Roofing New replacement roof, standing seam roofing, roof joists

Roofing New replacement roof, standing seam roofing, roof joists


Expert: Dan Merrill — 5/1/2007


QUESTION: Your the engineer next best to an architect — I’m planning a new roof to completely replace the one i have. (200 year old wood shingle base with lots of tar on top.) I’m in the city of Philadelphia, PA, in a townhouse. The roof is pitched about 3′-4′ to 1′ several chimneys. what is the best roofing material for this type of roof? I would like metal. or something that will last forever. I’m prejudiced against tar. Also, i would like the installer to use wide enough joists that will allow me at sometime in the future to take the ceilings out and put the ceiling along the bottom of the roof joists. (vaulted) Would 2×10’s be the correct size? I am also planning skylights.

ANSWER: Our favorite roofing product for this type of project is steel Standing Seam roofing, we prefer a 24 gage product for strength & longevity. A good product with Kynar paint should be expected to require a fresh coat of paint in 30-40 years, life should be perpetual if maintained in such a way.

The rafter criteria for pitching a roof such as yours usually isn’t the strength required, but the thickness of insulation desired.

Most codes require 10" of fiberglass insulation with a 1" air-gap above the insulation. This would require the use of standard 2x12s if they are strong enough for your span.

If you are removing the flat roof / ceiling, be sure your contractor knows this & sizes a proper ridge beam and supports it, as there will no longer be a horizontal tie-in between the rafter spans. And whatever you do, don’t forget the moisture barrier on the inside surface of the insulation, very important. We use a 6 mil or 8 mil poly.

The Standing Seam Steel roof will greatly outlive your skylights (15-20 years), for this reason we only use Curb-mounted skylights. The skylights can easily be replaced in the future without damaging or altering the roof. It is also very easy (less expensive) to replace curb-mounted skylights, it literally takes 10 minutes (or less) to remove & replace them.

I am with you on the tar, we will not use the stuff at all.

Is your roof already pitched 3 in 12? Or is that what you are improving it to?

If you install a steel standing seam roof with a double-lock seam, the female side of the lock can be sealed with an adhesive / weather stripping that will allow its use down to 1 in 12 or even 1/2 in 12 pitches, depending on manufacturer.

As a side note, we are using more & more of the sprayed-in foam insulation. Some do not require moisture barriers. They are fast & add strength & greatly lessen outside noise.

Roofing New replacement roof, standing seam roofing, roof joists

I hope these answers were what you were looking for.


what about flashing to brick walls and chimneys. I have two large chimneys and there are party walls here between townhouses brick flashing is very important. I’m familiar and have done step flashing. The ridge beam may not be necessary as i have a shed roof maximum span is 18′ in the front tower. I think there are 2X6 200 year olds holding it up now. wo the ceiling i think the 2X12’s should work but i may need an architect/engineer for the final job.

thx for the great advice.



Most lumberyards that sell rafters & laminated beams have charts that can recommend minimum rafter size & spacing required for your roof span. It’s free & it is reliable.

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