Construction of a gable roof, part 3
How to find the total length of the rafters?
– you should find the height of the rafters for each 30 cm of the length of the run, under this figure on the angle bar we find the length of the rafters for each 30 cm of the length of the run;
– we multiply this numeric value by an integer 30 cm sections of the length of the run;
– from this result half the thickness of the ridge purlin of the roof should be subtracted by adding the length of the end of the eaves rafter.
Example:
Find the length of the rafters for the following roof:
roof slope (i)= 1/4 (14° or 25% or 0,25),
roof run (a) = 300 cm,
roof span (2 x a) = 600 cm,
projection of the end of the eaves rafters = 30 cm,
ridge purlin thickness = 50 mm = 5 cm,
number of episodes 30 cm of running length
a/30 cm = 300 cm/30 cm = 10 episodes
Solution:
– consider the drawings,
– height at each 30 cm of length = roof slope (i) x roof span (2 x a) = 1/4 x 600 cm = 150 cm : 10 (episodes) = 15 cm high in one section,
– the length of the rafters on each 30 cm of the length of the run = 33,6 cm,
– length of rafters = length of rafters on each 30 cm of the length of the run x number of segments 30 cm on the length of the run = 33,6 cm x 10 (episodes after 30 cm) = 336,0 cm,
– subtract half the thickness of the ridge purlin:
336,0 cm – 1/2 x 5 cm = 336 cm – 5/2 cm = 336 cm – 2,5 cm = 333,5 cm
– add the length of the end of the eaves rafter:
333,5 cm + 33,6 cm = 367,1
– the full length of the rafters is 367 cm.
Maypots
Maypots are logs placed on each pair of rafters. The introduction of the collar loops reduces the stresses in the rafters between the support point and the vertex. The thickness of the material used for the collar beam depends on the roof span. Overall a board with a cross section 25 x 150 mm is sufficient, however, in the presence of a usable attic, larger sizes must be used, about sections 50 x 100 mm or 50 x 150 mm. The mayopies then serve as ceiling beams, that is, the beams of the apparent floor. The height of the collar flies will also change, because the normal usable height of an attic room is approx 2,40 m.
