YOUR RIDGED ROOF: What you need to know

A ridged roof has declined rafters that rest on vertical wall-plates on top of each wall. Rafters

are series of sloped beams that extend from the ridge to the edge of the roof. They are

designed to support the roof deck and its associated loads. The top ends of the rafters meet at

the ridge beam. Purlins are then fixed to the rafters and used in supporting a roof.

A purlin is any longitudinal, horizontal, structural member in a roof. Horizontal purlins support

the roof covering. Heavier under-purlin support longer rafter spans. Tie beams, which are also

called rafter ties, are connected between the lower ends of opposite rafters to prevent them

from spreading and forcing the walls apart. Rafter ties are designed to tie together the bottoms

of opposing rafters on a roof, to resist the outward thrust where the roof meets the house ceiling

and walls. Collar beams may be fixed higher up between opposite rafters for extra strength.

Collar beams is a horizontal member between two rafters and are very common in domestic

roof construction. A collar beam is often called a collar tie but this is rarely correct. A tie in

building construction is an element in tension rather than compression and most collar beams

are designed to work in compression to keep the rafters from sagging. A collar near the bottom

of the rafters may replace a tie beam and be designed to keep the rafters from spreading, thus

are in tension: these are correctly called a collar tie.

The rafters and tie beams serve to transmit the weight of the roof to the walls of the building.

There are a number of structural systems employed to facilitate this:

1.) The use of wall-plates set at the top of the wall

2.) Hammer-beams, which spread the weight down the wall and create equilibrium between

outward and upward thrust. Hammer beams are short beams projecting from the wall on which

the rafters land, essentially a tie beam which has the middle cut out.

3.) King posts, which transfer the weight of the roof ridge, and various types of trusses. A king

post is a central vertical supporting post used in architectural, bridge, or aircraft design

applications.

The main consideration in ridged-roof construction is to hold the roof down during severe storm

or weather. Every component of the roof has to withstand the uplift forces of high wind speeds.

This is not normally a problem in areas not prone to high wind.

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