Expert: Richard Taylor, AIA - 7/3/2007

Question
We added a small glass room on the back of our house, but did not use sill plate sealer between the foundation and the seal plate.  Now, of course, water leaks under the sill plate.  Can you recommend something we can use now to seal gap between the sill plate and the foundation to prevent leaks?  Thank you.

Answer
Hi Nancy - thanks for your question!

The sill sealer is only one component of the system that prevents water infiltration at this particular junction of building materials.  The sill sealer is the least important part of the assembly.  You need to be sure the entire system is in place.

Here's what you need to do (assuming it's a wood-framed room with siding):

Remove the siding and sheathing from the outside to expose the joint between the sill plate and foundation, and another 6 inches or so above it.  Clean the joint, then place a foam backer rod into it.  Cover the joint and backer rod with a good quality exterior silicone caulk.

Re-install the sheathing over the wall and plate.

Next install metal flashing (with a drip edge) to cover all of the exposed sheathing and down over the foundation at least an inch.

Replace the siding, making sure it laps down past the top of the foundation.

If this is an "all-glass" room (maybe a prefabricated unit with aluminum framing) then all you can do is rod and caulk the joint between the bottom frame and the foundation.  But I'd call the installer/manufacturer first.

Finally - in either case - make sure the grade is at least 8 inches below the top of the foundation and sloping away, and that all the gutters and downspouts are open, working, and carrying water away from the house.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com
www.rtahouseplans.com
www.dontbuyahouseplan.com