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IZZY 01-09-2012 01:44 AM

Buckling plywood underlay
Bought a house five years ago with about a 500 square foot covered patio roof. The roof consists of 2x6 timber rafters supporting 3/8 inch 8x4 foot plywood sheet underlayment, roofing felt and asphalt shingles. The problem is the plywood has buckled mainly along the joints creating numerous leaks. Time to replace the roof.

First question is what went wrong? Second, how do I prevent that from happening again? Third, should I consider using other materials besides what is there?

A few things to consider. I live in a desert region. We get moderate rain from Jan through April and little thereafter. Summer temps reach a consistent 95 to 105 degrees. There is no covering on the bottom of the roof. I can stand on the patio and see the bottom side of the plywood underlayment. I appreciate any suggestions.

nealtw 01-09-2012 07:21 AM

3/8" plywood is a little light for the roof sheeting and there should be "H" clip in the center of the joint between the rafters. Adding blocking at the joints when the roofing is off may help but changing to 7/16" osb or better would be the best fix.

EZHangDoor 01-10-2012 09:50 AM

I agree you need thicker plywood or OSB. I would recommend 1/2" thickness. Don't forget to properly space the sheathing, if it is placed too close together, that will also cause it to buckle. Check the manufactures specs for edge and length proper spacing.

nealtw 01-10-2012 04:13 PM

"h' clips provide spacing for the sides of the sheet and 1/8 inch between sheet ends.

EZHangDoor 01-11-2012 08:33 AM


Originally Posted by nealtw (Post 66139)
"h' clips provide spacing for the sides of the sheet and 1/8 inch between sheet ends.

Good point. Often times H clips are not required per code on 1/2" or thicker sheathing. That creates a problem when you don't get that proper spacing they create.

Jdmrenovations 01-11-2012 03:13 PM

Since assuming has gotten me &%&*) in the past, I'll ask...what spacing are the rafters?

Also, what's the pitch of the roof?

Roofmaster417 01-12-2012 01:02 AM


Originally Posted by Jdmrenovations (Post 66170)
what spacing are the rafters?

Excellent question. 2"x6" rafters are solid.,Depending on length and on center width and bracing.Close enough and 3/8" and 7/16" would be fine.Wider would require 1/2" or 17/32".

IMO a complete open overhanging/Carport structure is all the more reason for proper spacing.3/8",1/2" or 5/8" or more will buckle over time without proper spacing.That holds true for carports especially.Constant exposure to the elements.

Houses are built as one.One component handles the other and settling or movements of the structure have been figured by architects and the compatibility formulas are present and are used.

The problems with carports or porch overhangs is the after market effect.(Built by Uncle Buck on the weekends).Improper fasteners and length will also give the appearance of buckling when its actually seam pops.(The seams are nailed with inadequate number of fasteners)(Not enough)

The house shifts and is always moving and sometimes the carports or porches settle their own way resulting in twisting or shifting of the porch/carport against the houses movements.So binding and contorting of too tight of seams for the plywood is the effect from improper spacing.

Maybe a silly question to some.,but is the plywood running parallel to the gutterline?.,I have seen plywood ran vertical.Obviously wrong but it does happen.

How wide are your overhangs from the post points?.,Post some pix.Assumptions are a.,.,well you know.

IZZY 01-12-2012 11:15 PM

First, thanks to all of you for responding. I'm trying to get my ducks in a row before I tackle this in the spring. I'd like the new roof to last longer than the 8 years this one lasted. Actually It should have been replaced 2 years ago so I'll call it 6 years.

Second, a correction and a few more facts that will answers questions. On a second look, it's not 3/8" plywood. It's 7/16". Found a better spot to measure that wasn't half rotted. There are 3 sky lights installed. The house roof is a terracotta tiled roof. There are (16) 20ft rafters, (8) 16 ft and (4) 3 foot rafters on the patio roof. They follow the outline of the house roof. They did their best to keep them on 16 inch centers but for whatever reason, a few are on as much as 20 inch centers. The plywood sheets are installed parallel to the gutter. Overhang from the 4 posts to the roof edge is 20".

"H" clip? Sounds like a spacer between the plywood sheets? Metal? I don't see anything like that. Maybe that's part of the answer.

What's the roof pitch? Um. Not sure how to figure that but using a 12" bullet level on the bottom of a rafter I've got a 1 1/8" gap on one end with the bubble centered. 1 1/8" x 20' gives me a 22 1/2" differential between the house connection and the gutter end. Does that help? Is that a shade over a 1/12 pitch?

"The problems with carports or porch overhangs is the after market effect.(Built by Uncle Buck on the weekends).Improper fasteners and length will also give the appearance of buckling when its actually seam pops.(The seams are nailed with inadequate number of fasteners)(Not enough)" - Most of the buckling is between the rafters where nails aren't an option.


Spacing between the plywood sheets was mentioned. I would imagine to allow for expansion/contraction. Does that mean when installing them butting them against each other is a no no? My problem with that is aesthetics. If I'm following this right when standing on the patio I'll be looking up at gaps between the boards correct? If spacing has to be done how can I avoid seeing the gaps - cheaply?

Is this buckling caused by excessive heat or water? We live on the edge of the Mojave desert in Southern California. The general rule is 8-12 inches of rain from Jan through April. Practically none after that. Snow? What's that? It gets very hot June through Sept. It will hang around 100 degrees and bump to 110 now and then. Most of the roofs around here are clay tile. It's tough to find an asphalt shingled roof. Here's a few questions based on all that:

I'm thinking asphalt shingles aren't the best thing to use around here. What say you? Could they have trapped heat buckling the plywood sheeting or is it solely a water problem?

If I do use asphalt shingles and if this is a heat problem, would installing some kind of insulating material between the shingles and the plywood help prevent the buckling?

Is plywood the best thing to use? Which type? Some type of engineered wood? Should I consider metal roofing? Some other type of roofing?

Never dealt with a roof. Luckily this is just the patio roof. If it were the house roof I wouldn't even consider a DIY job. I've gutted and remodeled a kitchen. Built a deck or two. Hopefully some of that experience will carry over to installing a new roof. Thanks again for taking the time to pass on the info. I'll work on some pics.

oldognewtrick 01-13-2012 06:32 AM

IZZY, a 1 1/2-12 is not sufficient for shingles or tile. minimum on shingles is a 2/12 pitch and tile a 4/12. The problem is gravity wants to pull the water through the butt joints of the shingles and there isn't much head lap on tile. If you must install either of these, use a high temp, peel and stick underlayment as a water barrier. This is probably the reason that your felt failed and allowed the wood to rot.

If you go back with tile I'd recommend using at least 5/8" Advanteck decking and clips. If you don't like the small line from underneath, think about 2x6" tongue and grove slat boards. The buckling you experienced could be a result of live roof loads. Each piece of tile weighs about 1 lb. and there are roughly 100 pieces in a 10 x10' area. You need to add structure to hold the weight and ensure 5 years down the road you're not dealing with this issue again. 2x6 rafters may be under spanned, you might want to ask an engineer or look on the net for rafter span calculators. Most tile MFG's will have this on their web sites.

joecaption 01-13-2012 07:57 AM

A picture would sure be helpful on this one and ansewer a lot of questions.
What you have sounds like what's called a shed style roof. There very prone to leaks because no one ever gives them enough slope, and do not tie them into the existing roof correctly.
That narrow a 2X on that long a run is undersized and will cause the roof to sag.
The fact you have sky lights adds to the chances of leaks if there not low slope lights.
Is there any drip cap around the outside edges? (most often an aluminum strip of metal bent at a 90 deg. angle to protect the outside edges of the sheathing)
Are these 3 tab shingles? There the cheapest one, most likly to leak and have the shortest warrentys in the business.
Some other choises for low sloped roofs are EPDM, it a one piece rubber membrain. They can come in white and black. In your area a white roof would stay far cooler.
Metal, can come in many colors, and some have warrentys in the 75 year range. (not the cheap junk sold in the box stores)
If you want to hide the under side of the rafters there's lots of options.
I use vinyl soffit and vinyl cove moulding made by Cereented. The best looking one is called beaded. In your case I would use what's called hidden vent. When it's installed you can not see the vents and it will look like you installed Tongued and groved boards. Any real siding supply house will have it.
I just hate to see someone try and use paneling on the ceiling. Over time it will sag and there's batten strips to hide the joints.
Real wood T&G will be a nightmare to keep painted and will give you no ventalation to keep the bottom side of the sheathing dry.
PS Far be it from me to correct anyone spelling but the other poster misspelled Advantec, there is no K on the end.
It's much stronger then plywood, comes tongued and groved for butt joint strength, and holds up far better when wet. No H clips needed.

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