將2x8s轉換為2x4的拱形天花板桁架系統?


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問題(請參見下圖):我可以安全地將2x8s與2x4國王柱式桁架/開放式after架系統配對,以實現隔熱的開放式天花板/拱形設計。

詳細信息:我有一個長18英尺,寬14英尺的車庫。我正計劃將其緩慢地轉換為成品車間,並且為了美觀起見,更希望使用開放式r架/拱形天花板。我希望將選項轉換為ADU,並且要花大價錢編寫所有代碼。能源法規要求使用R49,除非它是拱形或單r設計,在這種情況下,您可以使用R38。由於我在屋頂和隔熱層之間留出1-2英寸的空隙,我想我可以在2x4 after子上加上2x8姐妹,然後使用6英寸的熱護套R13.1(每2英寸)獲得R39.3並且仍然有足夠的空間容納氣隙。

除了2x6s的國王柱,整個國王柱桁架系統完全由2x4構建。有4個間距為4英尺的桁架,然後有after子,間距為24英寸(最後一個桁架距後壁2英尺)。我位於華盛頓州斯波坎。

我只是擔心2x8的重量會超過屋頂的負荷,尤其是在有開放r子的地方。是這樣嗎,還是可以嗎?關於如何實現這一目標還有其他想法嗎?

謝謝!

這是我正在處理的照片:

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更新#1

這就是我要追求的目標,最終,我明白,當一切都說完之後,只有鐵after和國王哨所才會暴露出來。

enter image description here

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This type of roof construction is what I call “stick built” That is to say individual boards are nailed together in the field in place. Another similar type is “beam and joist”.

The main difference between the two systems is the beam and joist can resist the spreading of the exterior walls when put in a vaulted design. Where as the stick built will “push” the exterior walls outward when the bottom chords are removed for a vaulted design.

I think you have two options:

OPTION #1: Install a “roof tie” from one side (2x8 joist) to the other side (2x8 joist) to keep the joists from spreading apart when you remove the horizontal 2x4’s to create the vaulted design. (If you like the ceiling vaulted up to the peak, the roof tie could remain exposed.)

OPTION #2: Install a ridge beam the length of the roof so the new 2x8 roof joists can rest on the beam. By installing the ridge beam, all of the vertical forces (Live Load and Dead Load) will act vertically on the ridge beam AND exterior walls...thus no need for the roof ties.

I like the idea of using rigid insulation so you can maintain a 1” or 2” air space between the insulation and the bottom of the roof sheathing for ventilation. However, remember the code says ventilation shall be “cross-ventilation”. In order to accomplish this you’ll need to either provide a ridge vent and cut the sheathing back so air can vent in each joist space...or cut holes through the ridge joist so air can circulate from side to side of building.

You are creating a “habitable space”. Therefore you’ll need to bring the entire structure up to code, including energy code (walls and slab on grade), structural code (wind and seismic shear resistance), exit/entrance door (3’ wide minimum), egress window (in sleeping room), bath and kitchen facilities, etc.

To answer your original question, yes 2x8’s at 24” on center sistered to the original rafters will work, but as described in Option 1 and 2, several conditions apply, (i.e.: add roof tie or add ridge beam).

**Update 1:

The new picture shows a several structural trusses with a ridge “beam” spanning from truss to truss. This is not what was described in the OP. Building structural trusses using 2x8 joists for the top chord is possible. Providing a horizontal lower chord with a vertical king post at the center could be similar to the new updated picture. However, the 2x8 joists would be buried in the roof structure in order to provide the air space required for venting.