Skylights: double up the rafters?

[Aaron Fude]

Hi,

My carpenter just told me that if I am having a skylight put in between two rafters, I should double up each rafter because the hole in the wall weekens the roof. Do you agree with that?

Thanks,

Aaron

[Toffee Monday]

Aaron,

I'd be looking for a new carpenter. This statement by s/he, speaks volumes about their knowledge.

[David Nebenzahl]

On 10/24/2008 3:40 PM Aaron Fude spake thus:

Yes.

[Toffee Monday]

Wrong information, for the question asked.

[aemeijers]

Not unless the skylight runs most of the way from the wall plate to the ridge. A typical residential skylight, no more than 24x48, will be no problem unless the roof is weirdly framed. It is usually a good idea to box the 2 rafters around the hole, though. Keeps the area stiffer, and makes the skylight less likely to leak. I'm old fashioned, I like to see a box frame extending through the roof, with the metal skylight frame several inches above the shingles. Makes it easier to change down the road when it fails, since the skylight waterproofing is above the main flashing where the shingles and box frame come together. It also reduces the chances of ice dams on the skylight. Unless you have a cathedral ceiling, you'll be framing up a tube down to the finish ceiling anyway.

-- aem sends...

[trader4]

Normally, the rafters are not doubled up. A typical skylight requires one rafter to be cut to make a large enough opening, and that is what is done. Then the opening is boxed in to the right size using a doubled cross piece the same dimension as the rafter at the top of the opening and at the bottom that is tied in to the rafters on either side. That provides sufficient structural integrity. In essence the cut rafter is still there and carrying load, it's just tied in now with the skylight framing.

[Lou]

I've been a carpenter for 30 years. I have yet to see a communities building code that doesn't require doubling up the rafters. It's typical for bigger towns and cities. Lou

[Mike Dobony]

Reasoning is incorrect, but I would double up the rafters to stiffen it up to guard against flex-induced cracking. I like to overbuild so I don't have to do any repairs later.

Mike D.

P.S. get off googlegroups. Many a wise man blocks them out and your post is only seen if a less wise person fails to filter them.

[EXT]

Even if it isn't code, there is nothing lost by over engineering a modification, much can be lost by doing the minimum or worse.

[Toffee Monday]

You miss what the OP said, about in between rafters. Doubling up the rafters will no absolutely nothing, but waste time.

If you're referring to me about googlegroups, you might what to learn about servers. I'm not posting through Google.

[Toffee Monday]

I'm very proficient in framing, you needn't attempt to tell me how framing is done.

You missed the OP's question, I suggest you re-read it. They're not cutting any rafters.

[Toffee Monday]

Absolutely not. I call you on this. You may have been a handyman for 30 years, a carpenter, no. There's no code which calls for "doubling rafters", in the OP's situation.

In fact, type into your favorite search engine on how to install a skylight.

[Norminn]

I'm not a roofer or carpenter, but have one thought aboutt this. Skylights often leak, so the reasoning may be that if there is a leak and rot later on, doubling up will help ensure strength and support of the roof.

[Tony Hwang]

Hi, My last house had a few skylights. Didn't have a reinforced rafters. I was the original owner and lived in that house almost 20 years. Did not have any problem anything to do with sky lights. Now I live in this house custom built in '94 per our plan. We did away with sky lights, we have all glass all season sun room now like a green house.

[tom]

No. Tom

[Joe]

If the roof is on engineered trusses, modifications are needed. Allay your doubts by getting sound professional advice (architect) and asking questions at your building inspection department. We can't give you good advice based on so little information. Good luck.

Joe

[Aaron Fude]

Hi,

This is where I'm putting skylights:

They are 22x54 and I will have 4 of them and a cathedral ceiling.

What would your recommendation be about that?

Thanks,

Aaron

[Toffee Monday]

Skylights leak, simply because they're not installed properly. I had one architect tell me all skylights leak, he wondered why I looked at him like he had 4 heads. Then I explained about installation.

Doubling up rafters offers nothing. If this were the case, every penetration would need doubled up rafters. Such as vents, soil stacks, chimney flues, etc. Take a peek in your attic, you'll not see a doubled up rafter at any penetration, unless you have an over-sized chimney spanning several rafters.

When installing skylights, you do need to put blocking between the 2 rafters.

People tend to think because the sheathing ends at a skylight, you need double rafters. If this were the case, you would need double rafters on the entire structure, since sheathing starts at the eave & ends at the ridge. In other words, it has to start somewhere, and end somewhere, and there's no blocking at the eave or ridge.

[Bob]

how can a hole in a wall weaken the roof?

[TWayne]

Defintely NOT, as you stated it anyway. But I think something was lost in the translation. I'd bet on it.