_Building a Deck on a Flat Roof

Discussion:

(too old to reply)

Jack Goldstein

2004-01-16 02:12:54 UTC

Hi gang,

I need to build a recreational deck on a newly-rebuilt
flat roof that has a 2-ply "rubber" torch-welded membrane.
The new roof is framed with 3/4" plywood screwed down
on to 2x8's, 12" o.c., and is *very* stiff.

I was originally planning on doing the following:

1. Lay 2x4 P.T. "sleepers" on the membrane, 16" o.c.
2. Screw 2x6 P.T. boards on to the sleepers perpendicularly,
using three #8x2.5" green deck screws at each "intersection".
3. Leave a "nail's width" between each decking board.
4. Wait a year, then stain the P.T. decking with an oil-based stain.

Recently, I have received several suggestions from friends and
am not sure which suggestions are good and which aren't. Here
they are:

1. Glue down the sleepers on the membrane.
2. Glue down 4" strips of 1" thick rigid foam insulation on the
membrane then glue the sleepers on to the foam.
3. Use 5/4x6 decking instead of 2x6 lumber.
4. Use 2x4 lumber instead of 2x6 lumber.
5. Don't leave any space between the decking boards.
6. Don't *ever* stain the P.T. boards.
7. Use cedar decking boards, not P.T.

Does anyone have any ideas regarding any or all of the above
7 suggestions? Many thanks.

ameijers

2004-01-16 03:13:47 UTC

Permalink

(snip)

Post by Jack Goldstein
Does anyone have any ideas regarding any or all of the above
7 suggestions? Many thanks.

Before you do ANY of the above, call the company that installed the roof,
and find out if the material they used was rated for any loads on the
material surface. Especially true if there is gravel over the membrane.
Proper method would have been to have some sort of sockets attached to roof
structure coming through weather line, suitably flashed and sealed, and then
float the deck above that. Sleepers on the membrane will likely shorten the
life, from temp and moisture and UV variations, and flex in the deck when
wind blows and people walk on it. Also, keep in mind that crud will build up
under the deck and need to be cleaned out at some point. If you go with a
sleeper setup, suggest making it in form of panels that can be lifted out
for cleaning. Yes, sleeper based roof decks are common and doable, but there
are tradeoffs.

Off the wall thought- maybe some of those big interlocking rubber squares
like they use in building entrances and around skating rinks and such? The
cheap ones like Sams carries probably aren't UV rated, but the commercial
ones may be durable enough for outside use. Easy to install, and easy to
pull up for cleaning, and easy on the feet (if you can find some other color
than black, of course.)

aem sends....

pbillly

2004-01-20 11:31:37 UTC

Permalink

This is just a bad idea. Flat roofs are inherently a bad design. Yes it
can be made to work but think about it. The extra maintance involved in
removing ANYTHING on a flat roof just to re-roof it, in just few years, will
cause a headache for you or anyone else who inherents your problem (like the
next owner).

My advise would be to put a peak on this flat roof and not think about
fixing anything for the next 25 or 30 years.

Post by ameijers

(snip)

Post by Jack Goldstein
Does anyone have any ideas regarding any or all of the above
7 suggestions? Many thanks.

Before you do ANY of the above, call the company that installed the roof,
and find out if the material they used was rated for any loads on the
material surface. Especially true if there is gravel over the membrane.
Proper method would have been to have some sort of sockets attached to roof
structure coming through weather line, suitably flashed and sealed, and then
float the deck above that. Sleepers on the membrane will likely shorten the
life, from temp and moisture and UV variations, and flex in the deck when
wind blows and people walk on it. Also, keep in mind that crud will build up
under the deck and need to be cleaned out at some point. If you go with a
sleeper setup, suggest making it in form of panels that can be lifted out
for cleaning. Yes, sleeper based roof decks are common and doable, but there
are tradeoffs.
Off the wall thought- maybe some of those big interlocking rubber squares
like they use in building entrances and around skating rinks and such? The
cheap ones like Sams carries probably aren't UV rated, but the commercial
ones may be durable enough for outside use. Easy to install, and easy to
pull up for cleaning, and easy on the feet (if you can find some other color
than black, of course.)
aem sends....

SQLit

2004-01-16 16:54:37 UTC

Permalink

Have you thought about how much weight is going to be up there? Depending
on the span 2 x8's are not all that strong. Last deck I helped build had a
span of 12 feet and we used 2x12's 12" o.c.
What about the vertical supports?

No sense have friends and you toasted then come crashing down. Remember the
deck in the public building here in the states that hurt some 400 people
when at the party the deck became populated to the point of collapse.

Yes I tend to be a bit cautious.

Phisherman

2004-01-16 23:17:42 UTC

Permalink

Roofs are not normally designed to carry a lot of weight. One way to
still build a deck is to put all the weight on the perimeter, as if
there were no exisitng roof at all. That means the deck floor would
be about a foot above your roof, and add some protection to the
membrane. What you suggest may damage the membrane and/or encourage
rot. I have stained my deck, and it still looks good after 7
years--stay away from the short-lived clear stuff or consider the
plastic-composite Trex stuff. Cedar, redwood, teak, white oak, or PT
are very good. Why not hire an engineer who is familiar with your
local building codes? An hour of his time may be well worth it in the
long run. I know of an instance of a flat roof over a kitchen--we had
a freak heavy snowfall one night and she woke up to a kitchen with the
ceiling caved in!

On Thu, 15 Jan 2004 21:12:54 -0500, "Jack Goldstein"

Post by Jack Goldstein
Hi gang,
I need to build a recreational deck on a newly-rebuilt
flat roof that has a 2-ply "rubber" torch-welded membrane.
The new roof is framed with 3/4" plywood screwed down
on to 2x8's, 12" o.c., and is *very* stiff.
1. Lay 2x4 P.T. "sleepers" on the membrane, 16" o.c.
2. Screw 2x6 P.T. boards on to the sleepers perpendicularly,
using three #8x2.5" green deck screws at each "intersection".
3. Leave a "nail's width" between each decking board.
4. Wait a year, then stain the P.T. decking with an oil-based stain.
Recently, I have received several suggestions from friends and
am not sure which suggestions are good and which aren't. Here
1. Glue down the sleepers on the membrane.
2. Glue down 4" strips of 1" thick rigid foam insulation on the
membrane then glue the sleepers on to the foam.
3. Use 5/4x6 decking instead of 2x6 lumber.
4. Use 2x4 lumber instead of 2x6 lumber.
5. Don't leave any space between the decking boards.
6. Don't *ever* stain the P.T. boards.
7. Use cedar decking boards, not P.T.
Does anyone have any ideas regarding any or all of the above
7 suggestions? Many thanks.

2004-01-17 02:31:51 UTC

Permalink

Many years ago Polaroid had a film manufacturing facility with a flat
roof. The roof had leaked. Their "intelligent" maintenance folks
plugged up the roof drains and flooded the roof to test for leaks. The
roof collapsed and numerous employees were killed.

Don't overload a flat roof unless you know what load it will withstand.

RB

Post by Phisherman
Roofs are not normally designed to carry a lot of weight. One way to
still build a deck is to put all the weight on the perimeter, as if
there were no exisitng roof at all. That means the deck floor would
be about a foot above your roof, and add some protection to the
membrane. What you suggest may damage the membrane and/or encourage
rot. I have stained my deck, and it still looks good after 7
years--stay away from the short-lived clear stuff or consider the
plastic-composite Trex stuff. Cedar, redwood, teak, white oak, or PT
are very good. Why not hire an engineer who is familiar with your
local building codes? An hour of his time may be well worth it in the
long run. I know of an instance of a flat roof over a kitchen--we had
a freak heavy snowfall one night and she woke up to a kitchen with the
ceiling caved in!
On Thu, 15 Jan 2004 21:12:54 -0500, "Jack Goldstein"

Des Perado

2004-01-17 00:55:00 UTC

Permalink

For another possibility, you might want to check out
www.envirospec.com

Des

Des Perado

2004-01-17 04:25:26 UTC

Permalink

Sorry, that should have been

www.envirospecinc.com

Des

Post by Des Perado
For another possibility, you might want to check out
www.envirospec.com
Des

DanG

2004-01-17 19:56:38 UTC

Permalink

As others have said, you need advice from a roofer about your
particular roofing.

I don't understand 2 ply rubber.
Rubber, EPDM, is usually applied as a single layer. Modified
bitumen which can be torched down is applied as a single layer. I
would think there would be some type of insulation under the
membrane which must be able to support the loading imposed by your
deck. Pitch pans, which I would recommend, need to be installed
through the roofing membrane and properly flashed or roofed in.

1. Glue down the sleepers on the membrane.
I defer to a roofer, but I don't think so.

2. Glue down 4" strips of 1" thick rigid foam insulation on the
membrane then glue the sleepers on to the foam.
I defer to a roofer, but I don't think so. Most foams deteriorate
rapidly in sunlight.

3. Use 5/4x6 decking instead of 2x6 lumber.
Legitimate solution. Will require joists closer together.

4. Use 2x4 lumber instead of 2x6 lumber.
Has to do with looks and number of fasteners - your call.

5. Don't leave any space between the decking boards.
I you install the boards tight together you will have the spaces
you want within 6 months. The lumber will be "green" and wet when
you install it - it will shrink, especially CCA. I don't know
about the new treatment.

6. Don't *ever* stain the P.T. boards.
Why not? You do need to wait till after a year.

7. Use cedar decking boards, not P.T.
No comment. Either will give you splinters.

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Keep the whole world singing. . . .
DanG

"intersection".

Post by Jack Goldstein
3. Leave a "nail's width" between each decking board.
4. Wait a year, then stain the P.T. decking with an oil-based

stain.

Post by Jack Goldstein
Recently, I have received several suggestions from friends and
am not sure which suggestions are good and which aren't. Here
1. Glue down the sleepers on the membrane.
2. Glue down 4" strips of 1" thick rigid foam insulation on the
membrane then glue the sleepers on to the foam.
3. Use 5/4x6 decking instead of 2x6 lumber.
4. Use 2x4 lumber instead of 2x6 lumber.
5. Don't leave any space between the decking boards.
6. Don't *ever* stain the P.T. boards.
7. Use cedar decking boards, not P.T.
Does anyone have any ideas regarding any or all of the above
7 suggestions? Many thanks.

Tom Baker

2004-01-17 23:43:18 UTC

Permalink

Codes, which represent reasonable minimum requirements for health and
safety, call for 40 pounds per square foot live load in a residential
living room. Public spaces are supposed to support 100 pounds per
square foot last time I looked. I should think parties would produce
loads close to the upper end ot the range. The deck, existing roof,
existing walls or columns and footings will have to carry that weight.
In addition, movement of a number of people will require resistance to
sway. If the deck is in a high wind or seismic area, those forces
should be accounted for.

The roof covering, as has been pointed out must resist the weights,
and any chemical reactions with wood or with stain.

The railings required for safety have to be of approved height with
approved openings, and resist pressures of folks against them. Codes
speak to these requirements.

Of the 7 suggestions, I think only #5 is a basic requirement. #3, #4,
& #7 are a matter of choice.

Tom Baker