DEAR TIM: The fireplace in my new home is a mess. Water is leaking into the fireplace and then into the basement. The interior firebricks are not level, some have gaps between them and where there is mortar the joints are one-half inch wide and sloppy. The builder says I should not be concerned as when a fire is built, the brick get covered with soot and the defects will not be visible. What is an acceptable level of quality? How should the firebrick have been installed? Diane McC., St. Leon, IN

DEAR DIANE: Hypothetically speaking, I wonder what your builder would say to a truck salesman who sold him a shiny, new truck and then said to him, "Don't worry about all those scratches and dents in the body. After a week at the jobsite, all of the mud and dirt will hide them."

Fireplaces are one of many places in a new home where quality can't be compromised. Cracks in granite countertops, dents in new drywall, blemishes in ceramic tile do not present life safety issues. But a gap or crack in firebrick that is poorly installed can cause a house to burn to the ground. Death and serious injury are real possibilities.

The temperature inside a fireplace that is containing a roaring fire might surprise you. It is not uncommon for temperatures to fluctuate between 1,200 - 1,500 F. The gases swirling inside the fireplace can seep through the tiniest cracks searching for another way to the atmosphere other than the chimney. If the mason was sloppy with the critical firebrick, imagine the level of quality in the body of the chimney masonry that you can't see. If other gaps and cracks exist, the blistering gases can ignite nearby wood framing in a matter of minutes.

Mortar joints between firebrick in a combustion chamber should never exceed 1/4 inch in width. In fact, a smaller joint is preferred. If traditional sandy mortar is used, it should be a high-lime content mortar that does contain some Portland cement. High-lime content mortars perform well when subjected to repeated episodes of high temperatures.

I prefer to use fireclay as the mortar between the high-temperature white refractory firebrick in my fireplaces. Fireclay is a powder that is mixed with water until you have a consistency like pancake batter. The edge of a firebrick is dipped in the fireclay batter and then the brick is set and tapped in place. The resulting joint is usually one-eighth inch or less in width. Fireclay is perhaps the most fire-resistant mortar you can use in a fireplace combustion chamber.

But keep in mind that the firebrick you see is but one part of a system. Behind the firebrick, one should find a minimum of 8 inches of solid masonry material. This concrete block or brick should be laid with precision making sure that all horizontal and vertical mortar joints are completely filled and solid. The smallest gap in the masonry can create a pathway for rogue, searing flue gases that are constantly trying to escape from the fireplace combustion chamber.

The water leaks you are experiencing may be from two sources. Rain falling down the chimney may roll off a poorly constructed smoke shelf that is not constructed to capture it. Or rain water could enter the exterior of the fireplace masonry that is outside your home. The leaks often happen during moderate to severe wind-driven rainstorms. These exterior brick can be sealed with a clear brush-on sealant that can bridge cracks up to one-eighth inch. The best part is the sealant is breathable and allows water vapor produced by burning fires to escape from the brick back to the atmosphere.

Fireplace construction is a strict science. If you think you just start laying brick and everything will be fine, you are dreaming. The shape of the firebox, it's internal dimensions and the height and width of the opening are all extremely critical. If the fireplace is not shaped and sized right, the fire will not burn properly and smoke can easily roll out into the room.

The part of the fireplace you can't see, unless you poke your head up into the chimney, is even more important. The dynamics of the combustion gases as they leave the firebox and make their way to the actual chimney are just as important. This area of a fireplace / chimney is called the throat, and it must also be shaped and sized in accordance with the visible part of the fireplace you see from your family room. Professionals need to build masonry fireplaces. This is not a place for building technology experimentation.

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Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. oteria morgan 12 Apr 2008, 08:50 how can tell if the leaking from your fireplace is from the chimmy or the masonry. it really leaks when we have moderate to severe rain, it's so bad the dry wall on either side of the fireplace gets wet. we had to repair the drywall above the fireplace twice and it need to be replace again. AsktheBuilder 04 May 2008, 23:30 Oteria, You can get up on the roof and simulate a driving rain with a hose. Just get the sides of the chimney wet, not the top, and see if the inside gets wet. tim 11 Nov 2008, 15:10 I am building an oudoor fireplace. Can I use regular mortar behind the firebrick? ( to stick the firebrick to the cement firebox surround) I will use refractory mortar to put the firebricks together. just wondering sean 11 Feb 2009, 08:53 I have a new house with a fireplace While closing the flue I noticed that up top near the flue on one side there is no morter on top of the last brick so it is not sealed. I can put my hand behind the brick and feel air flowing. Should it be sealed San 03 Mar 2009, 07:32 My wood burning fireplace in my 17 yr old is very drafty! While I feel a draft coming from the closed damper, the majority of the draft is coming from underneath the brick hearth! Underneath the front of the hearth is a vent like opening and in the winter, the air is strong enough to blow out a candle! The hearth is not solid brick to the floor. I am stuffing insulation, towels, rugs, etc. to cover this opening but it doesn't help. I want to permanently seal it off but I'm not sure if it will affect future use of the fireplace. There is a switch next to the fireplace that controls a blower. Not sure if the vent opening underneath the hearth is associated with the blower. Please help! Also, who should I call to inspect this? Home Inspector? Chimney Inspector/Cleaner? Masonry professisonal? ted powell 19 Mar 2009, 00:39 fireplace construction mary m 25 Apr 2009, 08:50 The brick on my fireplace seems to be deteriorating. I am getting a fine (and sometimes not so fine) coating of red dust on my mantel each morning. I had a roofer in and he thought that the outside flashing was fine. Do you have any suggestions as to how to take care of this issue? Thank you. Tamara 04 Jun 2009, 20:41 Masonry continually cracks on the exterior block..leaks water to inside of FP...have sealed multiple times, always re-cracks. Odor in home due to this also. What is your best suggestion on repair? Rock it? Tear it down? It is very wide FP...Live on mountain and feel like Chimney and Home shifts during seasons. Tamara kenya fluellen 01 Jul 2009, 18:31 I would like to repaint the inside of my fireplace black. Do I need a special paint? Amanda Mardlin 05 Aug 2009, 07:07 I was told that my woodburning fireplace insert needed bricks inside cemented to the sides.Right now it is just the steel sides. It is an older insert, would this help it burn warmer, or stay warm longer?      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.