3 tab shingles on a very low sloped roof

Notice the roof valley at the left side of the screened porch, which is an addition. The slope of it is low. Was built with a permit, I’m told. What are the rules on the use of asphalt shingles on low-sloped roofs. I seem to remember that < 4/12, 2 layers of felt should be used (could not confirm that in this case). At what slope are shingles not allowed? What would you estimate the slope of this roof to be? If you consider this a poor installation, what is a better alternative?

Yes, there were stains on the framing inside the porch at this valley.

082609 012.JPG 082609 056.JPG 082609 061.JPG

082609 012.JPG

I have heard of special methods being used for 3 tab asphalt shingle and low roof pitches (less than 4 pitch) that includes adhering the shingles and changing the reveal so that in the roofer would be nailing through 3 layers of shingles (prevents wind lift off).

Personally I don’t like to see them on anything less than a 4 pitch, and certainly not in the a place like you have shown in the picture where water can run up under them from the adjoining roof slopes. At bare minimum this should have a self adhering membrane under it, but if it were my house I would change this section of roof.

Check with specific manufacturers in your area for any exceptions, and explain to the homebuyer that it may be a cause for future headaches.

Recommend buying a couple of buckets of tar and just leaving them up there so that they won’t have to carry them up and down the ladder every spring and fall, especially if there are already signs of water intrusion.

How I figure slope is with a four foot level.
This roof definitley is wrong. If you used a moisture meter on the stains and they showed above average moisture content, this will help support the case the roof is wrong.
I do not care how many layers of shingles, felt. and tar is applied, during a hard rain it will always leak. Too much water draining on it on too low of a slope. Even a 4/1 still might not be enough pitch. Sad but true.

Asphalt shingles should never be applied to roof slopes below 2:12. The measuring process is quite simple. You measure the rise from a one foot span of the roof slope. A 2:12 is a 2 inch rise from one end of the 12 inch span to the other end.

I would rather see EPDM used here.

Moisture problems are imminent.

I agree with David. Shingles should not be applied on 2/12 or lower, and I would rather see rubber also. Water will not shed off that set up fast enough the way it is right now, you know that from the staining and possible moisture intrusion already. Recommend repair by qualified roofing contractor.

Something like this…


Less than 4/12 requires special underlayment installation. Shingles can’t be instlled on less tha 2 1/2 and 12

The only function the three tab shingles perform at that low a slope is decorative - that’s a legitimate use of shingles when you wish to preserve a consistent appearance, but they do not function as a weather barrier in such installations, that function is being performed (it’s being performed at all) by the underlayment and/or flashing below the shingles, which in this case must be a material with self-sealing properties to prevent water intrusion at the shingle’s fasteners. I notice a ladder at the edge of the roof, were you able to lift shingles at the bottom of that “valley” and determine what sort of underlayment or flashing was present?

There’s also the problem of the concentrated flow water moving across the low sloped portions of the roof: it’s going to scour granules off the shingles, and I almost never see proper provisions for water control where the flow leaves the roof at such installations - this should be considered a “flat roof” for purposes of water removal, for example it needs a properly designed and flashed conductor head feeding an appropriately sized downspout discharging a location that can handle the flow rate, and you rarely see this done properly in residential construction as it does not “look right” to homeowners.

There is also going to have substantial water flow along the roof-wall junction on the wall below that window both from the downspout discharging there and from fields of the adjacent roofs, unless that junction has been *very *carefully flashed water penetration there is almost certain, and as the builders did not hold back the wall cladding above the roof surface you can’t establish the flashing method or extent at this location.

There is also a number of other issues here, for example as you’ve noted at some locations water is flowing down from adjacent roof fields is flowing across shingles in the low slope areas perpendicular to their long dimension, you’re not going to find that weave documented in any manufacturers installation instructions, and so on.

For these reasons this is going to be a “leak prone” installation, with a number of critical construction details invisible and at a typical home inspection. I have a number of pre-written disclaimers for these situations, and I make it very clear three times: verbally when a client and I are looking at the property, in a general disclaimer regarding roofing in the report, and in a specific disclaimer for each such construction detail in the report, that I cannot determine if such a roof is or will remain weather tight, especially under extreme weather conditions such as prolonged or wind driven rain. (That’s if there is no evidence of leakage at the time of inspection, if there’s already leakage I use the same disclaimers, only noting instead I can’t determine how quickly the problem will get worse).

IMO, based on my experience these are potentially high liability situations, and one of those cases where it’s extremely important to practice “defensive” inspection and reporting, especially as problems may not manifest themselves until considerably after the date of inspection or under unusual weather conditions.