About our data
How Do We Check the Facts on This Site?
Every rule, figure and estimate here is read at its source and dated. This page explains the sources, the math and the limits.
- Primary sources for rules
- Census margins of error
- Dated, linked estimates
Where do the facts on this site come from?
Permit and historic rules come only from the municipalities' own websites and codes; housing-age figures from the Census Bureau; costs from dated third-party publishers. Each is read at the source and shown with a checked date. Where a source could not be reached or confirmed, we leave the fact out rather than guess.
Source types
Rules and fees
City, township and borough websites, ordinances on eCode360, and the Historical Commission's Rules & Regulations. Quoted wording is copied from the source.
Housing age
American Community Survey 5-year estimates, table B25034, by zip code tabulation area.
Costs
Published estimates from Angi, RoofVista, Adilay Roofing and Industrial Roofing Contractors, each kept with its date and unit.
Roofing practice
National Park Service Preservation Brief 29 on slate roofs.
How the census percentages are calculated
The share of homes built in 1939 or earlier is the count in that category divided by total housing units. Its margin of error uses the Census Bureau's formula for a proportion: the square root of (MOE of the part squared minus the share squared times the MOE of the total squared), divided by the total; where that is negative, the ratio formula with a plus sign is used. Margins are at 90% confidence.
How we treat conflicting cost figures
We show every figure with its unit and date instead of averaging them. Where one publisher contradicts itself, as with a low headline average beside a much higher per-project range, we show both and say so. None of these are our prices.
Who can read this site
Our robots.txt lets search engines and AI search tools read every page, including OpenAI's search and training crawlers, Perplexity, Google and Bing. We do this so answers to roof questions can cite the dated sources here. A plain-text summary of the site is at /llms.txt; Google says it does not use that file, so it is a courtesy to other tools.
Method questions
How often are the facts rechecked?
How do I report a mistake?
Sources and checked date
- U.S. Census Bureau, ACS 5-year summary file, B25034
- Philadelphia Historical Commission Rules & Regulations
- National Park Service, Preservation Brief 29
Last checked 2026-10-29.
Rules change. Confirm with the city, township or borough permit office before you rely on any permit or historic-district detail here.
Spotted an error? Tell us
Tell us where the house is, how old the roof is and what it is doing. If you have photos, say so and we will ask for them by email. Outside our core area? Send the form and ask.
We answer form requests Monday to Friday, 9am to 5pm. There is no phone line, so the whole conversation stays in writing.