How Important Is Roof Overhang?
February 3, 2025 11:29 PM Subscribe
We recently purchased a prefab accessory dwelling unit including installation in our backyard. It is a simple rectangular shape (30' front/back x 20' sides) of 600 square feet (which is the maximum for our area) with a flat roof gently sloping to the back. Originally it was designed with a one foot roof overhang all the way around, however due to very recent zoning change, roof overhang is now considered as part of the square footage. The builder is proposing removing the overhang on the front and sides and retaining it on the back to hold the eavestrough.
We live in a typical northeast climate with the full four seasons. An alternative would be to reduce the interior size of the unit by 90 square feet, a significant 15% reduction in an already compact two bedroom unit. How important is roof overhang to protecting a house from the elements? Would a compromise solution of a smaller overhang make a difference? Other options?
We live in a typical northeast climate with the full four seasons. An alternative would be to reduce the interior size of the unit by 90 square feet, a significant 15% reduction in an already compact two bedroom unit. How important is roof overhang to protecting a house from the elements? Would a compromise solution of a smaller overhang make a difference? Other options?
I am with flabdablet although he put it nicer than I would have. The only question I would discuss with the builder is whether the permanent overhang should be in the back or the front. Without seeing the plans and the property, I would think that an overhang over the entrance makes more sense than the back. Obviously, the sun direction and structural issues are relevant.
Also, removing the overhang around a decent part of the structure could also affect runoff and drainage of precipitation. Not sure what sort of foundation the structure will sit on, but I would be cognizant of how the rain or snow melt will seep into the foundation.
posted by JohnnyGunn at 12:45 AM on February 4 [3 favorites]
Also, removing the overhang around a decent part of the structure could also affect runoff and drainage of precipitation. Not sure what sort of foundation the structure will sit on, but I would be cognizant of how the rain or snow melt will seep into the foundation.
posted by JohnnyGunn at 12:45 AM on February 4 [3 favorites]
Also, removing the overhang around a decent part of the structure could also affect runoff and drainage of precipitation.
Presumably that's why the builder is suggesting keeping the overhang at the back, which is what the roof slopes down towards. When (not if) the guttering overflows, you don't want the flood actually running down your walls because that's how you get seepage into the interior that turns into mould.
posted by flabdablet at 1:19 AM on February 4 [3 favorites]
Presumably that's why the builder is suggesting keeping the overhang at the back, which is what the roof slopes down towards. When (not if) the guttering overflows, you don't want the flood actually running down your walls because that's how you get seepage into the interior that turns into mould.
posted by flabdablet at 1:19 AM on February 4 [3 favorites]
Overhang is important where the roof water falls off of the roof, otherwise it will sheet down onto the wall and perhaps find a way inside.
I would have a similiar but less severe concern with leaking at the front of the house, but an overhang protects the join between the top of the wall and the roof in other ways, depending on your "weather direction" and wind.
Finally, making in-field modifications to a prefab house, which is presumably designed as a system, has a peril of its own. Flashing is different on a wall with no overhang, and a building shipped with an overhang is likely to lack that flashing.
posted by the Real Dan at 1:23 AM on February 4 [6 favorites]
I would have a similiar but less severe concern with leaking at the front of the house, but an overhang protects the join between the top of the wall and the roof in other ways, depending on your "weather direction" and wind.
Finally, making in-field modifications to a prefab house, which is presumably designed as a system, has a peril of its own. Flashing is different on a wall with no overhang, and a building shipped with an overhang is likely to lack that flashing.
posted by the Real Dan at 1:23 AM on February 4 [6 favorites]
Is the builder proposing removing the overhang from the already constructed building or removing it from the design prior to construction? They are very different propositions and I can't determine which is being suggested.
posted by deadwax at 2:01 AM on February 4 [1 favorite]
posted by deadwax at 2:01 AM on February 4 [1 favorite]
Response by poster: Is the builder proposing removing the overhang from the already constructed building or removing it from the design prior to construction?
It's still in the design stage.
posted by fairmettle at 3:09 AM on February 4 [1 favorite]
It's still in the design stage.
posted by fairmettle at 3:09 AM on February 4 [1 favorite]
In that case a I'd say a competent designer and builder can likely make it work with or without overhang, particularly as you are keeping the lower edge with the gutter, which is likely the most important. Whether the people you are dealing with are competent we don't know and I'd be having a chat with the designer about any climatic concerns (is south facing shading, presuming you are in the northern hemisphere, necessary for summer, etc, etc). But, yes it can be made to work structurally.
You can't add back what is designed out after construction though, at least not at the level of eaves or other significant structure. Some responses above assume it's already built I think.
posted by deadwax at 3:20 AM on February 4 [3 favorites]
You can't add back what is designed out after construction though, at least not at the level of eaves or other significant structure. Some responses above assume it's already built I think.
posted by deadwax at 3:20 AM on February 4 [3 favorites]
Depending on the nature of the ground there, from slope to water retention, the overhang may also impact foundation longevity, depending on construction. I'm not a builder, so I don't know what preparations may have gone into design, but theoretically the ground around houses should always have just enough of a slope for the water to travel away from the foundation. I would personally be a little concerned about this, were I you, and would ask the builder about impact of removal of the overhang on water getting to the foundation.
posted by cupcakeninja at 6:23 AM on February 4 [3 favorites]
posted by cupcakeninja at 6:23 AM on February 4 [3 favorites]
Our two-foot plus overhang is a significant factor in alleviating the heat beating down onto the windows and the side of the house in the summer. Given your NE climate, this probably won't concern you but be aware that it does affect the amount of light and potential glare you may get.
posted by BlueHorse at 8:31 AM on February 4 [1 favorite]
posted by BlueHorse at 8:31 AM on February 4 [1 favorite]
I'll see if i can find a chart from Japan used to rate different overhangs in terms of normal rain -however, roofs are not required to have overhangs and some house styles lack them (flat roofs for one) but pitched roofs also can lack overhangs.
Is it a big deal? IMO, the styles with a hatted roofs look odd without overhangs, but pitched ones can look ok, and according to modern construction techniques, walls vs floor vs roof are all designed in a very similar manner in terms of weather and water shedding, and a roof overhang is a design choice, not a requirement.
posted by The_Vegetables at 10:18 AM on February 4 [1 favorite]
Is it a big deal? IMO, the styles with a hatted roofs look odd without overhangs, but pitched ones can look ok, and according to modern construction techniques, walls vs floor vs roof are all designed in a very similar manner in terms of weather and water shedding, and a roof overhang is a design choice, not a requirement.
posted by The_Vegetables at 10:18 AM on February 4 [1 favorite]
House styles that don't have overhangs from Treehugger . Lots of modern houses have pitched roofs without overhangs.
posted by The_Vegetables at 10:34 AM on February 4 [1 favorite]
posted by The_Vegetables at 10:34 AM on February 4 [1 favorite]
It seems pretty common on tiny homes to have the eave overhang on one side, like thus. Obviously yours doesn't need to be this dramatic, but I don't think it will look odd.
posted by hydra77 at 11:54 AM on February 4 [1 favorite]
posted by hydra77 at 11:54 AM on February 4 [1 favorite]
The reason your county or municipality is counting overhang as being the same as the footprint of the building is because they are calculating the size of your building using drone footage. They don't want to have an inspector checking each building from ground level. But many buildings do use an overhang to protect their wall and their foundation. - As I write the back of my house is encased in ice which is damaging the siding. I have a flat roof and while there is eaves troughing, it's not enough to prevent ice sheets from forming when there is a lot of rain in the winter.
Your builder may have ideas how to mitigate the potential damage, and will use materials and construction that will result in less vulnerability to damage. But there is still a reasonable chance that the lack of the overhang will make your building less durable, and also may increase heating and cooling costs. You should go into this with your builder.
posted by Jane the Brown at 6:53 PM on February 4 [4 favorites]
Your builder may have ideas how to mitigate the potential damage, and will use materials and construction that will result in less vulnerability to damage. But there is still a reasonable chance that the lack of the overhang will make your building less durable, and also may increase heating and cooling costs. You should go into this with your builder.
posted by Jane the Brown at 6:53 PM on February 4 [4 favorites]
Traditionally, overhangs also include underside vents to aid airflow through the attic space. If there is an attic, I’d be curious how this will be accomplished with no overhang.
posted by Thorzdad at 8:14 AM on February 6
posted by Thorzdad at 8:14 AM on February 6
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Anything designed to be easily removed in response to potential nastygrams is also going to be easy to put back again once the heat's off. This kind of arbitrary zoning change can fuck off into the sun.
posted by flabdablet at 11:51 PM on February 3 [7 favorites]