Flat Roof Span Question

[Mutty]

Morning All,

I am trying to work out joist calculation for a flat roof that I want to build for my garden office. The joists will have 50mm to 0mm furrings, and osb3 18mm and 1.2mm epdm rubber.

The roof span is approximately 3.12 meters (depth) by 5.4 meters (width).

It's a double block garden office with 100mm cavity (yes overkill). I plan to have overhangs 0.3 meters at the back and 0.5 meters at the front overhangs and 0.1 meter overhangs in the sides

This brings the total roof measurements to 4.52 meter X 5.6/7meters.

But I'm tight on height restrictions so looking to see if I can get away with 5x2 roof joists instead of 6x2 Joists. The 5x2 joists will be 400mm centre and I am looking to have 4 rows of noggings.

Is this possible or is there a high chance or roof sag? If there is that possibility I see people double up on the joists. But how many joists need doubling up? Would just doubling up the centre joists suffice in this case?

Also for this roof size can I go down on osb boards?

 

[Dereekoo]

Based on 5 x 2 C16 with standard snow load of 0.75kn/m2 and assuming weight of roof 0.25kn/m2 then deflection would be about 9mm with an allowable of 10mm, 1mm less if c24 used. This is based on the simply supported span but since you have overhangs they would tend to help reduce the defection. 6 x 2 would reduce the deflection by 4mm. Ignoring snow load would reduce the deflection by 6mm

 

[cdbe]

My garage has 9mm OSB at 450 centres. Bit springy on the odd occasion my 17 stone walks on it but that's all.

 

[Mutty]

@Dereekoo Sorry you gone into abit technically detail, but is the deflection amount good or bad as I don't get that bit.

But if the deflection is not good due to the size, I was to buy C24 5x2 and if these were slightly under spec as the span is wider, then looking at doubling up the centre joist, and possibly the end joist or miss the next two next to the centre and double up the next two. So having 1-3 extra joist. Also have 4 rows or nogging and have 4x2 noggings to allow air flow as I will eventually have it insulated as a cold roof.

Does the revised details make it more secure and not prone to sagging?

 

[Mutty]

My garage has 9mm OSB at 450 centres. Bit springy on the odd occasion my 17 stone walks on it but that's all.

Ah I see looks like 18mm is the way forward with the OSB3.

 

[cdbe]

Ah I see looks like 18mm is the way forward with the OSB3.

It is if you like playing football on your roof, otherwise 9 or 12mm is fine.

 

[Dereekoo]

@Dereekoo Sorry you gone into abit technically detail, but is the deflection amount good or bad as I don't get that bit.

But if the deflection is not good due to the size, I was to buy C24 5x2 and if these were slightly under spec as the span is wider, then looking at doubling up the centre joist, and possibly the end joist or miss the next two next to the centre and double up the next two. So having 1-3 extra joist. Also have 4 rows or nogging and have 4x2 noggings to allow air flow as I will eventually have it insulated as a cold roof.

Does the revised details make it more secure and not prone to sagging?

The maximum amount of sag that TRADA span tables allow for is 1/360 of the span so in your case with a 3.3m effective span of your support joists gives an acceptable sag fig of 10mm (rounded up), the actual calculated sag is 9mm so it is up to you whether you are ok with this industry standard (which only happens when you have snow on the roof) or you want to reduce the amount of sag for whatever reasons you may have. Doubling up joists would reduce the sag in the immediate area but after a couple of joists to one side in would be of no benefit. Not sure what you mean by joists slightly under spec, if they are smaller or span further or of a lesser grade then the calcs change and as you are on the cusp of deflection you may not find this acceptable, its all down to what the client (you) wants. As an aside one set of noggins at mid spanis recommended in TRADA tables unless you want more for OSB fixing purposes

Ah I see looks like 18mm is the way forward with the OSB3.

Why, how often are you going for walkabouts on the roof and CDBE joists could be spanning further or just under sized

 

[Mutty]

@Dereekoo So from your reply, using the 5x2 joist is within tolerance without adding any extra joists and having a single row is nogging is within tolerance.

I just don't want a situation further down the road where over time sagging could occur, particularly if I can add extra support measures now ie doubling up in key area that are vulnerable to sagging ie the centre.

Also I am not going to be walking on the roof, just want a stable roof that is adequate for the EPDM and the joists. Also can you clarify this question

CDBE joists could be spanning further or just under sized

 

[noseall]

My garage has 9mm OSB

 

[Dereekoo]

Also can you clarify this question

CDBE joists could be spanning further or just under sized

Whats the question?

 

[Mutty]

What do you mean when you say this layman terms.

 

[catlad]

5X2 will be ok.

 

[Mutty]

@Dereekoo how does adding 4 rows of nogging and adding one double joist in the middle of the roof affect the sag figures?

Is this something you know how to work out?

Thanks in advance

 

[Dereekoo]

I am saying that CDBE could be experiencing bounce because the joists are under sized for the span they are being used for or the span is to long for the size of joists, which basically is the same thing.
4 toes of noggins make no difference to the sag.Doubling up one of the joists in the middle of roof makes little difference to the rest of the roof sag

 

[Mutty]

I am saying that CDBE could be experiencing bounce because the joists are under sized for the span they are being used for or the span is to long for the size of joists, which basically is the same thing.
4 toes of noggins make no difference to the sag.Doubling up one of the joists in the middle of roof makes little difference to the rest of the roof sag

So what options would you recommend? As I do want to continue with 5x2.