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I'm looking for a good way to shield the tops of exposed horizontal beams. These are 4x12's that extend about 18" (cantilever) out from the bargeboards (gable end fascia). The main problem is that the tops are level and it's a place for water and dirt to accumulate. I'm thinking of spraying on some tar, just on the horizontal portion, or more expensive, something like "Low Profile Copper Beam Caps".

Thanks for any ideas.

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The top of the beam is just part of the problem. The beam soaks up way more moisture from end grain exposure. Also, whatever you use to cover (protect) the beams, you need it to allow to dry out each time after a storm too.

Where I live, our community college had a similar problem and a local architectural firm designed a stainless steel cap to protect the tops and ends of the beams. However, after about 3 years the beams had even more dryrot and the problem accelerated, because moisture blew in and became trapped between the beam and flashing.

Even pressure treated material can rot. Good design does not included leaving wood structural members exposed in wet climates.

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I think the low profile copper caps are an excellent choice to cover the tops of your exposed beams. I would recommend that the beam tops be clean and dry before applying the caps. You could even tar them down on the beam top but that makes removal hard in the case you ever need to remove them for service and/or re-use.

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