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Every carpenter, electrician, plumber, HVAC contractor, and home inspector should know the rules for allowable cuts and holes into floor joists. Cuts and holes are known in the trades as notching and boring, respectively. The rules for notching and boring come from section 502.8 of the International Residential Code, which addresses cutting, drilling, and notching. Today I’m only focused on traditional dimensional lumber, like 2x8s or 2x12s. I’ll discuss holes in manufactured floor joists, aka I-joists, in next week’s blog post.

Rules for boring/drilling holes

To start, here’s the diagram that comes right out of the codebook.

You can put holes in floor joists anywhere along the length of the joist, but you can’t put them any closer than 2″ from the top or bottom edge of a joist. The maximum size of a hole is 1/3 the depth of the floor joist. Dimensional lumber will always be a little smaller than the stated dimension unless you’re dealing with some ancient lumber. For example, a 2×4 actually measures 1-1/2″ x 3-1/2″. For floor joists, here are the maximum hole sizes for common floor joist sizes:

2×6 (5-1/2″) ~ 1-13/16″

2×8 (7-1/4″) ~ 2-3/8″

2×10 (9-1/4″) ~ 3-1/16″

2×12 (11-1/4″) = 3-3/4″

If you’re dealing with rafters, these same rules apply.

Rules for notching

The codebook has two diagrams for notching; one at the ends of the joist, and another for notching anywhere else. Let’s start with the easier one, notching at the ends. You’re allowed to notch 1/4 of the joist depth at the end of a joist. For some common sizes, here are the maximum notches allowed:

2×6 (5-1/2″) = 1-3/8″

2×8 (7-1/4″) = 1-13/16″

2×10 (9-1/4″) = 2-5/16″

2×12 (11-1/4″) = 2-13/16″

For anywhere other than the ends, we use a different diagram.

You’re not allowed to do any notches in the middle third of a floor joist span, and you’re not allowed to notch the bottom side of members over 4″ in nominal thickness. For anything else, you’re allowed to notch 1/6 of the joist depth deep, and 1/3 of the joist depth wide. Here are the maximum numbers for some common sizes:

2×6 (5-1/2″) ~ 7/8″ deep, 1-13/16″ wide

2×8 (7-1/4″) ~ 1-3/16 deep, 2-3/8″ wide

2×10 (9-1/4″) ~ 1-1/2″ deep, 3-1/16″ wide

2×12 (11-1/4″) = 1-7/8″ deep, 3-3/4″ wide

Summary

When any of these rules are violated, it’s a judgment call. When these rules a fudged just a little bit, I usually don’t make a big deal about it, especially if it looks like it has been that way for a long time. These numbers always assume the maximum loading for a floor, along with the maximum spacing of a floor joist. But when these rules are egregiously broken, that’s when it’s time to call in a carpenter for repairs. And note, I didn’t say to call in an engineer for further evaluation. This is basic floor framing, and any qualified carpenter ought to be able to make repairs without having to do a bunch of difficult math. And just for fun, here are a few classics we’ve found over the years.

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What size shed can you build without permission?

Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.

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Permitted Development For Outbuildings

New rules regarding whether or not you can extend or add to your home without having to apply for planning permission for an outbuilding came into force on 1 October 2008. The New limits and conditions for what is allowed without the need for planning permission apply largely to the dimensions of the proposed addition, its position in relation to the house and its proximity to the boundaries of the property. Under the new regulations constructing an outbuilding that is not within designated land is considered permitted development, not requiring an application for planning permission, providing it meets the following limits and conditions: An outbuilding can not be constructed on land forward of a wall forming the principal elevation. Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof. Maximum height of 2.5 metres in the case of a building, enclosure or container within two metres of a boundary of the curtilage of the dwellinghouse.No verandas, balconies or raised platforms. No more than half the area of land around the "original house"* would be covered by additions or other buildings. In National Parks, the Broads, Areas of Outstanding Natural Beauty and World Heritage Sites the maximum area to be covered by buildings, enclosures, containers and pools more than 20 metres from house to be limited to 10 square metres. On designated land buildings, enclosures, containers and pools at the side of properties will require planning permission.

Within the curtilage of listed buildings any outbuilding will require planning permission.

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