Roof Slope Question

[CurlyDave]

Rebuilding my burned out house, I am having to jump through a lot of hoops the Town is putting up.

One is that in many places I can not increase the overall height of the house, not even by an inch.

The problem that I am facing is that my old roof was built 100 years ago from 2x4 rafters. To get the right insulation in I have to go to 2x10, which is 6 inches thicker. Combine this with 2x10 floor joists instead of 2x6 and a couple of other problems, and I have to lower the roof peak by ~12".

One relatively painless way to do this is to just lower the roof slope from 5 in 12 to 4 in 12.

Does anyone have any experience here. I suspect that a 4 in 12 roof is going to be more leak prone than a 5 in 12, but is it a big difference or a small one?

 

[dex3361]

4/12 is the standard in WV and I haven't had any issues with water. Most all of the trusses are built 4/12 as well. Its easier to walk on too.

 

[buckeyefarmer]

part of mine is 4/12, not that uncommon.

 

[dave1949]

Hi Dave,

I really don't know what the weather is like in your area. I don't think you should have a problem with 4/12 pitch though. Is a standing seam metal roof an option? Pre-cut or on-site formed and cut to length results in no seams except if you have a valley.

Here, where snow retention and ice damming is common, roofers put Grace brand Ice & Water Shield down first, then the roofing, even under metal if you want to have a really water proof roof and also be protected from the melt water that can back up behind an ice dam at the eaves. This is a great product, the actual roofing material is almost an after thought. Of course, it adds to the overall cost of the roof.

I have mentioned this product before, I don't have any connection to it, just believe in it.
Dave.

 

[CurlyDave]

The house is in the SF Bay area. In the past 30 years it has snowed once or twice and the snow only lasted one morning.

Rain comes through in "cells". When the cell is over you it can rain for a few minutes at a horrible rate, maybe 10" per hour, but the cell moves on after a few minutes and there is relative calm. Overall sustained max rate I have ever seen is more like 3-4" per hour and that is rare.

Cost is a big concern, as the house was for sale at the time of the fire, and will be going back on the market soon. Metal roofs are very uncommon in this area and would be a detriment. I was thinking more of 40-year composition shingles over double 30 lb. paper.

 

[Charlesaf3]

4/12 is fine, but one the edge, if that makes sense. Go ahead and do it, but cover it in grace ice and water shield and do an unvented roof assembly - google Lstibureks plans etc. Easier to build and will save you energy as well.

Under 4/12 I'd treat it like a flat roof - epdm it.

 

[JB4310]

That is a very low pitch roof, near the limit for shingles, I actually think 4/12 is the cut off point where you have to increase underpayment specs, something like 2 layers of felt needed to achieve warranty requirements.

Do you get snow, that's the killer on low slope roofs, when it's slushy due to melting and rain or when it freezes at the eaves. put the 2 of those together and you have a real potential for leaking on any roof, but more so on lower slope.

I know how those planing and zoning boards can be but they always have an option of applying for a variance, and in your case I think you have very strong grounds for them approving it, especially if it's only a few inches and it could affect the integrity of the building if not approved.
TPZ and building dept are usually pretty close to each other, I would talk to the building people who understand this stuff and if they are reasonable they may recommend to the zoning people that that's what should be done, if you're lucky maybe the building dept would require the greater pitch.

EDIT: I read you are just planning on selling and want to keep this simple, and you don't get snow.
So the easy and cheaper way to go is lower the ridge a foot like you said, you may have to double the felt, though 30 pound is pretty good stuff, that's what I used on mine. The ice and wind rubber is more for the problems associated with ice damming at the eaves and in valleys, probably don't even need it. or if you did you'd need it over the entire roof. End Edit:

Good luck, JB.

 

[Pops15]

4/12 roof pitch was the standard on ranch and tri-level homes built in the 50's, 60's, 70's and 80's here in the mid-west. I don't see any reason why you'd have any problems using that pitch as long as your flashings are done properly.
Shingle manufacturers vary on the recommended minimum pitch. Every brand that I've ever used say 4/12 is ok. Some are guaranteed on pitches as low as 3/12 but I'd suggest using an ice and water shield on the entire roof with that flat of a pitch.
ALWAYS use ice and water shield along the eaves no matter what the pitch.
Why 2 layers of 30# ? There are varying schools of thought on what weight of felt to use. 30# is popular in warmer climates. 15# works better in areas prone to hail storms (it's less "cushy" so the hail doesn't damage the shingles nearly as much). If you're going to use 2 layers of 30# why not just use one layer of 60#? Better yet why not just use ice and water shield on the entire roof?

 

[dave1949]

The house is in the SF Bay area. In the past 30 years it has snowed once or twice and the snow only lasted one morning.

Rain comes through in "cells". When the cell is over you it can rain for a few minutes at a horrible rate, maybe 10" per hour, but the cell moves on after a few minutes and there is relative calm. Overall sustained max rate I have ever seen is more like 3-4" per hour and that is rare.

Cost is a big concern, as the house was for sale at the time of the fire, and will be going back on the market soon. Metal roofs are very uncommon in this area and would be a detriment. I was thinking more of 40-year composition shingles over double 30 lb. paper.

In that case, as long as 4/12 doesn't look out of place in the surrounding homes, double 30lb and 40 yr shingles sounds like a pretty nice roof to me. I watched a stiff west wind blow heavy rain UP a valley in my 5/12 roof in Ohio once, so stuff happens. At least I found out why I had a slight leak.
Dave.

 

[bp fick]

4/12 is not the limit on shingles. As others stated, it is THE standard pitch on ranches all over America, and the simple two story "colonials" too. Compromises abound in almost every building project and having to go to a 4/12 isn't a deal breaker in any way.

It's fine. It just wouldn't be my choice for snow loading, which in your area is not a concern.

+1 on the ice and water shield, though.