I have a flat roof connected to the main pitched roof. The dimensions of the flat roof are (roughly) about 15X15. I maintain the roof pretty well. It has (currently) no leaks. The roof is about 75 years old.

There is a slight pitch to the roof. There are three vents on the roof. These have no problems. The underlayment of the roof is "Tongue and Groove", yellow pine. Asphalt roofing rolls, and 20 LB felt, are in good shape, and the roofing rafters are on 15.


I recently installed a large casement window that allows easy access to the flat roof.
I am interested in installing a small deck area of about 40 feet, using interlocking teak wood tiles (12X12).
This is a completely non invasive install! (hopefully)

These tiles have a plastic interlocking grid system that is about 1/4 of an inch high.
Will the roof be OK?

I figure the most weight that the deck will be dealing with is me and a lawn chair.

(figure 200 lbs.!)

 

Flexing of the roof due to walking about up there will result in leaks.

 

I would look into flat roof floating deck construction. Basically it's building a framework to support the deck which spreads the load and separates the deck from the roof. Especially important with roofs not specifically designed for foot traffic.