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Old 06-16-2013, 03:04 AM   #1
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Code concerning porch joists near grade


We are looking into building a 3 season porch here in S/E Wisconsin. Today a carpenter/home builder came out to look at the site. He suggests using a frost wall, topped with a 2X6 treated sill plate, and "I-joists" sitting on the sill. He said the joists don't have to be treated lumber because they're not exposed to the weather. I thought they needed to be treated because they will be pretty close to grade? The floor height is only about 16" above grade. Assuming 12" for I-joists and 3/4" plywood floor, that only leaves about 4" between bottom of joist and grade. Also, the top of pier footing should be a few inches above grade, correct? That puts bottom of joist about 1" above grade. It looks like I may need to use 9" joists instead of 11" correct? Thanks.

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Old 06-16-2013, 05:44 AM   #2
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Code concerning porch joists near grade


The size of joist required is a function of the loading (this is code based) and the joist spacing. If for reasons of clearance you cannot use the required depth joist, you need to space them more closely, or use a stronger joist (better species of wood, engineered lumber, or in extreme cases steel joists).

As to whether code requires PT lumber in your case, you could always ask your local building inspector, who would be the code expert in your area. You are almost certainly going to need a permit, so you will need to file plans prior to construction. In my experience, the building inspector will probably be happy to talk to you about your job.

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Old 06-16-2013, 08:06 AM   #3
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Code concerning porch joists near grade


Code or not anything closer then 16" from grade needs to be pressure treated.
Has more to do with lack of air flow and rising moisture coming up under ground then not being exposed.
Any closer to grade would never past in my area.
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Old 06-16-2013, 08:23 AM   #4
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Code concerning porch joists near grade


This may interest you.

From the IRC:

Quote:
SECTION R317 PROTECTION OF WOOD AND WOOD BASED PRODUCTS AGAINST DECAY

R317.1 Location required. Protection of wood and wood based products from decay shall be provided in the following locations by the use of naturally durable wood or wood that is preservative-treated in accordance with AWPA U1 for the species, product, preservative and end use. Preservatives shall be listed in Section 4 of AWPA U1.

1. Wood joists or the bottom of a wood structural floor when closer than 18 inches (457 mm) or wood girders when closer than 12 inches (305 mm) to the exposed ground in crawl spaces or unexcavated area located within the periphery of the building foundation. 2. All wood framing members that rest on concrete or masonry exterior foundation walls and are less than 8 inches (203 mm) from the exposed ground. 3. Sills and sleepers on a concrete or masonry slab that is in direct contact with the ground unless separated from such slab by an impervious moisture barrier. 4. The ends of wood girders entering exterior masonry or concrete walls having clearances of less than 1/2 inch (12.7 mm) on tops, sides and ends. 5. Wood siding, sheathing and wall framing on the exterior of a building having a clearance of less than 6 inches (152 mm) from the ground or less than 2 inches (51 mm) measured vertically from concrete steps, porch slabs, patio slabs, and similar horizontal surfaces exposed to the weather. 6. Wood structural members supporting moisture-permeable floors or roofs that are exposed to the weather, such as concrete or masonry slabs, unless separated from such floors or roofs by an impervious moisture barrier. 7. Wood furring strips or other wood framing members attached directly to the interior of exterior masonry walls or concrete walls below grade except where an approved vapor retarder is applied between the wall and the furring strips or framing members.
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Old 06-16-2013, 10:42 AM   #5
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Code concerning porch joists near grade


Installing a moisture barrier on the ground prior to construction may help.
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Old 06-16-2013, 11:00 AM   #6
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Code concerning porch joists near grade


You and your builder have options. One thing to keep in mind is that 3-season porches often turn into year-round living space. Also, what comfort level do you want in or on your porch? How carefully to treat that damp space under the porch is a question to answer correctly now rather than later.

Edit: What type of flooring or decking will go on the porch? If it's a continuous floor, i.e. not decking with gaps, then you want that space underneath to be dry.

Last edited by CarpenterSFO; 06-16-2013 at 11:03 AM.
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Old 06-16-2013, 11:45 AM   #7
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Code concerning porch joists near grade


Thanks guys. We are not sure on the floor yet. Possibly porcelain tile, but I think it may be too cold. We will probably just use portable electric heat.

I'm also getting mixed opinions on whether to use a frost wall or concrete piers. The house has a full basement with concrete block walls. The porch will butt up against 2 right angle walls, so there would be 2 frost walls, 48" deep. An architect told me a frost wall with concrete slab floor will always be cold(floor) even with 2" insulation all around. He suggests concrete pier footings with the largest joists that will fit, for the most insulation. I know a frost wall will be more expensive, but is it the best choice for a 3 season porch? If so, should we do a concrete slab floor also, or joists sitting on top of the frost wall? A concrete guy that came out yesterday said that if I'm using wood framing, there's no reason to use a frost wall. He suggests concrete pier footings. I'm just confused. Another benefit of the frost wall would be keeping critters out from under the porch, correct?

Last edited by Jim McC; 06-16-2013 at 11:58 AM.
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Old 06-16-2013, 08:11 PM   #8
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Code concerning porch joists near grade


if this was a project in my area I would recommend to my client a frost wall (48" to bottom of footings) with concrete walls on the two sides. I would remove the fill within the footprint. a concrete slab would be placed on top of the footing. floor joists (whether I-joists or solid sawn lumber) would be placed on top of a double top plate. Insulation would be installed between the joists with the vapor retarder towards the warm-in-winter side of the floor. If using porcelain tile was being used I'd go with a L/720 deflection limit for my joists (1/2 the deflection as typically allowed by most building codes). I'd provide access through the block wall into the enclosed crawl space under the room. I would use 2" rigid insulation along the perimeter of the frost walls.
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Old 06-16-2013, 08:35 PM   #9
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Code concerning porch joists near grade


Thanks. Do you agree with the architect in that if we use a frost wall and concrete slab floor, the floor would always be cold(even with 2" insulation for floor and frost walls)?
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Old 06-16-2013, 10:03 PM   #10
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Code concerning porch joists near grade


no, I do not agree ........

i'm not taking about having the grade close to the floor, I'm taking about having a slab set on top of the footing with the wooden floor set level with the rest of the house. the enclosed crawl space will remain at the same temperature as your basement and with proper insulation in the joist bays you'll be fine.
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Old 06-17-2013, 04:55 PM   #11
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Code concerning porch joists near grade


typically (in my area) if you use a slab on grade (with a frost wall) you are required to have a thermal break between the frost wall and the slab. this is typically accomplished with 2" of rigid insulation between the edge of the concrete slab and the frost wall. we usually install rigid insulation under the slab within 48" of the frost wall.

if you have a concrete slab touch an exposed concrete foundation then you will have heat from the slab pass through the foundation. remember heat travels from hot to cold.

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