Yes, you can change a commercial flat roof to a pitched roof but first you will need to install roof trusses or a tapered insulation system to create enough slope to ensure proper drainage. Which you need to install depends on whether your roofing system resides on a small commercial building or a large older one.

Small Commercial Buildings

If your building has a small footprint and dimensions that can accept standard-size manufactured roof trusses, then an easy way is to install the trusses on the existing parapet walls. You will obviously need the help of an engineer to first verify this is feasible for the building. You will also have to comply with local building codes.

I have seen it done on smaller commercial buildings.

Larger or Older Commercial Buildings

A commercial “flat roof” should not be dead flat. The proper industry term for a flat roof is a “low slope” roof because, even though the roof appears flat to the naked eye, there should be enough pitch built into the rooftop to allow water to evacuate and not pond.

A flat or low-slope roof should have a slope of 2/12 or less, meaning the roof should rise no more than 2 inches for every 12 inches of horizontal run. Most commercial flat roofs today have a slope of ¼ on 12, meaning they rise ¼ inch for every foot of rooftop.

Some larger and older commercial buildings have completely flat rooftops because the roofing membrane installed at the time needed standing water to perform well. If you have a large commercial building with a dead flat rooftop, the only feasible way to add pitch is to add a tapered insulation system.

Tapered insulation systems use a series of insulation boards stacked in a pattern on the rooftop to create positive roof slope and drainage. Depending on the layout of the building and whether there are existing drains on the roof, the system can be quite costly to install.

In Summary

So in review, if you have a commercial building with a small enough footprint for standard roof trusses, you may be able to convert it from a completely flat roof to a low slope roof with say a 3:12 or greater roof pitch. If your building is large and standard trusses won’t fit, the most practical, but unfortunately, costly solution is to install a tapered insulation system which will turn a dead flat roof into a low-slope one of about ¼ or 1/8 on 12.