Why do builders make double rafters on a hipped roof and insert trim boards between them?

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During the construction of the rafter system of the hipped roof, you can see that the diagonal rafters are double.

The roof rafter system must be designed for snow load. Depending on the length of the rafters, their cross section is chosen. As a rule, it is 150x50 mm or 200x50 mm. Now this can be done in any online roof calculator.

But often, boards of two sections are brought to the object - for the rafters and for the lathing. Therefore, to strengthen the diagonal rafters, they are made from a double board. Diagonal rafters of a four-pitched roof perceive the main load. But why do the builders insert pieces of boards between them, the so-called "crackers" or "lugs"?

The diagram on the left shows the installation rule for these inserts.

In theory, this technique is necessary when building rafters. And in the photo below (building a roof on my house) you can see that the whole diagonal is made this way.

"Crackers" do not affect the strength of the diagonal rafters. This technique is explained by the following reason.

Boards arrive at the facility damp. If they come into contact with each other, then a fungus will start between them and may even destroy the wood. It may rain, which will also provoke the appearance of bio-damage in the place of the tight connection of the rafters. And so the boards are ventilated and dry. The only thing is that the places where "crackers" are installed to some extent can "turn blue" from the inside. But this is not as critical as if all the diagonal boards began to be covered with fungus in the place of their close contact.

Something tells me that not all builders understand why they make rafters according to such a scheme. They know that it is so accepted, that everyone does it.

Even in this design, it is easy to fix the support. For reinforcement under snow load, props are required if construction is carried out from a board of 150x50 mm.

There were examples when builders erected kiln-dried timber roofs. Previously, such lumber was more or less affordable. But in this case, the boards must also be mounted through the inserts. Initially dry wood does not guarantee that it will stay this way forever. Condensation can form in the attic and the boards will get wet. If they do not dry out quickly, then the biosecurity will not cope and a fungus will also form, destroying the wood.

You can read the article about preventing condensation in a cold attic: A common problem: in a cold attic, condensation or frost forms and the walls get wet. Simple ways to prevent it