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How to Design a Roof – Part 3

The plank-and-beam roof may, of course, have a ceiling beneath the roof concealing its structure. It would be a shame, however, not to take advantage of its potential to enhance the rooms below it, since the heavier structural members can be quite attractive. If the planks are to be exposed to interior space, a means of insulating the roof from above must be devised.

Many designers, it should be noted, consider the 2 inch, 3 inch, or 4 inch thick planking to be insulation enough. In cold climates the 2 inch planking should be beefed up with 1 inch or more of rigid insulation.

If you use thicker insulation board, you will have to use very long roofing nails, not always immediately available. The finished roofing is applied above the insulation, as shown in Figure 5.

Post-and-beam construction

Figure 4 - Post-and-beam construction

plank and beam roofing construction

Figure 5 - Plank and beam roofing construction

One major structural problem must be surmounted when using this kind of construction for the roof – the problem of outward thrust, as shown in Figure 6. The load of the roof, the weight of the rafters, planks, finished roofing, and snow loads, pushes down and outward, tending to flatten the roof and push out the walls at the point of roof support.

This problem is not encountered in framing a roof with an attic, because the ceiling joists act as ties to connect the opposite supporting walls and counter the outward thrust, as shown in Figure 7. Since we have eliminated the ceiling and therefore the joists, we must seek some other remedy for this non-static condition. The rafters may be tied together at the top with collar beams that become part of the exposed roof, as shown in Figure 8.

post and beam outward thrust

Figure 6 - Post and beam outward thrust

Post and beam outward thrust contained with ceiling joists

Figure 7 - Post and beam outward thrust contained with ceiling joists

Post and beam outward thrust contained with collar beam

Figure 8 - Post and beam outward thrust contained with collar beam

Continued......

Part 1 Part 2 Part 4 Part 5 Part 6 Part 7 Part 8