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How to Design a Roof – Part 8

Flat Roofs:

The word "flat" in the case of a roof is often a misnomer. Roofs, even the ones that are used for decks, are, in most cases, not designed or constructed perfectly flat. A small pitch is usually an integral part of the flat roof design to facilitate the removal of water from the roof. Without any pitch, water and ice tend to build up on the roof. If the roofing membrane is undamaged, the puddling will cause no harm. But if there is even the tiniest of cracks in that membrane, there is a great chance of leakage.

There is a good deal of variety in the design of flat roofs. They may or may not have an overhang. They may be constructed out of posts and beams or out of closely spaced joists. They may or may not sport an “exposed” ceiling.

The various framing patterns are shown in Figures 18. 19 and 20.

 

Joisted flat roof supported by stud walls

Figure 18 - Joisted flat roof supported by stud walls

Joisted flat roof supported by post-and-beam

Figure 19 - Joisted flat roof supported by post-and-beam

Roof beams supported by post-and-beam

Figure 20 - Roof beams supported by post-and-beam

The joisted flat roof is structured like a floor. Building codes must be checked for the design loads on the roof.

Because the roof is flat, and must support snow loads, the ceiling, the built-up roofing, and whatever mechanical pipes and ducts are hung from it, the loads are not as small as those used for pitched roofs. The depth of the joists will be dependent on the design loads and on the spans. Please see the following joist charts:

  1. 40 psf and 12 inch spacing
  2. 40 psf and 16 inch spacing
  3. 30 psf and 12 inch spacing
  4. 30 psf and 16 inch spacing

For post-and-beam construction the methods outlined for the use of Table 5 must be followed to select the beams. Choose 2 inch, 3 inch or 4 inch plank as outlined in step 5 of the method to design for plank-and-beam construction. Bear in mind that this table is designed for live loads of 20 and 30 psf respectively.

Illustration 21 shows the correct method for framing a corner and overhangs in flat roof construction.

flat roof corner and overhang framing

Figure 21 - Flat roof corner and overhang framing

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