DEAR SAM: My roofer says that water is seeping through the porous top of my chimney, which is why my fireplace wall is stained. I have been given conflicting advice on how to cap the chimney. I used a thin layer of "Top and Bond," but that has cracked.

ANSWER: You may be able to make this repair successfully yourself, but I usually recommend that roof work be left to experienced persons. If you want to try it, here's what to do:

If the present cement cap has worn away so that the top-most layer of bricks is visible, you will need a new cap. Measure the outside dimensions of the chimney top and build a wood frame around it with 2x4s, using 16 penny spikes to nail the ends together into a rectangle. For added sturdiness, 1- by 2-inch strapping can be nailed to the top, avoiding the flue lining. The form should extend about two inches above the brick but should be lower than the tops of the flue lining or linings.

Depending on the thickness of the new cement cap, you will need these ingredients: 35 pounds of Brixment or an equal brand of mortar cement; 50 pounds of clean brick sand, and one-half pound of Medusa Powder for waterproofing. Mixed together, they will give you a cubic foot of mixed cement.

Pour the cement carefully into the form. Level the cement, with a slight bevel to the tops of the flue linings. The cement should set within eight hours and the wood form can be removed.