Is it possible to build a tree-resistant roof?

[spotter]

Tallahassee, where my 1960's house is, historically has a low probability of being hit by a strong hurricane. Nevertheless, I and some neighbors have removed large pines from our properties. Other neighbors don't want to cut their pines, most of which are 2-3 feet in diameter at breast height. I am going to retrofit my roof for 110 mph wind speed by adding hurricane clips at the rafter/wall junction, reinforcing the gable ends, putting shutters on the gable vents, eliminating the ridge vent, and by using wind-resistant shingles or metal roofing.

But even if I have all this wind protection, a neighbor's 100' high pine can still fall on my roof. If I added a new roof frame over the old one to get a second deck, say 4 feet above the current one, would that be a feasible way to decrease the risk of catastrophic water damage from a tree breaching the roof? I'd appreciate any ideas on how to make a tree-resistant roof.

Thanks,
Jim

[concretemasonry]

spotter -

It sounds like you may be tring to qualfy for the huge insurance discounts offered by the insurance companies. This is a voluntary program that is in addition to the minimal code requirements.

Any time a 100' tree fals on a roof, it will usuallt destroy is and the second roof below that.

the clips are good and few people recognize the weak point in the gable ends. The codes are minimum standards, so some people can get away with a lot even they meet the code requirements.

Don't forget about the garage door. That is one of the main reasons for the wind destroying a home. - Wind gets in and the roof goes. After that every goes unless it is partially reinforced masonry.

Dick

[spotter]

No, I'm not that concerned about insurance cost. I'm trying to avoid a miserable year of living in a half-destroyed house with my possessions messed up while I try to get an insurance settlement and reconstruction. I believe I have all the vulnerabilities covered (there is no garage) except for the possibility of tree impact. Isn't there anyone who has suggestions on ways to build a stronger roof frame to mitigate the tree risk?

[Joe Carola]

N Isn't there anyone who has suggestions on ways to build a stronger roof frame to mitigate the tree risk?

You can try and build a tree proof house , but who's to say what you need. You can frame a roof over your existing roof with 5-1/4" x 20" glulams for rafters 6" on center on the whole house and sheath with 2x12's and 6- layers of 1" plywood. Who knows?

You would have to have someone come out and measure the height and width of the tree. They have to measure how far away from the house the tree is in order to see how much of the tree would hit the roof. If the tree is 20' away from the house and is 60' tall,3' wide, what would the impact be and the speed of the tree falling hit it at from that distance.

If the tree was 10' away from the house and fell, it wouldn't have enough speed from falling and the impact and the part of the tree might not cause that much damage. I just did a repair on a house that a tree fell on. The tree I think was 80' and 4' wide. The tree covered the entire front of the houses and branches covered the back of the house.

I just had to replace 2 rafters and fascia and soffit. That was all the damage because the tree was so close to the house it didn't have enough speed to gain any strength to blow out the whole roof and cause severe damage. Had the tree been 20' away, the tree probably would've been laying on the first floor.

[Richard A Hetzel]

I did a repair of tree damage a few years ago. The tree was about an 18-inch oak, located 20 feet away and a little bit uphill from the house. It caved in the center of the roof, fell through the attic, and drove a door jamb of a second floor door down through the ceiling like a nail. The bottom of the door jamb ended up about a foot above the dining room table.

I used to design telephone buildings. One was located in a less-than-ideal neighborhood, so the phone company wanted us to protect the roof-mounted air-conditioning unit, without which the building could not function. Our question to them was "Protect against what" A rifle? A bazooka? A grenade? A mortar? A howitzer?" Their answer was "Oh, forget it", and I'm sure they went out and bought riot insurance.

Same thing here...protect against what? You tell me what, and then I can tell you how. The answer could well be "buy insurance".

[Don_P]

Mine for this spring was a large poplar tree right beside a vacation cottage. The top broke off and drove through the roof right over the bed. nobody home luckily. A couple of broken rafters, about 4 sheets and some ceiling, soffit and fascia.

I had one a couple of years ago, a hemlock planted by the clients great grandmother after "the war" it was over 100 years old. I had recently restored the old homeplace. The tree did little real damage. A good bit was the fall itself but some of it was the beefed rafters, I had just sistered onto undersized sagging ones and had covered over the oak sheathing with new ply. I think having a heavy deck distributed the load to the stronger rafters. It was a new standing seam roof. It antiqued it some for us but really didn't hurt it. I stood the seams back up replaced some snow guards and the gutters and its been doing fine.

[spotter]

OK Richard,
I'll be more specific. Assume that the tree is an 80' tall loblloly pine and it's 40' feet from the house. It uproots and hits the roof diagonally to the run of rafters. Say that the diameter of the trunk where it hits the house is 15". Assume that the tree limbs snap as they strike the roof before the trunk does.

By the way, the last couple of days of TS Fay have had several houses (older, small houses) virtually cut in half by falling trees here in Tallahassee. And that was with wind gusts of 50 mph and under, but on soaked ground.

Regarding insurance, they don't pay for pain and suffering, which is what I'm interested in avoiding. Before someone suggests moving to a different house, I am considering that, but there are major risks there of ending up with two houses in a depression era housing market, plus the costs of moving. So, I am trying to determine whether by spending 20K on a reinforced roof I could improve my odds significantly of weathering a hurricane without water damage.

[spotter]

Let my ask another different question, since a lot of you fellows have experience with actual tree damage. What is the most dangerous "angle of attack" of the tree on to the roof: Parallel to the rafters (= perpendicular to the long dimension of the roof), diagonal to the rafters as in the post above, or on to the gable-end fascia?

[Richard A Hetzel]

I don't think it matters much. If the heaviest part of the trunk hits the roof, you're going to have damage. The tree I spoke of hit almost parallel to the rafters, but one tree episode does not an expert make.

In a high wind, trees can attack from almost any angle, so the problem can't be considered with a static tree and a simple fall. The tree could fall the other way, then spin around and be thrown toward the house like a bullet, missing the roof entirely and smashing through the wall. It could do anything. Fall and swat at your house as if it were a baseball bat...anything. So back to my other question: what would you like to protect against?

It's even possible that anything you put on your roof could increase the possibility of direct damage from a hurricane, with no tree involved. Who knows?

[concretemasonry]

spotter -

You are trying to micro-analyze something that is variable and you have no control over.

After 500+ hurricane damage verifications, I learned the worst damage (beyond the obvious storm surge and normal hurricane winds and neglecting the later flooding followinf Katrina) are the tornadoes that are spawnwd ahead of and on either side of a hurricane.

Hurricanes have both a good side and a bad side, but the maximum winds are opposite, but substantial. A tornado, on the other hand, has far greater winds and can cause total destruction of damage coming from any direction. If you are across the street from a tornado, you are lucky. If it hits you, everything is gone.

If you are foolish enough to try to ride it out, it makes little difference if a tornado gets a direct hit, since everything will be gone. In that case, you should go to the FEMA site and look for "safe cell" requirements (document 315, I believe). This is the basis for the severe area code requirements in the special Florida coastal areas. Altough this was designed for tornado survival in the rest of the country and not just damage from a puny hurricane, it contains some good information.

Richard is right that you can never select a specific tree and try to predict what wil happen. Consult the insurance company requirements for the preferred reduced insurance rates as a practical (not code, but better) guide for more protection. - If you think you will lose shingles, do not use fiberglass insulation in the ceiling or walls because of the ability to hold moisture and cause mold. Also get rid of the turbines. - This is based on people's problems after Katrina, Rita and Willma.

Dick

[spotter]

I am not proposing to pick a specific tree. I simply gave Richard what he asked for in his first reply and then he slid around the question again. By the way Richard, your baseball bat scenario is pure fantasy. 110 mph winds cannot pick up a large tree trunk. My second question was about the likely degree of damage depending on the angle of attack. It was not useful to inform me that there will be damage.

Joe, thank you for actually answering what is an unusual question. You're the only one who tried to respond to what was asked.

[Richard A Hetzel]

I apologize, but I don't think there is a human on Earth, professional or otherwise, who could calculate the possible forces from a specific tree and design a structure to resist them. Also, any structure you might put on or over your roof would only be dependent on the existing structure below it, and all it might do is transfer the blow to a weaker structure, and therefore damage from one place to another. I think you are asking questions presuming there can be a precise answer, and there just cannot.

If you think the baseball bat analogy is fantasy, then you haven't seen films of hurricanes and tornados, and what can fly through the air in such winds.