Couple Roof, 3m at 25 degrees?

theexis

Hi,

I've built concrete block room in the garden and am now ready to roof it. I would like to use a couple roof, 25 degree pitch, covered with Nordman tilesheeting and including a few Velux windows.

Any book I've referenced suggests that couple roofs are fine up to a span of around 3.6m (the room is 3m) but mention nothing about resonable pitch and depth (room is 7m) where wall spread will not be a problem.

I will be constructing the roof with 100x50 rafters against 100x75 wall plates @ 400mm intervals. Would it be safer to include a few collars along the length of the building anyway? Does any of the above sound like a crazy man talking?!

Johnniep

Hey theexis,

Do I understand correctly in that you want to construct a vaulted ceiling. If this is the case, I have attached a page showing the permissable spans for timber rafters based on section sizes.

As you can see, C16 timber (typically available) will span the 3.0m which you need! The minimum pitch of the roof will depend on the roof covering. Talk to Nordmans tech department for the minimum pitch they can cater for.

The only thing with this form of construction is that the the lack of triangulation in the roof form. To cantain the thrust generated by the roof, you either need to introduce collars as you mentioned, or another possibility would be to introduce a band beam at wall plate level to resist the horizontal loads. Either is valid and depends on the look you are going for.

Hope this helps,

J.

Mellor

I have seen info on couple roofs that said they can go up to 3.6m.
BUT if it was me I would rather use a collar roof, for two reasons, one is general peace of mind, the other is that I think they look nicer. The majority of the ceiling will still be sloped, and velux are not a problem.

It is also fine for 25 degree pitch. Nordman say they can go to pitches as low as 4 degrees. So your covered there too.

For a 3m wide room @ 25 degrees. The rather span is <2m. so you would be fine with timbers alot smaller than the ones you quoted. From IS 444, C14 36x100 @ 400 centres will do it. If you already have them not worry.

theexis

Mellor wrote:

From IS 444, C14 36x100 @ 400 centres will do it. If you already have them not worry.

Thanks I already have the timber for the rafters but not the ridgeboard - what would be the appropriate size to use for the 50x100 rafters? I was planing on getting 44x150. Is it possible to get timber 7m or would you typically join a few to create a long ridgeboard?

Mellor

You can get 7m but isn't as easy to find as smaller pieces. Getting a 3m + 4m and joining them might be easier for you.