A miter seam in a countertop near a seam is a risky situation. Here, pros consider a case of joint failure. July 3, 2008
Question
Are there any guidelines on "properly sealing" a post-form top at the seam and what sealant to use?
The installer put DAP in the seam (fair amount) and two months later the seam was buckling. The sink is only located 8" away from it in the front. Can a properly-sealed seam buckle from constant moisture (kitchen towel placed on top of if to drain dishes) or what other factors could have caused it?
If the seam is sealed properly is it fair to say that it still may let moisture in that will cause damage to the substrate? If so I will need to enter a new clause in my warranty.
Forum Responses
(Laminate and Solid Surfacing Forum)
From contributor H:
If the seam is 8" away in front where is it in the back? Isn't it a butt joint or is it a mitre? Damp dish towels or any constant moisture will eventually get in regardless if sealant. Is a dishwasher under the joint?
Over tightening connection bolts will cause joint problems. The substrate crushes over time and has to go somewhere either bulging the top or blowing out the bottom. We keep joints of any kind as far from sinks and dishwashers as possible.
From the original questioner:
It is a 45 degree miter joint. The sink is 8" away from the seam in the front and about 35-36" in the back". Tops have been installed less than three months ago. We have never had this problem before.
From contributor X:
In my opinion water is a factor. Its getting into the seam, thus swelling the material. Miter cuts of the joints are also flawed. Your excessive use of seam fill is a no-no. Miter cut joint is too close to the sink area which allows water to close to the joint.
Who's at fault? Everyone from the designer to the seller, mfg., installer and consumer. The solution is to make a new top. Pre-form the top again and hope for the best or different style top where the joint is not accessible to water. If it were up to me I would sell everyone on a square lip top. Who is going to step up and bite the bullet and assume the new cost of the top?
From contributor J:
Contributor H and X are both right. You can replace that top and it will eventually fail again. That layout sounds like a good candidate for a shop or site-built laminate top with seams as far away from the sink as possible. Call it a personality flaw, but if I can buy a big enough piece of laminate to avoid a seam all together I will no matter how much waste it produces. I pass the cost on to the customer, but they get a much better product in my opinion.
From the original questioner:
Thanks to you all for all your responses. Unfortunately, since we did not have the proper provisions in the warranty to cover the water damage on the seams, we will have to pay for a new top.
Can anyone please tell me if the miter seam of a pre-formed top that is perfectly sealed with proper sealant can buckle from water if it is applied constantly over three 3 months? So far it sounds like no one is really sure. Thanks again for all your knowledge and help.
From contributor A:
Yes the seam can swell from the factors you've presented. I believe your first mistake was using DAP to bond a post-form mitre. You should use wood glue, even a type 2 water proof kind. Any gap left over in the mitre should be filled with seam fil, or a latex based caulk.
If you make your own tops I'd talk them out of a replacement top in post-form. Assuming the return legs are under 60in you could offer them something without a seam.
From contributor J:
I don't think there's any question that a joint can buckle in much less than three months if it's exposed to constant moisture. I saw a properly installed post form top that swelled-up over night when chicken had been left to thaw over the seam.
I agree that the joints are not water tight, nor is an HPL bond perfect. Water can penetrate the seam between the HPL and the core over time and then you've lost your seal.
However, the TYPE III Adhesives are a good item to use at that seam. They are a one-part polyurethane and they offer the best shield for keeping water out. Also, using a spline or biscuit to keep the top surfaces aligned will help keep the joint tight. If you cannot avoid water in the area of a seam make sure you pitch the top so that standing water won't stay right on top.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor T:
Informing the consumer is the key. Explain to them how laminate is made. It consists of layers of brown craft paper overlaid with a colored paper covered by a thin layer of clear plastic. This thin layer of plastic is not water proof, only water resistant and what is underneath it - layers of colored paper. Most often the substrate is particle board and we all know what happens when that gets wet.
I totally agree with the adhesive solution mentioned by several that have commented. The polyurethane glue (such as Gorilla Glue) ise probably the best adhesive to use but can cause its own problems when drying if you are not used to it. It will swell and fill voids and excess glue above the seam has to be removed carefully. For the beginner I would recommend a good grade of water resistant wood glue such as Durabond or even Elmers wood glue (yellow).
I once had a customer that complained of swelling in a post form countertop joint. Upon inspection I quickly determined that it was the result of water intrusion. I informed the customer that "I was sure she mopped her countertops with a dishtowel soaked with water from her sink". She admitted yes that she had done that since she was a young girl helping her mother in the kitchen. She then admitted that her sister that bought the house next to her, and had the same problem. I then replied "you had the same mother who taught you both the same bad habits".
