Roof framing made easy

IX placing this little work before the student of Architecture or Building Construction, I would state that it is not intended for those uneducated but for those who, desirous of becoming proficient in the higher principles of construction, wish to study and apply the best methods in actual daily practice. With the assurance to the student, that he will find the contents, i studied, will return him full remuneration by his becoming more valuable on account of his increased knowledge, I send it forth confidently. The cardboard models will prove the accuracy of the methods described. The articles being originally published in The Carpenter, are now issued edited and revised.

The entire work is dedicated to my wife, by whose aid and encouragement I have been enabled to persevere and succeed in technical principles.

With a view of explaining the principle of the truss and its practical application in the construction of roofs and bridges, I have commenced with this chapter.

Let A B and A C be two rafters resting together at the ridge or point, as A. Even by their own weight, these two rafters would have a tendency to slip at the points B and C, and to sink at A. If a tie rod or beam be stretched from B to C, and the rafters, A B and B C, be made stiff or rigid, and the tie, B C, not liable to stretch, then A will be made a fixed point. This is the ordinary roof of two rafters in which the tie, B C, is the attic floor beams, and which form may be used for houses of small span.

When the span is wide, so wide in fact that the tie, B C, being unsupported in the centre, tends to sag by reason of its length, then the conditions of stability are injured. Now if from the point or peak A a string or tie be let down and attached to the middle of B C, as D, it will then be impossible for B C to bend or sag down, as long as A B and B C are the same length. D will be also like a stationary point if the suspension or tie A D be of iron or wood and not stretch. But the span may be increased, or the size of the rafters A B and A C diminished until the rafters tend to sag, and to prevent this, "struts," as D E and D F, are set in, reaching from the stationary point D to the middle of each rafter, or to the centre of its length, as E and F; thus making E and F stationary points, provided the struts E D and F D remain their full length.

By this means they "truss" or tie up, the point D, and the frame, A B D C, is a trussed frame, or in the term applied in carpentry, a "truss." Similarly, if D C be long its centre can be suspended from the fixed point E by a suspension rod, as E G.

In every truss there are two principal strains exerted on the members. These are termed Compression and Tension. For this simple truss the rafters A B and A C are in Compression, or being pushed together. A D and B C are extended, or in Tension. Those which are in tension can either be made of wood (as wood is not: liable to stretch) or of wrought iron rods, but never of ropes, or any material likely to stretch easily.

From the above, the student will understand that: the rafters, by their not being subject to compression or crushing, and the tie rod or beam, not being liable to stretch, or, in better words, subject to tension, and the suspension rod complete the truss, thus preventing the sagging of the centre of the tie beam.

In modern roof construction, engineers, as a rule, use timber for rafters and struts and iron for tie and suspension rods; these materials being light and easily put together ; and I am sure many readers will meet roofs of this class.

In the ordinary form of house roof shown at Fig. 2, the rafters are in compression, the ties, or attic floor beams, in tension, and the collar beam is in compression, as it takes the place of the struts, yet gives the head room.


CHAPTER III - Hip and Valley Roofs.

The next roof which I produce is one of the hip and valley class, or a main rectangular building, with an L or addition. A B C F D E, Fig. 5, is the plan of the building and the outside line of the wall plates. The roof is of half pitch or square pitch as some mechanics call it, which means that the height of the roof is equal to half the width of the house. The house has two gables, one on each end of the main part with a hip on the L, and the intersection of the L roof with the main roof produces two valleys. E I D is the plan of the hip and E I D, is the elevation of it shown on the elevation Fig. 6, where the general view of the constructed roof is shown. Q J, and J F, (Fig 5), are the valleys on the plan.



In framing this roof the simplest way is as follows: To obtain lengths and bevels of the common rafter, produce the ridge line G J H to L and K. Join A K, and K Q ; also B L, and L C. A K, will be the neat length of the common rafter, if no ridge board is inserted; but if there be a ridge board, half its thickness must be sawn off the length on the bevel. K is the bevel for the top or peak cut and A, the bevel for the cut on the plate. Any ordinary mind will see the simplicity of this method.

For the hip rafters which will stand over the seats E I, and D I, produce the line D I, to M, and set off on it the height of the pitch I M, equal to K G. Join M E; M E will be the exact length of the hip rafter required, and the bevel at M will fit the top cut, and that at E the plate cut. In regard to the cuts for the jack rafters, which run up the hips and valleys, it might be said that the top cuts against the ridges for the rafters which run up the valleys have the top cut the same as the common rafter top cut. The bottom one which nails against it, can be readily determined by the following simple method: Produce the ridge line J I, to N, and make
D N, and N E, equal to M E, the length of the hip. W is the jack on its seat or as it will appear in position. X is the exact length of it from the plate line to the hip, and the bevel at X will be the exact bevel for all jacks both on hips and valleys ; being reversed for different sides, right and left hand.