Saturday, June 18, 2011

Adding a Shed Dormer to a 1920’s Era Bungalow

My Client's 1920's Era Bungalow
My Client's 1920's Era Bungalow
I am working for a client right now designing a shed dormer addition to their 1920’s era bungalow in the Ravenna district of Seattle.  The challenge was to find an economical way to expand their home to make way for a new baby.  They also want to add the modern amenity of a Master Suite.
Computer model of the house used in the design process
Computer model of the house used in the design process

Existing Developed Condition

Their second floor is currently developed below the roofline where you might imagine an attic to be.  It has a bedroom open to the stairway, a small closet and an office you access through the bedroom.  This pair of rooms is 13′ wide by the length of the house.  The walls between the room and the attic are 5′-10″ tall and the ceiling slopes from there up to 8′ and then flattens out.  The challenge is to create a master suite and a nursery on that floor while minimizing the structural (and actual) disturbance to the house.

Bird's eye view of the existing configuration
Bird's eye view of the existing configuration

Proposed Configuration

Here’s what we’ve proposed.  We’re adding a 15′ wide shed dormer to each side of the roof.  This adds about 180 square feet of useable floor area that they don’t already have.  We’ll add a landing at the top of the stairs, put the master suite in the dormers and keep the nursery in the room that exists in the front of the house.  In the future the nursery can become a private office.
Concept floor plan
Concept floor plan

Issues

Structural Issues - The number one question I get asked in this context is, “Can my house support a dormer?”  I generally don’t answer that question.  The question I answer is, “Is there a cost efficient way to add a dormer to my house?”  The answer to this question involves minimizing the intrusion into the existing finished structure.  The less we have to alter the existing lower floors the easier and cheaper the dormer will be.

Perspective view of the house with the added dormers
Perspective view of the house with the added dormers

Floor Support

Generally speaking there are 2 structural components that need to be addressed in the dormer design; how to support the additional load on the floor and how to support the new roof.  Often in older homes these “attic” like stories have under sized floor joists.  There are a number of solutions to this problem, all of them including adding additional structural elements.  In this case the floor joists aren’t undersized and are adequately supported by the walls below.   We won’t need to change anything to make the floor work.

Bird's eye view of the new configuration
Bird's eye view of the new configuration

Roof Support

 The roof of a traditional dormer (built at the time of original construction) was typically supported on the floors and interior walls below it.  The chances are you don’t have sufficient structure immediately below the proposed dormer to support it.  The solution that first leaps to mind, and that I most often see in projects I didn’t design, is to replicate the traditional system of support.  This involves adding beams and supporting structure below the dormer walls and providing support for those beams down to the foundation.
While I’ve done that myself on a number of projects my preferred solution is to get the roof loads out to the outside walls rather than trying to pick them up by new structure inside the house.  I use a couple of different methods of accomplishing this and the Ravenna project demonstrates the simplest, most cost effective one.  In this case the entire roof of the dormer is built from common trusses.  Over-framing is added to project the dominant existing ridge “through” the dormer.  This gives the impression that there are 2 shed dormers projecting from the roof, which is what you want a dormer to look like.
View without the dormer roof structure
View without the dormer roof structure
This obviously only works with shed dormers.  It also only works when “both” dormers are the same width and directly across from each other.  Here you can see that the trusses only rest on the outside walls and impose no loads on the floor or inside walls.
Roof truss configuration as described
Roof truss configuration as described
The only other consideration is to verify that the headers over the existing windows below are sufficiently strong to support the added weight.  This is usually not a problem on small windows like we have here.

Plumbing

Simplicity Issues - As above, these issues involve minimizing the intrusion into the existing finished space.  The first of these issues is plumbing.  In an old, existing finished structure it can be difficult to run waste piping from the new bathroom to the existing lines.  Our solution in this case is to locate the new bathroom roughly above an existing bathroom in the floor below.  Some of the lower floor ceiling and wall plaster will need to be removed to provide access but in this case we’ve been able to limit it to a closet adjacent to the existing bathroom plumbing wall.

Demolition

The next simplicity issue is to have simple lines of demolition.  In this case we can demolish the entire roof within the new dormer area while preserving all of the roof structure outside of the dormer area.  It’s just 2 straight lines to cut and remove.

The Chimney

One final issue is the chimney.  It’s currently too short after the dormer has been added.  It’s also in an awkward location.  One solution is to eliminate the wood burning fireplace and to install a gas insert into the existing box.  In this case the chimney can be torn down to just below the roof overhang.  The overhang can be extended a little to cover it and a small vent can provide exhaust for the insert.
The alternative is to tear down the chimney to the 2nd floor line and to build a new reinforced chimney back up to the appropriate height.  The illustration suggests that you might only need to add a couple of feet to the existing chimney height.  However, chimneys of this era were not reinforced and have been through several severe earthquakes.  The part of the chimney above the roof is most vulnerable to deterioration and is probably not capable of supporting any additional height.  In addition, because of the relationship of the chimney to the dormer,  a small cricket will need to be added to the roof to facilitate drainage behind the chimney.
In my opinion, this is the most cost efficient way for my clients to add the additional space they need for their growing family.  The dormer is consistent with the original architectural style.  We’ve minimized intrusion into the finished structure below and we’ve met their programmatic goals.  I’ll post pictures of the finished project when it’s completed later this year.

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