Lean-to porch roof question on pitch

[Easy Livin’ 3000]

Replacing an old lean-to roof on a porch. The old roof had a steep pitch, and I'd like to replace it with a much less steep pitch, so that I can comfortably stand on it to clean the gutter on the second story roof. I only need the pitch to be steep enough so that water runs off of it.

I'd also like to use asphalt shingles with synthetic underlayment. I asked a roofer what they do when a pitch is less steep than the shingles spec., he said they just add a second layer of underlayment


Two questions, first what pitch would you recommend, and second, would the second layer of synthetic underlayment be sufficient to make the shingles work to keep the water out, or would I need to cover the deck with a membrane before putting the shingles down.

The roof will be 10'x 20'.

 

[EatenByLimestone]

Ice and water shield over the roof deck. Keep the pitch 4/12 or steeper. You can comfortably walk on pitches up to 6/12 with asphalt shingles. If you're still unsure of footing, boots like Cougar Paws will let you walk on 9 /12 pitches without issue.

 

[Easy Livin’ 3000]

Ice and water shield over the roof deck. Keep the pitch 4/12 or steeper. You can comfortably walk on pitches up to 6/12 with asphalt shingles. If you're still unsure of footing, boots like Cougar Paws will let you walk on 9 /12 pitches without issue.

Thanks!

If I want it with less pitch, I guess I need to go with a flat roof product. I have a couple rolls of bitumen melt-down for flat roofs that I could use. If I use that, what would a minimum pitch be, just so the water runs off?

I even considered using the bitumen melt-down as my underlayment, and just putting the shingles on for aesthetics. Would that work?

 

[EatenByLimestone]

I think you’d be poking lots of nail holes in your membrane when you covered it with shingles. Ice and water shield is designed to seal around those punctures. Flat roofs have drainage installed to control the amount of weight sitting up there. Be careful not to encourage water to flow back at your house. You could be opening yourself up to thousands of dollars in damage.

Have you looked into gutter guards? Some of them do a wonderful job of keeping junk out of the gutters. Some not so much.

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valorgutterguards.com

 

[ctreitzell]

I will be building a lean-to roof against my existing brick-built garage. it will be used as a wood shed so will have three open sides with the roof being supported by posts. The roof will be tiled to match the house and garage. The shed will be 3.0m wide by 3.6m long, so the rafters will be spanning the 3.6m length. The roof will be 15 degrees. A couple of questions, was thinking of using a 200x100 timber lintel sat on the two timber posts (200x200) which will give a clear span of about 2.6m for the rafters to sit on. Will this suffice? I think I'll probably need a purlin but this too will be mounted on posts and due to restrictions on the ground will be about 1.0m back from the lintel, giving a max span on the rafters of 2.8m. Will a 200x75 purlin suffice for this? Rafters will be 150x50 spaced at 600mm. Any advice is much appreciated.

I'd say the Americans are going to struggle to guide you on your proposed metric sizing. Also, UK dimensions are opposite to the American way
USA= 2x4, 4x6 etc
UK= 4x2, 6x4 etc

I'm American, but have been in Europe and the UK since '96.

I'm not sure your lintel or purlin is sufficient, you should quadrupple check that. If you are thinking of using 2x6 rafters with a purlin, you might be able to get away with 2x4...but not at 2.8 meters...all that needs checking, tho.

Further, are you sure these dimensions are available at your timber merchant? I typically ensure dims available before planning my build.

I'm lucky, I now have a close friend who is a structural engineer and I would ask him...I'll see if he can provide me a span chart for dead loads...

further terminology:
USA = Lumber
UK = Timber

in America, we shout "Timber"! At the moment a tree we are falling is cut enough to fall. The opposite is certainly not true...Ive never heard an english person use the term "lumber" when referring to milled tree.

 

[ctreitzell]

there is this:
https://www.omnicalculator.com/construction/wood-beam-span

and this is a lot easier:
https://www.mycarpentry.com/rafter-span-tables.html

@lamywaby maybe you should start your own thread?