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Question DetailsAsked on 10/15/2014

is there a waterproof underlayment that will prevent water penetration on a shingled roof?

Ice build up on my low pitch shingled farmers porch leaks through the roof both at the roof edge and where it meets the houses roof

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2 Answers


There are a large number of manufacturers that make ice and water protector. I copied and pasted one of the links from a product from Tamko's site. Make sure that whomever installs the product verifies that the decking that it is being adhered to is in good shape so it seals well.

Moisture Guard Plus®
Provides an extra layer of waterproofing protection to help prevent damage from ice damming and windblown rain. A self-adhering modified-bitumen underlayment that creates a watertight barrier by sealing to the deck and around nails to provide continuous protection year after year. Helps protect the entire deck of a properly ventilated roof or just in problem areas where water collects or drainage is slow.

Answered 5 years ago by ExteriorUpgrader


Basically 4 common types of underlayment from least to most resistant -

1) treated kraft paper, which is basically a waxed brown paper bag paper like the oiled or waxed paper used to protect metal items like guns in shipment which is designed to shed water but has a very short life under shingles so rarely used now (and not to code for occupied structures). Also very hard to walk on when roofing without tearing it.

2) roofing felt (tarpaper), typically in 15 and 30# weights (though that is no longer the actual weight as they have dramatically reduced the thickness and durability),

3) roof wrap or underlayment with is basically a treated woven plastic sheet (Tyvek being one brand) - NOT the same housewrap though sold under same brandnames typcially but more water-repellant, and comes in both self-sealing (around nail holes) and not,

4) "ice and water shield", which is the bitumastic product mentioned in the other comment, and seals very well around nails - though of course can still leak at nail holes if nails rust out or are pulled.

Roofing felt would be the usual underlayment for a shed, but since you are low slope then the best product if you have proven infiltration issues would be ice and water shield, though if the added $0.25-.50/SF added cost is a burden then a self-sealing synthetic underlayment at maybe $0.10/SF more than felt is next best. My decision would be based on how drastic it is if you get a leak or two down the road - with synthetic, it is likely to be minor drips through nail holes only, not flooding as can occur when felt starts getting old and cracked.

At the bad spots - from about 3 feet uphill of the eaves to the edge of the roof (so typically about 5-6 feet total width along roof edge) you should have ice and water shield anyway if you are in an icing susceptible area, and at all roof/wall junctions it should run from the roof up onto the wall under the siding. In low snow areas you can get away with a full-width (typically 3' wide roll) roof sheet run down alongthe interface and rolled up about 6 inches at the wall to provide a waterproof valley membrane. In areas with heavy slushy snow buildup or real thick snow drifting against the house, then it might be run from an additional foot or two to as high as 10-15 feet up the wall to protect against water from melting snow penetrating the house there, overlapped and sealed of course at the bottom over a turned-up roof sheet, which in that case would normally be the roof waterproofing turned up couple of feet onto the wall (under the siding of course). Doing this can be tricky because any lap seams from sheets turning up at the house tend to open up with time due to the bend/curve, so what I do in this case is run the roof ice and water shield horizontally on the roof as usual - at least 5 feet wide leading up to the house and wrapped at least a foot up the house wall so roof water can't get into the roof/house interface, then overlay and seam with a longitudinal 3 foot wide piece running down the junction half on the roof and half on the wall (acting like a valley flashing), then do the wall as high up as needed with sub-horizontal ice and water shield running parallel with the roof slope instead of horizontal to avoid any seams intersecting the junction area, overlapped over the valley flashing piece down to the point it turns onto the roof. This provides a positive interface waterproofing with zero seams at the interface "corner", which is the normal leak location. How high you go up the wall depends on how highthe snow builds up there.

One thing - make sure the house wrap/underlayment above the ice and water shield overlaps the ice and water shield - otherwise any siding leakage running down the siding underlayment cannot get to the surface, and will try to run down behind the ice and water shield and get trapped in the walls or roof. With this sort of installation you still need flashing under the siding, to direct the siding runoff onto the top ofthe roof shingles instead of letting it get under them by running down the ice and water shield to the intersection, then under the shingles.

One other thing - which might or might not work for you if you have snow buildup against the house issues - and especially if using roofwrap rather than ice and water shield - if you have access and not too high a roof, you can get 15-22 foot aluminum long roof rakes at Amazon and Harbor Freight and such for under $50, and if you keep the snow raked off then ice damming cannot occur. I do this several times a year on one section of my roof where a series of vents and exhaust ducts all line up from ridge to near the eaves and cause chronic icing. Just be sure not to get violent with the rake and damage the shingles, and remember you don't have to get it perfectly clean - especially in melting conditions, because once the shingles are exposed they heat up and melt the rest real quick if sun exposed). Also, if you rake one area you need to go basically to the ridge or very close with the clearing, because otherwise just doingthe lower edge where ice damming usually occurs just moves the melting point up the roof to where there is bare roof exposed just below snow and then causes ice damming over the main roof area rather than at the overhang where and water leak damage is usually more limited.

FYI - here is the product sheet on WR Grace's product (which is my favored brand) - Carlyle makes a similar one too, as do most roofing mateials manufacturers -

Answered 5 years ago by LCD

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