Q: We are designing a pitched roof for our 1950s-era home that has an almost-flat torch-down roof. We want to update the look and get a...

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We are designing a pitched roof for our 1950s-era home that has an almost-flat torch-down roof. We want to update the look and get a better roofing system. The engineer and construction company are closely affiliated, which has me a bit concerned. The initial design calls for roof trusses to basically rest on the top of the existing exterior walls with a small “knee wall” built into the truss to boost them over the existing roof framing.

The claim is that this will allow them to cut cost, complexity and risk of water damage during construction by keeping the existing roof and ceiling basically intact. In other words, they are pretty much building over the top of the existing roof structure.

Is this something that might be problematic? Do we run into venting, or other issues? We are hiring the construction company to only do the basic framing and roofing, while we are going to handle (or hire independently) the rest of the project. Any advice would be appreciated.

When possible, keeping the old roof structure in place is actually a good technique. There’s really no need to rip off the old roof framing unless you want to change the internal ceiling heights or alignments.

Doing so only adds complexity and expense (for example, all the wiring up there). If you can build over an existing structure without complication, then more power to you.

Aesthetically this foot-high “knee” may look a bit odd from the street, but enclosed eave soffits and large roof overhangs will cover it completely, making it appear that the roof actually starts conventionally at the top of the external wall.

Venting is an issue any time you add a pitched roof. Chimneys, flues, plumbing and air vents all need to be extended.

The real sleeper is the electrical service mast: If it penetrates the roof, you need to alter it. This requires a visit or two from the utility company (expect power outage for a short time) and your electrician, and a new mast and service wire from the transformer to the meter, if it cannot be spliced.

You will need siding for the gable ends (if any), paint for that and all the new fascia, and bargeboard trim. Add to that the money to buy, install and paint the new eave soffits and venting.

Adding insulation in the new attic is something to consider while you have the building opened up. Air movement and ventilation between the old roof, new attic space and the exterior is something to discuss with the engineer because it will depend on the insulation and venting configuration.

Despite best intentions, I can almost assure you that with the multitude of holes that will be opened between the two spaces to facilitate all the utilities (don’t forget an attic hatch!) that a previously sealed roof system will need to become completely opened and ventilated to the upper attic.

The long and short of it is that you will end up cutting holes everywhere, allowing free flow of air between the spaces, then insulating and ventilating the upper attic conventionally. Finally, you inevitably will have ceiling damage; budget money for repair to the ceiling.

Regarding your recent article on air filtration: Your description of the issues relating to an electronic air cleaner is exactly what I am experiencing. But I am not able to find pleated 4-inch-thick filters at the hardware store. Where can I get one?

Your store may be able to special order one. Many heating contractors carry them. You could also check the Internet.

Darrell Hay is a local home inspector and manages several rental properties. He answers reader questions. Call 206-464-8514 to record your question, or e-mail dhay@seattletimes.com. Sorry, no personal replies. More columns at www.seattletimes.com/columnists.