special common rafter shortening amount

[chirpychippy]

Hello all, I am currently stumbling again on a measurement for the special/crown rafter on an equal pitched hipped roof. The hips are single cheek cut at an 1½ thick. Now the common rafter is shortened by half the thickness of the ridge and if this were a double cheek hip then the ridge would grow by half the thickness if the common rafter x 2 there fore the special/crown rafter would be shortened by ½ the thickness of the common rafter . So, the mathematical length of the ridge on this roof is 5 inches to this I add half of the ridge thickness plus half 45 degree thickness of the single cheek hip totalling 2 inches either side of the mathematical length. Does this mean I have to shorten the special/crown rafter by said 2 inches for it to be the correct length??? Thank you in advance.

 

[noseall]

Not a clue.

It would be simpler to argue Einsteins' special theory of relativity than to solve the above!

 

[chirpychippy]

Lets try again, the building is 11625 by 11500 which gives me a ridge length of 125mm because of this short ridge I am going for the single cut cheek hip as apposed to the double cheek cut. there will only be one pair of commons coming off of the ridge and four hip rafters with single cheek cuts to the ridge. I have to extend the ridge from its mathematical length of 125mm to allow for the hip connections.

My roof framing book tells me I have to extend the ridge by half the thickess of the ridge plus half the 45 thickness of the hip = 50mm, so accumulated = ridge length of 225mm.

Because I am extending the ridge by 50mm both sides then to my mind that reduces the run for the crown rafter by 50mm or 2 inches?

 

[freddiemercurystwin]

What'd he say?

 

[Nige F]

If the roof is going up fine apart from this one piece - assuming it`s standing in situ - I`d be up there with a piece of string , or 2 and using that - but then I`m a plumber who could never master measurements - sometimes a piece of string is a usefull measure .

 

[chirpychippy]

Don't worry I will send you s pic I am doing it tomorrow, cheers all the same

 

[noseall]

Are you building (more or less) a pyramid hip?

Are you on about this junction and instead of cutting a double pointy (in blue) you are going for a single cut at the junction......?

Are you going to use a plywood gusset at the junction?

 

[catlad]

I do not understand what a crown rafter is ?

 

[Harbourwoodwork]

yes it would shorten the run of the crown rafter therefore shortening the length ,it makes it a bit harder to do the calculations using this method of having the extended ridge, I am more familiar with with the hip with double plumb cut cut at the top ,this method will push the first common rafter back along the ridge to allow for the longer cut of the hip ,so the clear run from the crown might be different from the run to the first common , I haven't done the maths ,on a roof with double cuts on hip the clear run from the crown rafter is the same as the common that makes the hip go in at 45 deg if you don't get that right the roof will be in a pickle also if the ridge is thinner than the crown rafter you will have to chamfer the the edges of the plumb cut
what is the pitch of the roof

 

[theoldun]

Chirpy, There are no maths involved with your problem, whether you are cutting single cheek or double cheek hip rafters as your hips do not come into the equation at this stage, how ever you will have major problems on a double hip roof of this size.
Will write it up in laymans language that you should understand. Pitch would have been helpful but assume 35 degree with 50mm ridge board. Overall run = 11500, half run = 5750 less half of ridge = 5725 x a 35 degree secant of 1.221 = length of rafter = 6.984 from top plumb cut to heel of birdsmouth. First problem. To support that length of rafter you will need two rafters per slope with a wrap round purlin supported of either spline walls or treble joists.

Back to your problem. Two choices Ridge board all as your post 125mm long, set central to length of building and supported with pair of queens at each end,. Half thickness of ridge, ply collecting gusset at each junction end. This will then make your two kings at each end the same length as your two pairs of queens supporting ridge board.
Or increase length of ridge board by thickness of ridge board and this will make kings and queens same length.

Bear in mind if you are at 35 degree your rafters will be 35 seat and 55 plumb but your hips will be 26.5 seat and 63.5 plumb. You can work out hip length with a secant, but we always take a physical measure and drop the hip 12mm below HAP, be it single or double cheek.
Your other major problem is have you load bearing spline walls to support ceiling joists?

Fifty years ago we would have thrown this size roof up with out to much thought, but these days we would truss it as it will work out so much cheaper.
Regards oldun

 

[Harbourwoodwork]

Chirpy, There are no maths involved with your problem, whether you are cutting single cheek or double cheek hip rafters as your hips do not come into the equation at this stage, how ever you will have major problems on a double hip roof of this size.
Will write it up in laymans language that you should understand. Pitch would have been helpful but assume 35 degree with 50mm ridge board. Overall run = 11500, half run = 5750 less half of ridge = 5725 x a 35 degree secant of 1.221 = length of rafter = 6.984 from top plumb cut to heel of birdsmouth. First problem. To support that length of rafter you will need two rafters per slope with a wrap round purlin supported of either spline walls or treble joists.

Back to your problem. Two choices Ridge board all as your post 125mm long, set central to length of building and supported with pair of queens at each end,. Half thickness of ridge, ply collecting gusset at each junction end. This will then make your two kings at each end the same length as your two pairs of queens supporting ridge board.
Or increase length of ridge board by thickness of ridge board and this will make kings and queens same length.

Bear in mind if you are at 35 degree your rafters will be 35 seat and 55 plumb but your hips will be 26.5 seat and 63.5 plumb. You can work out hip length with a secant, but we always take a physical measure and drop the hip 12mm below HAP, be it single or double cheek.
Your other major problem is have you load bearing spline walls to support ceiling joists?

Fifty years ago we would have thrown this size roof up with out to much thought, but these days we would truss it as it will work out so much cheaper.
Regards oldun

Hi Oldun iv'e only come across this with a double hip, modern spec a bit over kill ,Iv'e only seen these on existing older roofs circa 1300-1600 just out of interest how do you work out the length of the extended ridge ? and what is the difference in the clear run from crown rafter edge to wall plate outer edge and clear run from wall plate edge on the hip end to hip end edge of first common rafter and does the geometrical line of the hip run down the centre of the ridge or down the edge?cheers

 

[chirpychippy]

Thank you very much to the Oldun and everyone else that has replied, I was nervous about this roof, but needn't have been as I pitched it today(or atleast Kings, Queens and Hips) and every measurement was mostly spot on or as near as damn it

Just for the record the pitch of the roof is 34 degrees. I extended the ridge to 225mm from 125mm to make the King/Crown rafter the correct length in relation to the Queen/Common rafter. The hips went in at 26 degrees on a single cheek cut

As far as the ceiling joists are concerned they are 9x3 C24 grade throughout which have been doubled up where the the ashlar walls are being formed to support the roof joists and hips.

I always really appreciate the feedback I am given, because certain cut and pitch roofs make me nervous, this being one of them. I have always done the double cheek cut on hips, for the some reason the single cheek cut threw me, thanks again Chirpy.