Calculate the length of a roof rafter.

Learning how to estimate the 35mm standard roof truss is not only left in the contractors hands. Dig deep to know what you need as enough materials. Let your roof contractor find you with ideal or starting knowledge.

Let’s start by letting you know that 35mm standard roof trussis one of the most common roof structures and measuring its length rafter is easy. We have all learnt Pythagorean Theorem at one point from our education backgrounds and this is enough for such a task.

Pitch. This is the highest point of the roof and is an important point when measuring the truss

The rafter run represents the distance the lumber runs on the width roof area. So we add the extra 2ft on each side to make 24 and divide by 2. One side of the rafter is 12ft run. Change to inches 12ft *4= 48inches

The rafter rise represents the distance the lumber rise on the height roof area in 12 inches. Every one ft rises 6 inch converts to ft 0.5 ft*12 inc converts to 1 ft. The total rise is 72 inches. The blue line is 72inches.

Let’s get the rafter distance now.

A2+b2=c2

48*48+0.5*0.5=c2

144.25=c2

Sq. rt of 144.25=c

Remember c is the rafter distance equaling to 576.0312”

4*12 rise rafter dimension needed

So how many roof trusses.

Length of the roof plus overhangs=27ft

On space center=24” converted to 2ft

Round off and add one extra truss.

(27/2)+1=15 truss count/ trussed rafters

120 lumber of dimensions of length 10 or 12ft

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Truss design spacing.

Roofers are carefully designing structural assemblies that include roof rafters which in truss designs are called top chords and ceiling joists.

Truss means reinforcing members of top and bottom chords to add strength to the truss. The design of a truss may allow use of a lower grade from lumber than rafters with comparable spacing and span.

Metal roofs

Metal roofs directly rest on the raft. Horizontal runs will tie together the rafters in one position and the metal roof. Remember that this roof is so light since it has got no added weight of the plywood ceiling.

This metal roof is lighter than asphalt shingles roofs. This keeps the space in the rafters a bit large. So the roof truss may still be 2 – 4 feet between the rafters except when we are having complicated designs.

Effects of rafter spacing

Spacing in rafters in a given building is calculated by engineers according to the building structure. These two factors below dictate the size and spacing between rafters.

• Type of wood used for rafters
• And the load that the rafters need to support

According to the factors above, the rafter number will either decrease or increase on a particular building.

How to calculate a roof truss.

A roof truss contains spaces and mostly made strong to attach the roofing materials like roof sheets, shingles. And many other categories. Roof truss is easy measured for gable roofs. The formulae is to Get the length of the roof and divide by 2.

Our example was 27ft length. And we can divide that by 2 and have 14 truss.

What is the standard spacing of a roof truss?

There are two common dimensions of a roof truss 12”, 24” and 16” in the roofing world.

24” inch is mostly used on a roof truss because the weight is split evenly to the next roof truss and the does not consume a lot of material like the 16” roof truss. It decreases the rafters by 50%. But remember a rafter spacing should be chosen according to the joist and rafter.

Estimating the number of rafters

The metric formulae to space rafter is ((length of the roof + overhang distances/rafter spacing)+1)*2.

For length of ((23ft +2ft+2ft/2ft)+1)*2=29 rafters

This is a formulae for finding the estimated rafters on the roof for our previous example of costs to to build a 2 bedroom house.

What size of rafters do I need for a 24” roof spacing?

If the materials used are not so heavy like a metallic roof, common rafters for 24’ spacing start from 2 by 4 lumber timber.

They are strong to hold the roof for a long duration. But if you are to use asphalt shingles, you need to use a more large timber dimension for safety and durability.

How far can a 2*10 rafter span without support?

It brings us to how far apart should rafters be spaced? A rafter space should not exceed 24 inch. Without support unless the designs are supported by walls. But they are recommended to have rafter ties and support for safety. 2*10 rafter spacing is not to exceed 24 inch without support.

How many rafters do I need for a 16 foot span?

The metric formulae to space rafter is ((length of the roof + overhang distances/rafter spacing)+1)*2.

16+2+2=18/2=9+1=10*2=20 rafters.

A 16 foot span will need 20 rafters for either a hip roof or gable roof.

Rafter tie vs. ceiling joists

Rafter tie is mostly a 2 by 4. 48 inch space to another center. It’s connected in an angle using nails to the rafter. It is connected to both sides of the wall preventing the wall from spreading. They can also be used in the opposite direction (length to length rafter ties). It’s recommended to use a 2 by 6 if the width of the roof is longer than 20ft. could also connect a brace to connect the rafter to the ridge.

Ceiling joists 16 inch from center, a few more nails than the rafter tie, its normally a 2 by 10 Douglas fir, then they are used to hold up a ceiling.

How much does one truss cost?

It depends on the length of the rise rafter and width. But with our previous 2 bedroom house plan of 24 ft width and 6/12” rafter rise , one truss will consume not more than 8 lumber timber.