy-efficient that unless the metal roofing is white, solar heat may distort it. However, on that issue, it is worth checking the recommendations of the manufacturer.

An ice-and-water protective membrane should be installed in all the locations you list except at the ridge, if a ridge vent is used. The practice of installing an ice and water protective membrane all over the roof is seen more and more. It is costly but it should prevent any leakage problem if there is some failure in the roof covering itself.