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Default Best way to get a shallow slope on a flat roof

Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per foot
is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush with
the wall plate and rest the base of the high end of the joist on the wall
plate. This will require the low end cutting at an angle to sit properly in
the joist hanger and the high end notching to sit onto the wall plate.
Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist
hangers. Both ends will need cutting at an angle to fit the joist hangers.
Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front and
back wall. Fit a profiled piece of timber above the joist. Simple hanging,
no angles cut, less lateral force on the walls (not that there should be
much anyway). Flat ceiling. Downside (if any) seems to be the odd shaped
roof void which will give a lot of air space above the insulation at the
higher end.

What dos the team think?

TIA

Dave R

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Default Best way to get a shallow slope on a flat roof

On Wed, 10 Mar 2010 11:17:27 -0000, David WE Roberts wrote:

Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per foot
is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush with
the wall plate and rest the base of the high end of the joist on the wall
plate. This will require the low end cutting at an angle to sit properly in
the joist hanger and the high end notching to sit onto the wall plate.
Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist
hangers. Both ends will need cutting at an angle to fit the joist hangers.
Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front and
back wall. Fit a profiled piece of timber above the joist. Simple hanging,
no angles cut, less lateral force on the walls (not that there should be
much anyway). Flat ceiling. Downside (if any) seems to be the odd shaped
roof void which will give a lot of air space above the insulation at the
higher end.

What dos the team think?

TIA

Dave R


One thing to watch ... I used galvanised steel sheet for covering my
shallow angle garage roof (existing wooden roof) and got water coming in -
it turned out that at shallow angles, the water can run back under the
lower edge of the sheet and far enough uphill to get into the structure. I
solved the problem by screwing a 3/8" piece of angle 3/4" back from the
edge as drip-edge. Luckily there was no problem with the high part of the
profile (despite it being flat on top) and so I could just run the angle
straight across the bottom.

SteveW
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Default Best way to get a shallow slope on a flat roof

David WE Roberts wrote:
Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per
foot is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush
with the wall plate and rest the base of the high end of the joist on
the wall plate. This will require the low end cutting at an angle to sit
properly in the joist hanger and the high end notching to sit onto the
wall plate. Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist
hangers. Both ends will need cutting at an angle to fit the joist
hangers. Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front
and back wall. Fit a profiled piece of timber above the joist. Simple
hanging, no angles cut, less lateral force on the walls (not that there
should be much anyway). Flat ceiling. Downside (if any) seems to be the
odd shaped roof void which will give a lot of air space above the
insulation at the higher end.

What dos the team think?

TIA

Dave R

I'd use method 3 with half the pitch 1 in 48 should be plenty especially
with steel which should have less tendency for ponding.
ISTR regs for flat roofs suggesting 1 in 80 - I tend to use 1 in 40/50
and have no problems.
The pieces of taper timber are called firring strips/battens specified
by the length and the timber they are cut from

e.g. N number firring strips ex 3.6m x 50mm x 100mm will give you
about 1 :40 fall
N number firring strips ex 3.6m x 50mm x 75mm will give you about 1 :50
fall.

The short sides of your block work will need to be angled to match. Cut
the blocks at an angle and finish off with a course of bricks. These
will be hidden behind the roof fascia.

Bob


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Default Best way to get a shallow slope on a flat roof


"David WE Roberts" wrote in message
...
Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per foot
is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush with
the wall plate and rest the base of the high end of the joist on the wall
plate. This will require the low end cutting at an angle to sit properly
in the joist hanger and the high end notching to sit onto the wall plate.
Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist
hangers. Both ends will need cutting at an angle to fit the joist hangers.
Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front and
back wall. Fit a profiled piece of timber above the joist. Simple hanging,
no angles cut, less lateral force on the walls (not that there should be
much anyway). Flat ceiling. Downside (if any) seems to be the odd shaped
roof void which will give a lot of air space above the insulation at the
higher end.

What dos the team think?

TIA

Dave R

What will it be? Shed or room?
If you intend to fit a ceiling, then (3) is correct way.
Fall should be 1 in 60 or greater. The "pofiled pieces of timber" are called
firring, nailed to top of joists.
If fall is in same direction as joists, their shape is a narrow
triangle/taper, no less than 13mm thick at thin end.
If fall is at right angles to joists, their shape is parallell (each piece)
but decreasing in thickness from top of slope to bottom.
Takes some thought and calculation. Remember measure twice cut once!!
You will need to consider direction of joists and covering together, it has
to be fixed into joists.


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Default Best way to get a shallow slope on a flat roof


"David WE Roberts" wrote in message
...
Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per foot
is adequate so a difference of 6" front to back should be O.K.).

I can see some obvious ways:

(1) Front and back wall level. Hang the lower end of the joist flush with
the wall plate and rest the base of the high end of the joist on the wall
plate. This will require the low end cutting at an angle to sit properly
in the joist hanger and the high end notching to sit onto the wall plate.
Sloping ceiling.

(2) Front wall one block higher than rear. Hang both ends from joist
hangers. Both ends will need cutting at an angle to fit the joist hangers.
Sloping ceiling.

(3) Front and back wall level. Hand the main joist parallel from front and
back wall. Fit a profiled piece of timber above the joist. Simple hanging,
no angles cut, less lateral force on the walls (not that there should be
much anyway). Flat ceiling. Downside (if any) seems to be the odd shaped
roof void which will give a lot of air space above the insulation at the
higher end.


Spookily (especially the dimensions)
http://www.diynot.com/forums/viewtop...&view=previous



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Default Best way to get a shallow slope on a flat roof

On Mar 10, 6:14*pm, John Rumm wrote:
Regarding insulation, you need to decide if you are going for a warm or
a cold deck roof. With a felted roof, warm deck[1] is quite easy these
days. Lay rigid PIR foam boards on the firings, and then ply screwed
down through the insulation, then felt etc on that. With corrugated
sheet that may or may not work as well.


John, I am going to install a warm deck on my garage. I was advised to
lay a sheet of 9mm ply below the celotex. This prevents them squishing
on the timber when you screw down the top decking. Also I was trying
to work out how to apply foil tape on the bottom of the boards - in
order to maintain the vapour barrier on the bottom side of the
insulation. Obviously you can't hinge the joints down and at a width
of 7 metres, trying to lift up an entire row could be tricky.

Im laying a deck with 18mm 8ft x 2ft T&G spruce ply. I am hoping that
will give a stronger deck than butt jointed ply.

Dave.

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Default Best way to get a shallow slope on a flat roof


"Bob Minchin" wrote in message
...
David WE Roberts wrote:
Looking at the roof design (I have looked back through previous useful
threads).

I am considering 6" (150mm) * 3" (75mm) at 600mm spacing.
The span is about 3.5m front to back and about 7.8m side to side.
The proposed roofing is metal sheet.
I am assuming the joists will span the short (front to back) span.

The issue now is how to engineer the slope (I have read that 1/2" per
foot is adequate so a difference of 6" front to back should be O.K.).

snip
I'd use method 3 with half the pitch 1 in 48 should be plenty especially
with steel which should have less tendency for ponding.
ISTR regs for flat roofs suggesting 1 in 80 - I tend to use 1 in 40/50 and
have no problems.
The pieces of taper timber are called firring strips/battens specified by
the length and the timber they are cut from

e.g. N number firring strips ex 3.6m x 50mm x 100mm will give you about 1
:40 fall
N number firring strips ex 3.6m x 50mm x 75mm will give you about 1 :50
fall.

The short sides of your block work will need to be angled to match. Cut
the blocks at an angle and finish off with a course of bricks. These will
be hidden behind the roof fascia.


I intend to put a wooden wall plate all round the top of the wall.
Any reason why I can't use a flat wall with firring strips on the top of the
wall plate to geve a slope and a seal?
It looks a lot easier than trying to cut a very shallow angle in dense
concrete blocks.

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Default Best way to get a shallow slope on a flat roof

On Mar 12, 1:15*pm, "David WE Roberts"
wrote:
It looks a lot easier than trying to cut a very shallow angle in dense
concrete blocks.- Hide quoted text -

The approach I took to get me to the stage of putting the roof timbers
on my garage are similar. At the top of the walls, screw some
shuttering either side. One side lower than the other to give the
desired shallow angle across the span. Use a pretty strong mix, level
to the shuttering. When its set, lay dpc strip on and put your timber
on that. I've got 150x50 regularised for a span of around 3 metres
side to side and i'm going with 550mm spacing. I'm also going to use
metal straps, from the walls bolted to the roof timbers- I think they
are called hold down ties? Supposed to prevent the entire roof lifting
off in strong winds.

Dave
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