Structural ridge: 9" x 2" enough for 13' (4m) span

[jamesd256]

Hi,

Thinking of finishing my garden building with a structural ridge. The span will be 13' (4M), and I have some 9" x 2" stock of sufficient length. I want to avoid rafter ties. The rafters will be 4" x 2" at 24" centres.

Is 9" x 2" enough? Should I screw and glue two together giving 9" x 4"?

The stock is quite rough sawn, so I was thinking of trying to get a smooth face to glue the two piecies if two are necessary. I have a belt sander, but was wondering about buying a power planer.

Any thoughts welcome.

Thanks.

 

[Porque223]

9x4 gives twice the bending resistance as 9x2, as does 11x2. You gain more by increasing the 9 dimension for a given volume of wood, assuming this member stands on edge.

 

[jamesd256]

9x4 gives twice the bending resistance as 9x2, as does 11x2. You gain more by increasing the 9 dimension for a given volume of wood, assuming this member stands on edge.

OK, interesting and good to know. Thing is, I have 9x2's kicking around.

 

[jeds]

Can't say without knowing details of the roof load.

 

[tony1851]

As per jeds, you need to know the area of roof it is supporting, not just the span.

Don't bother gluing the beams together, just screw or bolt them. (Glue has different strength propeties than screws/bolts and the 2 together does not equal the separate strengths combined).

 

[peterperfection]

For the sake of a few quid (peanuts) go for 'overkill'. Bolt a few 12x2s together for the ridge and 'rip down your 9x2s and use them for spars .
Job done !!.

 

[jamesd256]

As per jeds, you need to know the area of roof it is supporting, not just the span.

Don't bother gluing the beams together, just screw or bolt them. (Glue has different strength propeties than screws/bolts and the 2 together does not equal the separate strengths combined).

Jeds, Tony,

Very good point! The floor plan of the building will be 8' by 13' (4 x 2.5 m to be precice). The pitch of the roof will be 12 degrees, and there will be 18mm ply on top of 4" x 2" rafters at 24" centres. The roofing material will be breathable membrane and standard felt. Minimal wind and snow load, situated in a very sheltered spot under a very large oak tree.

Going back to the beam question, presumably the increased sideways resistance of two 9" by 2"'s over a 11" x 2" would be of some value?

 

[tony1851]

The beam will be supporting about 5 sq m of roof, right?

Taking some snow load into account for a worst-case scenario and assume a total load (snow +dead load) of 0.8kN/m^2.

Total load = 4kN.

Bending moment = 4 x 4/8 = 2kNm.

Using two 9x2s, this will produce a bending stress of about 2.5 N/m^2, which is less than one-half the allowable stress, so you will be OK for bending stress.
The other main problem you have to watch with timber is excessive deflection. But your deflection under that load should be no more than about 6mm, which on that span will be fine.

So go for two 9x2s screwed or bolted together.

 

[jamesd256]

The beam will be supporting about 5 sq m of roof, right?
.

Great, thanks for the info. I will use your calculations as a starting point for more research. Learning!

When you say 5 sq m, I calculate my total floor area to be 10 sq m (4 x 2.5)

Taking into account the 12 degree pitch and the overhang I'm going for, it seems to me I'll have rafters of about 1.5m, giving about 6 + 6 = 12 sq m total roof area. Perhaps you meant 5 sq m each side?

 

[tony1851]

Assuming your cross section is as shown on the sketch, your ridge will be supporting approximately 1/2 the roof, ie 1/2 from each side. The overhang over the walls is of no consequence - in fact, the greater the overhang, the less load there will be on the ridge.

A 12 degree slope only adds a very small amount of area, and the load I assumed of 0.8kn/m^2 (about 16lbs/ft^2) is well over what you are assuming.

The two 9x2s = no problem.

Untitled

• tony1851
• 20 May 2013
• 1

 

[jamesd256]

Tony, thank you so much for helping out, very grateful.

Just for fun and to follow your calculations :

I reckon the rough dead weight of the roof to be:

Ply (18mm): 110kg
Felt + membrane: 25kg
Rafters (100mm x 50mm): 42kg

But I'm lead to believe the self weight of the rafters can be excluded from the loading calculation, right? Mass of roof 135kg. Area between walls and ridge at 12 deg = 10.2 m2

If so, my load over 10.2 sq m is roughly 0.13 KN/m2

I found the basic (flat roof) snow load in Essex is 0.5

12 degrees gives a pitch of 2.5 in 12, or roughly a 1.02 slope factor.

Snow load for a pitched roof is flat snow load x slope factor, giving:

0.51+0.13 = 0.64 KN/m2

So, total load over 5.1 m2 = 3.84

...Ok, I give up for tonight, can't find any references for simple bending moment calculation.

Thanks again for inspiration, enjoying learning about this stuff.

 

[Porque223]

Very soon you will need to know vectors, which have a magnitude and a direction. A sci calculator or a spreadsheet helps in this case.

 

[noseall]

Very soon you will need to know vectors, which have a magnitude and a direction. A sci calculator or a spreadsheet helps in this case.

Eh?
He isn't building a particle collider.

 

[AdamCH]

The ridges in my house are made from T&G floorboard. Guess building regs have changed a bit.

 

[jamesd256]

Very soon you will need to know vectors, which have a magnitude and a direction. A sci calculator or a spreadsheet helps in this case.

Eh?
He isn't building a particle collider.

Hehe, how do you know what I'll be doing inside the shed once it's built?

Seriously though, I think he means to follow Tony's calculations further rather than to build a shed.

Since Tony was kind enough to share his knowledge, I thought I would follow it up and learn something. Aspirations to build a house one day.

Luckily I know what a vector is because I did a bit of the maths back in the day...just need to dredge it up.