Do flat roofs usually sag?

[Esteban]

Do flat roofs on teardrops usually, or often, sag? If so how can this sag be avoided? I've seen two teardrops locally with flat roofs. They both sag about a half inch or so in the middle. It could create a water ponding area and make the roofs more prone to leaks. Is this typical for a flat roofed teardrop?

One builder said he used stout roof rafters. Both are 5 ft. wide. One is skinned with plywood and epoxy. The other is skinned with aluminum over plywood.

I'm starting the construction of my teardrop. I'm concerned because my Grumman II inspired profile is almost flat for about 3 feet above the sleeping area. This part of the roof has about a 1/4" per foot, front to back, taper. If it'd help to keep the roof from sagging I can taper it a little more from the center of the roof to the front. The back part of the roof tapers more and will have support from cabinets and the partition wall between the galley and sleeping area. So it worries me less.

I plan to build the roof with 1-1/2" high roof spars on 1' centers. I may also put blocking in between the roof spars along the center of the roof hoping that will stiffen it.

Currently I have a 5'x10' trailer frame and just cut out a side profile pattern from 1/2" plywood. I'm going to begin building the floor today. The teardrop will be about 5'3" wide by about 11' long. I don't want my trailer to have a saggy roof.

[Dale M.]

It may depend on construction.... IF you use a wide flat spar the strength factor is in wrong direction.... IF you use a tall narrow spar the cross section strength is better...

Why not build a little arch in top by using a spar that has a height of something like 2 inches at ends and 3 at middle.... This should pre-stress spar and give a little arch to allow water a "runoff" ability. Also weigh of roof material is a consideration. If you are not walking around on roof, you don't need heavy materials in roof construction.

Dale

[Miriam C.]

I have a small pond that gets on my flat roof partly because I put trim up and made walls. I would make the roof slightly rounded if I could do it again.

Ponding might not be such a big deal in summer but in winter it is sure to freeze and even the best sealed trim will move over for ice.

[caseydog]

If you want to use a roof vent, you will want a area of flat big enough for that, but the rest can curve how ever you want.

And, I agree that the way you position your spars will matter when it comes to sagging, and that using a lightweight skin will help. My TD has a lightweight, yet very rigid skin. The spars just form the shape -- they really don't support any weight.

[bg]

In Glen-L's plans for the Sequoia, they show making your roof ribs with a upward curve cut into them. I think it's a 1/2" or so rise in the 5 foot width of the roof.

[mikeschn]

If you are building a flat roof, you have to pay attention to the wood grain in your spars...

A spar typically has a bow (be it ever so slight). You must orient the spar so the bow faces up.

If you orient the bow so it faces down, your roof will sag, and you will collect water on the roof. Then the water will find a pinhole, and your spar will start to rot... Soon the roof will fall in.. and you will have a trailer without a roof.

Another option is to crank up the tongue jack so that the water runs off. But I always orient my spars so the bow faces up.

Mike...

[48Rob]

What Mike says...

In addition... while most of the time we toss aside those pieces that are not straight...roof spar selection can benefit from choosing those which are slightly bowed in the center.
An inch tops, will settle over time to near, but not just, straight, leaving just enough pitch to help carry water away.

Obviously you can cut a little pitch into the spars if you are making them from larger stock, to achieve the same result.

Either way, Grain orientation is VERY important.

Rob

[Esteban]

Some additional information on the teardrop I'm building.

Years ago I salvaged a WWII era warehouse. I have plenty of well seasoned tight grained old growth 2"x8" (warehouse) rafters that will be cut up for spars. This lumber should be stronger than new lumber that's mostly sapwood.

The typical spars will be 1-1/2" high by 3/4" wide. Most have a crown of about 1/4" which will be oriented upward against the load. I could cut the spars for the flat top part of the roof so that they're wider, than ones in the curved area of the front, to give them more strength.

Maybe, for the flattest areas needing the most strength, I'll glue together two 3/4"x1-1/2" pieces to make one stronger 1-1/2x1-1/2" spar that'd then have unmatched grain so weak spots, if any, would be minimized.

The roof will have a Fantastic Fan centered in the flattest part of the roof. The only roof penetrations will be for the fan and for attaching the hurricane hinge for the hatch. The top roof skin will have two top layers of 1/8" plywood covered with epoxy and 6 oz. fiberglass. The fiberglass should seal the roof very well. The walls will be fiberglassed (1/4") plywood too and I'll round over the transition from the roof to the wall. It'll all be painted with epoxy boat paint.

The center part of the roof will have 1-1/2" of rigid foam and the 1-1/2" high spars spaced on 12" centers. The spars will all be installed with crowns facing up (or outward). The bottom layer of the roof will be 1/8" plywood for the cabin ceiling.

I'm hoping this construction method will counteract the tendency I've seen of flat roofs on teardrops to sag, and possibly to leak.

[G-force]

I am also doing a Grumman 2, however it is true to the plans so it only has a 13" flat section, everything else is part of the top two elipses. Because of this, I went with 1 1/2 x 3/4 poplar spars, 1/8 oak ply inside and out. It feels pretty rigid to me so I am hoping for no sagging. Honestaly, if I was to do it again, I would probaly double them up next time to make sure.

[bobhenry]

I ripped my 2x2 spars (1 1/2" x 1 1/2") out of southern yellow pine 2x12's ( these are usually #1 grade)
Look at the grain pattern when you buy them and select only the straightest grain possible with absolutely no knots. After cutting them let them relax for 2 days. any modest crowning (bowing) place it upward as gravity will flatten it out. Some may twist like a cork screw some will go wild and bow like a long bow. I cut 6 more than I needed and 4 went crazy. I was able to use them later in short sections as cabinet frames and other uses but not as spars.

[aggie79]

Be aware of compound curves. It will be difficult to get sheet goods for skinning the roof to bend horizontally and longitudinally at the same time.