Converting flat roof to pitched - timber span

4 Oct 2012
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United Kingdom
Hi I am in the process of converting my porch flat roof into a mono pitched roof using plain concrete tiles. The existing roof is 1.8m deep and 3.8m wide.

My concern is that the front 6x2" beam where all the weight will now be is only supported each end by brickwork, as the front of the porch is set back around 500mm (to provide shelter from the rain).

I was thinking of doubling up the 6x2" beam with another laid horizontally over the top of the existing one and bringing the rafters down past it using a birds mouth cut. Would this be ok or do i need to do something else?

I am planning to make the 6x2" rafters 600mm centres.

Being a novice I just want to make sure it is safe and doesn't sag when the tiles are put on!

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If I understand right you want to use 2 no 2x6" spanning 3.8m? then I would say its not strong enough and will sag after a while.
If you really want to use timber you could make a flitch beam by sandwiching a piece of flat steel 12mm x150mm between your 6"x2" and bolt them together say every 16" with 12mm bolts.
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The flitch beam looks interesting but fairly expensive.

Would a 75mm x 200m or similar beam be OK to use?

Also does it change anything it I make the rafters 400mm centres instead of 600?

To my rough calculations you will be loading 5 to 6 hundred kgs on that beam if using concrete tiles, and my instinct says over time it will sag.
OK that doesn't seem like a great option then.

If I was to do it properly and used an RSJ what size would it need to be?

Use a 7" or 8" x 3" timber purlin.

Is this roof open/vaulted?

If not you can supplement the roof support by fixing 18mm ply gussets onto say every other rafter/joist, forming a rigid triangle that must butt tight against the upright wall and pole plate.

This method relys upon a good fixing of the pole plate and you would need to add an angle bracket from pole plate to gusset at the top.

This will help prevent rotation if done correctly.
Hi the ceiling will not be open/vaulted.

From what i can understand you are recommending something similar to this? (Please excuse the poor diagram!)

The only bit i dont fully understand is "This method relys upon a good fixing of the pole plate and you would need to add an angle bracket from pole plate to gusset at the top. " Can you elaborate on this a bit more?


Here are some more photos that might also help:
The ply gusset would need to have its 'back' against the wall to be effective and link up to the pole plate. It is there to resist movement away from the wall thus preventing the outward point of the triangle from tipping or sagging.

No rotation - no sag.
Ok thats shed a bit more light on the situation I think your plan of 2x 6x2" will be ok seated over the door and windows will be fine, if you can not get a suitable pitch then you can put osb over your new joists then staple some bitumin roofing felt to that and then counter baton and then your tiles. Not forgetting your insulation.
Ok so something like this is what I was thinking initially.

I guess i could also triangulate it off the other beam that is resting above the window/doors if I wanted.

Would they be better horizontally than like this though?

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