Flat Roof

[cfz164]

Good afternoon,

Could i ask you chippies some advice please?

I am going to be building a timber framed garden room, fed up of the kids under my feet! HA.

It will be 4m wide and 5 m long.

It will be within boundry limits, so no planning permission means under 2.5m which im cool with.

So in my head i have my concrete base down, and walls up in 2m panels bolted together in 4x2 treated timber.

Now onto the roof.

Im thinking:

Adding another 4x2 on top of my top wall plate.

Using 8x2 for the roof at 400 centres and then 4x2 to nothing furrings screwed ontop to get my slight gradiant.
Cutting a 2 inch notch out of the 8x2 at both ends (about 200 in off each end as i want a roof overlap front of back of the walls) and sitting it over the extra 4x2 header i put on.

So the roof is in essence 10 inch high at the front as ill "lose" 2 inch from the 8x2 with it notched into the extra header - this will keep me nicely within my height restriction then allowing me to still use correct size timber for the roof.

Does this sound normal, workable, ok?
If not how should it be done?

I appreciate any advice you can give in your spare time, i appreciate your all very busy people.

Thankyou

 

[cfz164]

sorry for the rather crude drawing - just thought it may be easier to see what i mean rather than some half wit trying to explain it!

Black - timber frame
Red - extra 4x2 header
Orange - 8x2 notched over extra header

 

[carlb40]

TBH i wouldn't bother notching the timbers as it will create too much work and i think reduce the effective timber size ( strength wise ) to 6 x 2. The joists nailed/screwed to the wall plates will be enough to stop the walls spreading. You can add joist hangers/ truss clips as well to give additional strength/ support.

 

[Carlow52]

whats the plan for insulating the roof and then ventilation of the enclosed airspace?
Rather than one long slope, perhaps make it a very flat A

 

[cfz164]

@Carb - could I use 6x2 + 4x2 furrings in your opinion for that span?
I don't want the whole roof height to exceed 10 inch as that takes me borderline 2.5m.



@carlow52 - insulating with kingspan - thickness depends on what size joists I go with...be 80mm I'd imagine to create a cold space roof.
ventilation in the roof space?? Didn't think of it.... Assuming I need something if you have mentioned it?

Walls will be wrapped with breathable membrane, didn't think of the roof though. What do you suggest?

I couldn't create a shallow pitch with 10 inch could I?



Thanks

 

[EBH1]

I would construct a warm deck flat roof, where the insulation sits on the top. There seems to be a real shift to warm deck roof which negates the need to ventilate the cavity. From memory this would need to be a clear 50mm ventilated cavity. On my flat roof we've just built (which was specified by building control) we have 150mm rigid PIR insulation on the top with a single ply membrane on the top of that. You can also use a 3 layer felt system rather than the single ply. You'll also need polythene on the warm side of the insulation. On the 3 x 4m flat roof section on our house the engineer specified 175 x 45 C16 timbers for the roof joists with 18mm OSB on top of that and 145 x 45 C16 for the walls.
Cheers,
Euan

 

[OLD]

No firings required just make stud work give you a slope.
Look up kingspan for flat roofs there warm roof system it does not compress when walked on worked for me with torched on covering.

 

[RobinBHM]

Just thought I would mention, between 15 and 30 sq metres a building needs to be a min of 1.0m from a boundary or made of non combustible material.

You could make the roof with 2no 8x2 set at 1/3 and 2/3 distance along the 5.0m and then infill with 4x2 cross ways on jiffy hangers. If you want to insulate with a warm roof the celetex could sit on the joists. Any firrings would sit under the insulation. There would be some thermal bridging at the 8x2 points but it is still timber.