Roofing/Flat roof


QUESTION: I am resident in NJ living close to NYC ( northeast) and need to fix by flat roof.
I have narrowed down to three proposals which are totally different purposes.
  Which one would you recommend and which one you prefer the least?
  Please tell me a few pointers  ( or things I should ask and make sure the contractor does the work ) and I am to put in in writing with my contractor and get a verification that  he has done right work for the proposal that you recommend

1)   Proposal 1: Cold Press
1.   Rip-off roof down to existing wood.
2.   Remove all damaged or rotted wood.
3.   Re-nail all decking boards.
4.   Install inch insulation board.
5.   Install base sheet.
6.   Install karnak #66 brush grade.
7.   Install sbs mop down roofing system..
8.   Install gravel stop flashing along all edges.
8.    Install 2x2 roof edging flashing.
9.    Install karnak #66 trowel grade on all seams of sbs & all edges of flashing.
10.  Flash all pipe penetrations.
11.  Flash around  chimney.
12.  Installing cricket behind chimney to divert water from chimney.
13.  Remove all debris from job site.

2)   Proposal 2: rubber torch down roof ( Allied product)
1.   Rip-off roof down to existing wood.
2.   Remove all damaged or rotted wood.
3.   Chimney to be re-flashed using Karnack 19.
4.   GAF ice water shield leak barrier will be installed to all gutter eyes abd o all valeys to protect areas of ice dams.
5.   Install inch insulation down + base sheet.
6.   Silver coating on the top after roof is done  ( do I need this step now or after 5 years?).
13.  Remove all debris from job site.

3)   Proposal 3: GAF liberty adhesive 2 PLY system
1.   Tear off old roof ( asphalt) .
2.   No redecking, ice and water shield since the adhesive has the sheilds.
3.   Install new vent pipe collar neoprene gasket flashing.
4.   NO ventilation, insulation or hip and ridge cap.
5.   Install aluminium gravel stop.
6.   Install Aluminium drip edge if needed.
7.   Install copper flashing at chimney

ANSWER: I assume this roof is over a portion of your home that has insulation under the roof deck (batt insulation between the roof joists above the drywall ceiling).

Since this is an occupied space the most durable would be the best, a 2 ply system. So the cold-process #1 is great so the odors are minimal and no roofing torch that might burn down your building. I assume the cap sheet, finishing layer has a granule surface like a shingle. If it is smooth it will have to be painted.

Also the GAF two ply system, #3,  is also preferable for the same reasons noted above.

Least favourite is the #2 torch down rubber. This is a single ply system with a glass base sheet. The top layer is the only layer to keep the water out. The seams have to be sealed 100%. The  cap sheet (torch on rubber) is smooth and has to weather at least 45 days to let the oils exude. Then it must be painted with a fibrated aluminum paint every 5 years (maintenance item). Also the use of a torch in inexperienced hands may cause fires.

No matter which one you choose , 1 or 3, make sure they can get a warranty from the manufacturer and they carry valid liability insurance to protect your home from damage (chemicals seeping into home, rain events causeing leaks and damaging the inside etc.

All the best.

---------- FOLLOW-UP ----------

QUESTION: I am fixing my flat roof  ( 312 X 161 = 506 .46 surface area) with a potential GAF product.
My previous roof is old asphat and am planning to tear it down.

I plan to use GAF liberty product

1)   How many sheets would I need?
2)   Can I apply 2- ply system- If so- Can it be done in one day without priming the wood ?
a.   Does the adhesive base sheet and cap sheet be applied same day?
b.   Do I need to treat the wood for a day or 2 ( prime the wood before I put the adhesive?
c.   After that, what extra things I need to be aware for the cap sheet?
d.   I have a chimney on flat roof- do I flush with metal or with what material? Do people really flush chimney of the FLAT ROOF with METAL?
e.   I do have pipes- what do I do? Collar it?
3)   If NOT- do I have to go for 3-ply system
If so- Can it be done in one day?
a.   Liberty Base Ma would be to nail down? What would be the size of nails
b.   Then liberty SBs SA base sheet ( does it have a glue?)
c.   Self attached GAF cap sheet on top? ( any precaution?)

Please provide the information as soon as possible

.1- 3 rolls of base sheet / 6 rolls of cap sheet approximately
.2- Do not know how competent you are. I did a friends carport with a self-adhering two ply about the same size and it took me two days and I am experineced (base sheet, base sheet flashings, cap sheet, cap sheet flashings) Priming is recommende to improve adheion to the wood deck.
.2a- The base sheet and cap sheet can be installed on different days. Hasve to complete base sheet flashing (chimneys, perimter drip edges, vents, etc.) before cap sheet can be installed
.2b- drying time for the primer is dependent on weather,  is less than 20 minutes approx. with direct sun.
.2c- Install membrane with no wrinkles and roll or smooth to ensure full adhesion. install membrane on roof so water runs over the completed side laps.
.2d- Membrane to be flashed with base sheet then cap sheet after cap sheet membrane is installed.
.2e.- either flash them in with membrtane or use preformed aluminum stack flashings.
.3-Two ply is sufficient if installed properly. completion time depends how good you are in installing low slope roofing.
.3a- standard cap nails. 1" long.
.3b- sheet has a self-adhesive back, no glue required, only primer.
Avoid wrinkles and roll to ensure adhesion. Stagger side lpas in cap sheet away from side lpas in base sheet.

If you really want to attemot it I suggest you go to the GAF site ( and download DIY instructions.

Good luck


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D. John Henderson


My 33 years in the building science business has involved air barriers, waterproofing, and flat & steep slope roofing systems.


My roofing encompasses commercial systems (BUR, mod-bit, single-ply) and residential applications (asphalt shingles, cedar shingles, slate and metal. I am cognizant of code stipulations in regards to FM and UL for commercial roofs.

Attended numerous courses offerd by Roof Consultants Institute, Roofing Industry Educational Institute, Factory Mutual Research Corporation, University of Wisconsin, and BURSI. Lectured for RCI and mentored at a local university for a Building Science Certificate.

Past/Present Clients
Government, commercial businesses such as The GAP, hospitals, school boards as a roof designer.

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