It's often been said that old walls hold secrets, a phrase that usually refers to mysterious deeds that took place within them.

But old houses can be persuaded to spill their secrets, at least architecturally speaking. What it takes is a smart and tenacious house detective to ferret out the hidden clues, and an injection of "happy accidents," as professional preservationist Diana Painter puts it.

Such was the case with Ernie and Deb Ongaro. When the couple bought their 2,000-square-foot house on Petaluma's Prospect Street in 2010, it was something of a cypher.

The house clearly was old, dating back to around 1865, when it was built by a family named McCleave. That would make it one of the oldest homes in the city. But sometime between 1871 and 1885, a new front porch was added that extended the full width of the house, and an addition was built in the back.

Between 1888 and 1894, another addition was built, the front porch was altered again and the house given a bit of a Victorian look. Another wing was added between 1894 and 1906 on the east side. Somewhere along the way, it acquired a dormer.

Fast forward into the 20th century. In 1963, it got another update, with new asbestos shingle siding, brick steps and a wrought-iron rail.

In short, the house had suffered so many changes the Ongaros weren't sure what to do when it came to restoration. What exactly would they restore it to? Should they try to take it to its 1860s vernacular or should it be made to look more Victorian, like many of the other homes in their Brewster/Oak Hill Historic district?

At one point, before they retained Painter as a consultant, they had even considered re-doing it in Greek Revival.

Their solution came in the mail. Out of the blue, said Ernie, they received a photograph from a woman in Oregon who said she had found it among her father's effects after he died. It was a picture of the Petaluma house taken in the 1940s when the woman said her parents apparently lived there. She knew where to send it because the address was on the back of the photo.

That photo provided important clues on which the entire project would turn. It showed a more Victorian porch, probably the same porch that had been on the house before the turn of the last century. So they decided to use it as their template, and bring the house back to the way it looked in that picture.

But other clues remained to be uncovered. Once they ripped off the cement porch, they discovered remnants of the old porch beneath it, key pieces like the old post, which they took to woodworker Philip Nereo of Windsor. Using that and the photo, he was able to recreate the rails and posts of the 1890s porch.

"Really, everything we needed was underneath the old porch," said Ernie Ongaro, a third-generation heating and plumbing contractor.

But their detective work didn't end with the porch.

The house had more secrets to spill. Underneath the old asbestos-shingle siding they found multiple clues to what the house would originally have looked like.

First, they discovered that the original redwood siding was intact. They also found clues to the original color. Although the siding had been painted white, they could find traces of an earlier shade of yellow. From Sherwin-Williams historical collection, the Ongaros were able to find a close match.

"Fortunately that (added) siding had protected the original siding over the years. So it was in beautiful shape," Ongaro said. "I was able to strip the paint down to the bare wood, prime and paint it."

Those asbestos shingles had also hidden another important clue for historical accuracy. When they were put on 50 years ago, the old trim pieces were removed. But because the trim had originally been installed first before the paint, they left marks that created a shadow effect, an exact outline of their shape. Petaluma woodworker Chris Lindeman was able to reproduce the original windows, window trim and some house trim based on those shadows and the historical photo.

"It's been a really fun project," said Ernie Ongaro, who has re-done other homes in Petaluma and still has a long way to go with this house, includng the roof, gutters, backside and interior. "But it's so rewarding. You really get attached to a house when you do a project like this."

You can reach Staff Write Meg McConahey at meg.mcconahey@pressdemocrat.com or 521-5204.