The size of the timber needed to span 2m, 2.4m, 2.5m, 3m, 3.6m, 4m, 4.8m, 5m & 6m depends on various factors, including the type of timber, load requirements, and the intended use.

In general, the size of the timber required for 2m span need to be 47×120, while the size of the timber for 3m span have to be 47×170. Likewise, the size of the timber for 4m span ought to be 47×220, while the size of the timber for 5m span need to be 75×220. In addition, the size of the timber required for 6m span ought to be 75×245.

There are many species of wood/ timber or dimensional lumber used in house construction as wood frame structure. It is used as floor joist, ceiling joist, pitch roof rafter, flat roof rafter, purlin and Binder.

There are different depth of floor joist made of timber are used in floor construction such as 47×120, 47×145, 47×170, 47×195, 47×220, 75×175, 75×220 etc made of different species of wood (southern yellow pine & Douglas fir) and different grade such as grade – 1, grade – 2, grade – 3. Southern yellow pine is strongest species of wood that can span more distance compared to other species of wood. Grade 1 lumber have higher strength than other grade Lumber due to fewer knots.

Ceiling joists provide support for more than one surface in your home. The first surface is, unsurprisingly, the ceiling, the materials of which attach directly to the joists. The other surfaces are the walls, which the joists span the distance between, providing a source of support to keep the walls from shifting.

Floor joists can be define as several small beams. Floor joists are typically a horizontal structural member that runs across an open space of building on supporting wall or studs. Their function is similar as beams, except floor joists are supported by wall, foundations, or beams framing.

Floor joists usually run between walls or rafters to support a ceiling or floor. They transfer the dead load of the subfloor and floor finishes and live load of people, furniture and furnishings to beams, walls, headers, or foundation walls. It is like the skeleton/ framing of a building that supports the load which the floor is built to bear. Joists are technically beams that typically spans relatively short distances compared to beams and are made of wood, steel, or engineered wood or lumber.

Spacing between floor joist for different sizes are kept at 300mm, 400mm and 600mm apart. The joist which are kept at lower spacing can allow longer distance to span as compared to joist spaced further apart. Standard spacing between floors joist is 400 mm centre to center.

What size timber to span 2m, 2.4m, 2.5m, 3m, 3.6m, 4m, 4.8m, 5m & 6m depending on different size of timber, species of wood, grade of wood, spacing between the joist, expected dead and live load of structure that coming to joist. The loading includes dead load of the weight of the frame structure & fixed weight and live load of People, furniture, appliances and a snow on the roof.

**What size timber to span 2m, 2.4m, 2.5m, 3m, 3.6m, 4m, 4.8m, 5m & 6m**

In general, the size of the timber needed to span 2m should be 47×120, while the size of the timber needed to span 3m should be 47×170. Likewise, the size of the timber needed to span 4m should be 47×220, while the size of the timber needed to span 5m should be 75×220. In addition, the size of the timber needed to span 6m should be 75×245.

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**What size timber to span 2m**

The size of timber needed to span 2m have to be **47×120 mm. A ****47×120 mm** size of timber as floor joists **can allow to span** **2m** (metres) when spaced 600mm apart for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2. So, for **2 meters** span you need a **47×120 mm** size of timber as floor joists at standard 600 mm spaced centre to centre.

**What size timber to span 2.4m**

The size of timber needed to span 2.4m have to be **47×145 mm. ****A 47×145 mm** size of timber as floor joists can allow to span 2.4m (metres) for permanent loads are assumed to be **50 kg/m2** and imposed load of **150kg/m2** when spaced **600mm** apart. Thus, for **2.4m** span you will need a **47×145 mm** size of timber as floor joists at standard 600 mm spaced centre to centre.

**What size timber to span 2.5m**

The size of timber needed to span 2.5m have to be **47×145 mm. A**** 47×145 mm** size of timber as floor joists can allow to spans **2.5m** (metres) for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced 600mm apart. Thus, for **2.5m** span you will need a **47×145 mm** size of timber as floor joists at standard 600 mm spaced centre to centre.

**What size timber for 3m span flat roof**

The size of timber needed to span 3m have to be **47×140 mm. **As per UK practices, you need the best timber joist size for a 3m span flat roof is 47×140 mm in which the depth of the timber joist is 140mm and the joist width is 47mm when spaced 400mm apart and permanent loads 50 kg/m2 and imposed load of 150kg/m2.

**According** **to** UK **practice** the best timber size for a 3m span flat roof is **47** **x** **140mm,** **where** the depth of the timber joist is **140mm,** **the** **width** **of** the joist is 47mm **at** **a** **spacing** **of** **400mm,** **with** **a** **dead** **load** **of** **50kg/m2** and **a** **live** load of 150kg/m2.

Typically, for 3m span flat roof, a** 47×170 mm** size of timber as floor joists are required at permanent loads 50 kg/m2 and imposed load of 150kg/m2 when spaced 400mm apart. Thus, for **3m** span you will need a **47×170 mm** size of timber as floor joists at standard 400 mm spaced centre to centre.

**What size timber to span 3m**

A **47×170 mm** size of timber as floor joists can allow to span 3m (metres) for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced 400mm apart. Thus, for **3m** span you will need a **47×170 mm** size of timber as floor joists at standard 400 mm spaced centre to centre.

**What size timber to span 3.5m**

A **47×195 mm** size of timber as floor joists can allow to span **3.5m** (metres) for permanent loads are assumed to be **50 kg/m2** and imposed load of **150kg/m2** when spaced **400mm apart**. Thus, for **3.5m** span you will need a **47×170 mm** size of timber as floor joists at standard 400 mm spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 3.5m span flat roof is **47** **x** 170**mm,** **where** the depth of the timber joist is 170**mm,** **the** **width** **of** the joist is 47mm **at** **a** **spacing** **of** 4**00mm.**

**What size timber to span 3.6m**

A **47×195 mm** size of timber as floor joists can allow to span 3.6m (metres) for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced **400mm** apart. Thus, for **3.6m** span you will need a **47×170 mm** size of timber as floor joists at standard **400 mm** spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 3.6m span flat roof is **47** **x** 170**mm,** **where** the depth of the timber joist is 170**mm,** **the** **width** **of** the joist is 47mm **at** **a** **spacing** **of** 4**00mm.**

**What size timber to span 4m**

**A 47×220 mm** size of timber as floor joists can allow to span **4m** (metres) for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced **400mm** apart. Thus, for **4m** span you will need a **47×220 mm** size of timber as floor joists at standard **400 mm** spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 4m span flat roof is **47** **x** 220**mm,** **where** the depth of the timber joist is 22**0mm,** **the** **width** **of** the joist is 47mm **at** **a** **spacing** **of** **400mm,** **with** **a** **dead** **load** **of** **50kg/m2** and **a** **live** load of 150kg/m2.

**What size timber to span 4.5m**

A **75×195 mm** size of timber as floor joists can allow to span **4.5m** metres for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced **400mm** apart. Thus, for **4.5m** span you will need a **75×195 mm** size of timber as floor joists at standard **400 mm** spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 4.5m span flat roof is **75** **x** 195**mm,** **where** the depth of the timber joist is 195**mm,** **the** **width** **of** the joist is 75mm **at** **a** **spacing** **of** **400mm.**

**What size timber to span 4.8m**

A **75×220 mm** size of timber **C16 class** as floor joists can allow to span **4.8m** (metres) for permanent loads are assumed to be 50 kg/m2 and imposed load of 150kg/m2 when spaced 400mm apart. Thus, for **4.8m** span you will need a **75×220 mm** size of timber as floor joists at standard **400 mm** spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 4.8m span flat roof is **75** **x** 220**mm,** **where** the depth of the timber joist is 220**mm,** **the** **width** **of** the joist is 75mm **at** **a** **spacing** **of** 4**00mm.**

**What size timber to span 5m**

A **75×220 mm** size of timber **C16 class** as **floor joists** can allow to span 5m (metres) for permanent loads are assumed to be **50 kg/m2** and imposed load of **150kg/m2** when spaced 400mm apart. Thus, for 5m span you will need a **75×220 mm** size of timber as floor joists at standard **400 mm** spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 5m span flat roof is **75** **x** 220**mm,** **where** the depth of the timber joist is 220**mm,** **the** **width** **of** the joist is 75mm **at** **a** **spacing** **of** 4**00mm.**

**What size timber to span 6m**

A **75×245 mm size of timber C16** class as floor joists can allow to span **6m** (metres) for permanent loads are assumed to be **50 kg/m2** and imposed load of **150kg/m2** when spaced **400mm** apart. Thus, for **6m** span you will need a **75×245 mm** size of timber as floor joists at standard 400 mm spaced centre to centre.

**According** **to** UK **practice** the best timber size for a 6m span flat roof is **75** **x** 245**mm,** **where** the depth of the timber joist is 245**mm,** **the** **width** **of** the joist is 75mm **at** **a** **spacing** **of** 4**00mm.**

**Conclusion**:-

For flat roof, **a 47×120 mm** size of timber can allow to span upto **2m** as floor joist. Thus you will need a **47×120 mm** size of timber to span **2 metres** as floor joist.